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PHYS THER
Vol. 87, No. 12, December 2007, pp. 1651-1666
DOI: 10.2522/ptj.20060378
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Research Reports

Event Standardization of Sit-to-Stand Movements

Bruce Etnyre and David Q Thomas

B Etnyre, PT, PhD, is Professor, Department of Kinesiology, MS-545, Rice University, 6100 Main, Houston, TX 77005 (USA), and Adjunct Professor, Physical Therapy Department, Texas Woman's University, Houston, Tex
DQ Thomas, PhD, FACSM, is Professor, Department of Kinesiology, Illinois State University, Normal, Ill

Address all correspondence to Dr Etnyre at: etnyre{at}rice.edu


Submitted December 15, 2006; Accepted July 19, 2007


   Abstract
 
Background and Purpose: Unlike gait analysis, no commonly accepted method for studying sit-to-stand (STS) movements exists. Most previous studies describing STS events used various methods to identify movement events while restricting sitting positions and movements. The present study observed natural rising from a sitting position using a simple method for measuring this common task. The purposes of this study were to compare commonly performed STS movements and to propose a standard system for defining identifiable sequential events.

Subjects and Methods: Ground reaction forces of 100 adults who were healthy (50 male, 50 female) were recorded using a force platform as each participant performed 4 methods of rising from sitting on a standard chair. The 4 STS conditions were: with arms free, with hands on knees, using armrests, and with arms crossed.

Results: For each subject, 11 recorded events from the vertical, fore-aft, and lateral dimensions were identified for all arm-use conditions. The only significant and clinically relevant force difference among arm-use conditions was that the armrests condition produced less average force than the other 3 conditions during the seat-off and vertical peak force events. Among average event times, the armrests condition showed significantly longer time to the vertical peak force event than the other conditions.

Discussion and Conclusion: Because these events occurred invariably in sequential order for every individual for all arm-use conditions in a relatively large sample of observations during natural STS movements, this method may be useful for establishing a standard method to assess and compare patient functionality and allow comparisons among STS research studies.


   Introduction
Top
 Abstract
 Introduction
Method
 Results
 Discussion
 Conclusions, Clinical...
 References
 
The sit-to-stand (STS) movement is a simple task for most people. The ability to rise unassisted from a seated position is one of the most fundamental movements among normal activities of daily living. The STS movement is an essential precursor to walking and has at least an equal magnitude of importance.

Describing and defining common human movement tasks often are difficult. People differ in shapes and sizes and have unique and distinctive movement styles. Individuals tend not to repeat movement tasks exactly the same way during each successive performance. Parameters for describing gait patterns during walking have been established with little controversy, but a commonly accepted method for describing STS movements has been particularly elusive.

Numerous studies have used widely varying methods to describe actions performed during STS movements and have produced equally diverse results. A common approach for establishing standardized events has been by constraining STS movements to control variability among individuals. However, by controlling variability, important components of the movement may be lost. Additionally, differences in experimental methods made comparisons difficult or impossible, and results often were contradictory.1 Early STS studies emphasized the importance of individual differences,2,3 but mostly they have been ignored since then.

Standardized events for the gait cycle have been fairly well established.4 Normal walking is classified as a continuous task, without a distinct beginning or end. For analytical purposes, however, the gait cycle, by consensus, has been standardized to identify initiation at heel-contact. Rising from a sitting position is a discrete task,5 with the initiation logically beginning from the seated position. The transition from sitting to standing empirically comprises horizontal and vertical momentum generated by movements of the head, arms, trunk, and body segments around the hip, knee, and ankle joints during performance of flexion and extension movements. Other than the time of initial sitting, however, researchers generally have not agreed on a single method for delineating STS events.6 Similar to methods currently used to describe gait pattern events, a commonly accepted description of distinct events required throughout the functional stages of the STS movement would allow reliable comparisons among research studies and simplify communication related to the STS phases for researchers and clinicians.

Most previous studies of STS events relied on kinematic measurement analysis (video recordings, electrogoniometers, accelerometers) to identify movement of body segments or joint angles to determine each STS event. No 2 movements, however, are identical between or within individuals. This resulted in widely varying methods for identifying distinct movement events of STS, which should be unequivocal for any STS performance. These movement events include seat-off, the beginning of the movement, and the end of the movement.

Although methods for analyzing STS movements mostly differed among studies, some events should be clearly evident. It would seem that the point of separation from the seat could be defined as an indisputable common event for any observation of STS movements. However, the seat-off event (also termed "lift-off," "seat separation," and "seat unloading") has been described in various ways among researchers. This transition point is important because it is when balance parameters change from a stable base of support (sitting) to a relatively unstable base of support (standing). This may be a critical event for understanding prevention of falls in elderly people.7 Methods used in past studies defined seat-off as the time at peak horizontal force,8,9 peak vertical force,10 initial vertical force,11,12 a point of thigh separation from the seat,13–16 maximum anterior head movement,17,18 or 100% vertical ground reaction force19 or through use of an instrumented seat (seat-switch or force transducer).3,20–33 It was argued that methods defining seat-off without an instrumented seat may lack validity.28 Using a force platform as an instrumented seat to measure the seat-off time, however, may differ, depending on whether initial unloading, peak unloading, or end of unloading is selected as the point of seat-off. Many studies did not report how seat-off was operationally defined or did not report the time of seat-off.

The beginning of the STS movement also would appear to be an easily identifiable event, necessary for establishing a point to measure the relative durations to other STS event times (eg, seat-off) and allowing valid comparisons among studies. Reported start times, however, often were based on varying definitions, identifying the start time as: initial fore-aft momentum25; the initiation of trunk flexion displacement, velocity, or momentum8,11–14,24,27,29,34–37; the first horizontal displacement of a marker on the head, vertebra, or shoulder2,3,15,16,22,32,33,38,39; or the first vertical force deviation greater than 10 N.20 Determining initiation from only one identifier or a limited number of identifiers may overlook initial movement contributions from other segments (eg, arm-use momentum). Most studies reported some statistical form of total STS duration, but many did not operationally define the initiation time, and definitions varied among studies regarding how the start time was defined. Nuzik et al,40 in their report of a study of 55 subjects, stated that no single body segment consistently identified STS initiation, due to individual differences among movement strategies.

Precisely defining the end of the STS movement also may be difficult because of postural sway movement during steady standing. Some studies8,11–13,17,18,23,25,34,35,41 simply described the end of the STS movement as a point of a fully upright or quiet standing posture. Most researchers, by observing displacement, velocity, acceleration, or momentum in the horizontal direction2,3,20–22,29,30,32,33,37–39,42,43 or the vertical direction,24,44 defined the end of the STS movement as related to some measure of minimal movement of the head, vertebra, shoulder, or hip. Other researchers14–16,27 defined the end of the STS movement as the maximum height of the shoulder, hip, or center of mass. Defining seat-off, the beginning of the STS movement, and the end of the STS movement from minimum or maximum body segment motions could result in varying measurements between and within subjects, depending on individual differences related to varying strategies and patterns of body segments used during subsequent performances of the movement.

Two fairly extensive reviews of literature examining STS by Janssen and colleagues6 and Kerr and colleagues45 reported that the variability of methods and measurement techniques among previous studies made it extremely difficult to compare experimental outcomes or relate outcomes to clinical applications. Kerr and colleagues45 concluded that, although valuable numerical data were derived from numerous studies, the extensive complexity of designs using highly technical equipment and considerable manipulation of the often limited number of participants in each study made it impossible to reasonably compare or apply the results.

Janssen and colleagues,6 in a more recent comprehensive review of 39 STS research reports, generally concluded that chair height and foot position, as well as the use of armrests, substantially influenced STS performance, but the limited number of participants and the widely varying methods and results did not provide relevant information for how individuals without impairments rise from sitting positions in customary ways. Although the conclusion of the review appreciated the necessity to control variables for experimental and comparative studies, Janssen and colleagues recommended that new methods needed to be developed for generalizability outside the laboratory because constrained-condition methods often were not useful for clinical application. Since the most recent of those reviews in 2002, more than 100 published studies included STS as a variable, but no consensus has been established for standardizing the events of the movement.

Movement analysis is most effectively described by dividing it into separate events or phases. The most frequently cited studies identifying STS events and phases generally described them in 1 of 3 ways: (1) flexion and extension phases,8,13,17,18,24,29,35,38,39,44 (2) 4 phases distinguished by trunk movement and ankle dorsiflexion,11,12,16,34,35,37 and (3) momentum, torque, or velocity event changes.23,25,30,36,46–50 These studies analyzed STS movements most frequently by determining events related primarily to joint angles rather than ground reaction force events. Because of the diversity of movements both between and within individuals as they perform repeated trials of the STS task, the variability of joint angle measurements may have contributed to difficulty establishing a common method for describing STS events or phases.

With literally hundreds of studies completed involving STS movements, it seems inconceivable that no standardization has been widely adopted. However, because each research group formed its own parameters for measurement, based mostly on joint angles and using available equipment with widely varied objectives, no consensus on a standardized STS method currently exists. This severely limits generalizability among studies and the feasibility for clinical application.

In order to analyze the components of STS movements in a scientifically acceptable manner, previous studies commonly constrained initial positions and movements to allow comparisons between normalized results or to compare the influences of the varied conditional parameters. Previous studies also varied inclusion of participants by age, sex, or pathology. Common parameters imposed by researchers of STS studies constrained: chair height, subject starting positions, rising speed, and use of the arms. Many studies adjusted the seat height to a proportion of each subject's knee height. Some studies required starting foot placement positions or required specific trunk, knee, or ankle joint angles. Rising speed sometimes was controlled by synchronization of movement with a metronome. More than half of previous studies recorded STS movements with the participants' arms crossed. Often those studies included 2 or more of these movement constraints. The constraints resulted in observations of the STS movement in ways not normally performed.

The primary purposes of the present study were: (1) to identify a set of the most useful invariably occurring ground reaction force events during performance of 4 different STS methods, (2) to compare ground reaction forces and event times of the most common normally used methods of rising (ie, arms free, hands on knees, and using armrests) with the constrained method of arms crossed, and (3) to compare ground reaction forces and event times between male and female participants to determine whether differences could be observed between sexes. A relatively large sample of individuals who were healthy was included in an attempt to begin to establish normative values for event forces and times during the STS movement.


   Method
Top
 Abstract
 Introduction
 Method
Results
 Discussion
 Conclusions, Clinical...
 References
 
Participants

Data for this study were obtained from 100 randomly selected volunteers (50 men, 50 women). Table 1 includes summary statistics of participant characteristics. None of the included subjects reported any recent history of any neuromuscular or orthopedic disorder.


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  		Table 1. Summary of Subject Characteristics for Age, Height, and Weighta

 
Instrumentation

The apparatus included a force platform, a seat switch, and a signal light, all synchronized to a computer. An AMTI force plate (model 3181*), positioned directly beneath each participant's feet, was interfaced through an analog-to-digital (A/D) circuit board to the computer. A switch was embedded in the seat of the chair in a position that closed the switch on contact from the participant's right buttock beneath the ischial tuberosity. The seat switch was connected to a channel of the A/D conversion board interfaced with the computer. A signal light (Light Response Control, model 58036{dagger}) was placed in direct view of each participant indicating when to begin the STS movement for each trial. An experimenter-controlled switch simultaneously illuminated the signal light for the participant to begin each trial and initiated force platform and seat-switch recordings. The sampling rate for each A/D channel was 1 kHz.

Procedure

Following informed consent, self-reported information was recorded from each participant regarding sex, date of birth, height, weight, and orthopedic or neuromuscular pathologies. Participants who reported having had a pathology within the past 2 years were excluded from the study.

A standard wooden chair (seat height=43 cm, seat width=54 cm, and seat depth=56 cm) with a back support (33 cm high) and armrests (24 cm high and 55 cm apart) was positioned adjacent to the force platform (Fig. 1). The force platform recorded ground reaction forces in the vertical, fore-aft, and lateral dimensions. Vertical force was recorded as the force values the body weight produced directly against the ground during the STS movement. The fore-aft force recordings were in a positive direction when force was applied backward, thus moving the body forward and in a negative direction as force was applied forward. In the lateral dimension, recordings deflected in a positive direction when force was applied to the right and in a negative direction for left forces.


Figure 1
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  		Figure 1. Experimental paradigm.

 
Participants assumed a self-selected comfortable seated position on the chair without resting on the back support. The only constraint was the right buttock area beneath the ischial tuberosity needed to depress the seat switch. In most cases, no repositioning of the participant was necessary. Each person performed each of the 4 STS conditions at a self-selected natural speed in response to the recorded visual starting signal. The start signal ensured a consistent initiation time for all trials, regardless of the participant's initial movement.

The 4 STS conditions were: arms free, arms crossed, hands on knees, and hands on armrests. Instructions to the participants for the conditions were: (1) in the arms-free condition, let the arms hang freely at the sides and rise naturally, (2) in the arms-crossed position, cross the arms over the chest and hold them close to the body while rising, (3) in the hands-on-knees condition, place the hands on the knees and rise naturally, and (4) in the hands-on-armrests position, place the hands on the armrests and rise naturally. Participants were instructed to begin the STS movement when the initiation signal illuminated. Order of performance of the conditions was counterbalanced among participants using a Latin-squares design. (For video clips demonstrating the 4 STS conditions, visit Supplemental Videos.)

Data Analysis

Force platform recordings in the 3 dimensions and the seat-switch voltage were plotted for each trial of the 4 conditions for each participant. Every plot displayed 6 distinct events identifiable in the vertical ground reaction force recordings. These events were identified, in order of occurrence following the visual signal to begin, as: (1) initial force change, (2) a counter force, (3) seat-off, (4) peak force, (5) post–peak rebound force, and (6) steady standing force. Figure 2 shows recordings for one participant (#073), an 86-kg (189-lb) man, during an arms-crossed trial with bordered labels added identifying the recorded traces and labels and arrows indicating the vertical events. The initial force change from a sitting position was defined as the first deflection from baseline of the force platform recording. The counter force was observed as a reduction of vertical force following initiation. The peak counter force event was defined as the lowest recorded vertical force value. Seat-off was identified by the seat-switch opening and the resultant voltage change (DC voltage on the computer monitor was offset to less than zero for clarity). Peak force was defined as the greatest vertical force recorded. A less-than-body-weight rebound force event was observed following the peak force. The peak rebound force was defined as the lowest force value following the peak force. Steady standing was determined as the first instant following the rebound recovery when the recording leveled to normal postural sway. Visual determination of the first point of steady standing was found to be more reliable than any algorithm to establish initial steady standing, such as within some arbitrary boundary (eg, within ±5% of body weight).


Figure 2
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  		Figure 2. Individual data recording for a single participant (#073, 189-lb [86-kg] man) during performance of a trial in the arms-crossed condition. Labels were added to identify the vertical force, fore-aft force, lateral force, and seat-switch traces. Arrows and labels indicating the 6 distinct events of the vertical force recording were added to the original recording. Labels from original plot: FX=lateral force, FY=fore-aft force, FZ=vertical force, SS=seat switch.

 
Three force events were identified in the fore-aft dimension: (1) start of fore-aft force, (2) peak fore-aft force, and (3) end of fore-aft force. Fore-aft peak force was defined as the greatest positive value force in the fore-aft force recording. Start of fore-aft force and end of fore-aft force times were defined as the points of least force in the negative direction immediately before and after the peak fore-aft force, respectively. Peak lateral forces were observed in the right and left directions, defined as the point of greatest force in each direction, respectively. The total of 11 event times and event force values from all 100 participants for each arm-use condition were recorded on a spreadsheet for further analysis. All recorded forces were normalized to percentage of body weight.

Event times were analyzed using the actual times (real times from raw force recordings) when each subject's force events occurred following the initiation signal (eg, time to peak force). Event times also were analyzed by percentage of time, calculated by dividing each actual force event time (eg, time to peak force) by the total time (ie, from the initiation signal time to the steady standing force event time) for each subject for each trial. Normalized forces as well as actual and normalized times were analyzed using separate 2-way (2 [sex] x 4 [arm-use conditions]) analyses of variance (ANOVAs). An alpha level of .05 was selected and a Bonferroni adjustment was made for the ANOVAs performed with each of the 11 events (6 vertical, 3 horizontal, and 2 lateral). With the P-value adjustment (.05/11), a significance level of .005 was established for all comparisons. When significant effects occurred, follow-up t tests were performed to determine differences between means.


   Results
Top
 Abstract
 Introduction
 Method
 Results
Discussion
 Conclusions, Clinical...
 References
 
The prominent finding of this study was the identification of distinct events during the STS movement that were consistent for all 100 subjects regardless of arm-use condition. Six events were invariably identifiable from the vertical dimension force recordings during rising. These events were, in sequence: (1) initiation, (2) counter, (3) seat-off, (4) peak, (5) rebound, and (6) standing. Fore-aft events also occurred invariably as the: (1) start, (2) peak, and (3) end forces. Lateral forces in each direction were identified as: (1) right and (2) left. Lateral forces were not sequentially invariable across participants. Although the vast majority of STS trials (94%) resulted in force to the right first, some trials (6%) produced force to the left first. Figure 2 includes 3-dimensional force recordings for a typical representative subject during the arms-crossed condition, and Figure 3 includes the averages over all subjects by events for each arm-use condition.


Figure 3
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  		Figure 3. Averaged normalized recordings over all trials and participants from the force platform and the seat-switch event for the 4 arm-use conditions by events. The arms-crossed condition was considered as a control condition, and the standard deviation was plotted for that condition. Positive deflections of the fore-aft traces represent backward force (ie, moving the body forward). Lateral force to the right is shown in the positive direction, left toward negative. FREE=arms-free condition, CROSS=arms-crossed condition, KNEE=hands-on-knees condition, and ARMRESTS=hands-on-armrests condition.

 
Event Ground Reaction Forces

The comparison of normalized force results showed significant main effects among arm-use conditions for the vertical seat-off, peak, and rebound events (Tab. 2). Average normalized forces were generally similar for the arms-free, arms-crossed, and hands-on-knees conditions for the 6 vertical events: initiation ({approx}20%), counter ({approx}12%), seat-off ({approx}60%–70%), peak ({approx}116%), rebound ({approx}80%), and standing ({approx}100%) (Tab. 2). Average normalized forces also were generally similar for all conditions for the fore-aft peak ({approx}9.5%) and lateral peak ({approx}3% right and –1.0% left) events (Tab. 3). Although the hands-on-armrests condition had average forces similar to those of the other 3 conditions for the initiation, counter, rebound, and standing forces, it produced significantly less average vertical force than the other conditions at the seat-off (36.6%) and vertical peak (99.7%) events. The average vertical seat-off event force for the arms-free condition (59.8%) was significantly less than the forces for the arms-crossed (72.2%) and hands-on-knees (70.5%) conditions. For the rebound event, the arms-free (79.6%) and arms-crossed (79.1%) conditions produced significantly less average vertical force than the hands-on-knees (82.4%) and hands-on-armrests (82.9%) conditions. No other statistically significant force differences among arm-use conditions were attained for vertical event means or for the fore-aft or lateral event forces.


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  		Table 2. Vertical Average Ground Reaction Forces (Normalized to Percentage of Body Weight) Over All Trials and Participants From the Force Platform and the Seat-Switch for the 4 Arm-Use Conditions by Eventsa

 

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  		Table 3. Fore-Aft and Lateral Ground Reaction Forces (Normalized to Percentage of Body Weight) Over All Trials and Participants From the Force Platform and for the 4 Arm-Use Conditions by Eventsa

 
A significant main effect between sexes was observed for the vertical seat-off, fore-aft peak, and lateral left force events. The average normalized seat-off and fore-aft peak event forces for the female participants (57.4 % and 8.3%, respectively) were significantly less than the forces for the male participants (62.1% and 10.7%, respectively). Female participants produced significantly greater average lateral force to the left (–1.2%) than the male participants produced (–0.3%). No other statistically significant main effects between sexes were observed. No sex x condition interactions were statistically significant.

Event Times

Using average actual times and normalized times allowed comparison of the averaged force events in the 3 dimensions relative to durations of real time and proportions of time among the phases between events. Normalized event times were calculated as a percentage of the total time from the start signal to the first steady standing. Averaged actual time results for the vertical, fore-aft, and lateral ground reaction forces are shown in Tables 4 and 5 and in Figure 4. Averaged results of the recordings in the 3 dimensions for normalized times are shown in Tables 6 and 7. Significant differences among arm-use conditions are identified at the bottom of the table event columns, with abbreviations for each condition.


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  		Table 4. Vertical Average Actual Event Times (in Seconds)a

 

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  		Table 5. Fore-Aft and Lateral Average Actual Event Times (in Seconds)a

 

Figure 4
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  		Figure 4. Force recordings over all trials and participants and the seat switch for the 4 arm-use conditions over the averaged actual (real) times. FREE=arms-free condition, CROSS=arms-crossed condition, KNEE=hands-on-knees condition, and ARMRESTS=hands-on-armrests condition.

 

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  		Table 6. Vertical Average Event Times (Normalized to Percentage of Steady Standing Vertical Time)a

 

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  		Table 7. Fore-Aft and Lateral Average Event Times (Normalized to Percentage of Steady Standing Vertical Time)a

 
Significant main effects for the average actual and normalized vertical peak force event times and the lateral right and left peak force event times were observed. The fore-aft start and fore-aft peak main effects also were significant for the normalized times. No other statistically significant differences occurred for the actual or normalized times among arm-use conditions.

Post hoc analysis of the times to vertical peak force showed that the hands-on-armrests condition was significantly later than the other 3 arm-use conditions for actual and normalized average times. The average normalized time to vertical peak force was significantly later for the arms-crossed condition than for the hands-on-knees condition.

Comparison of the normalized fore-aft start times showed that the hands-on-knees condition was significantly earlier than the arms-free and arms-crossed condition times and that the hands-on-armrests condition was significantly earlier than the arms-crossed condition. Normalized fore-aft peak results showed that the arms-free and hands-on-knees condition times were significantly earlier than the arms-crossed average time and that the hands-on-knees condition time was significantly earlier than the hands-on-armrests condition time.

For the lateral right actual and normalized event times, the hands-on-knees condition time was significantly earlier than all other arm-use condition times. The lateral left hands-on-knees actual time also was significantly earlier than in the other 3 conditions. The normalized lateral left hands-on-knees time was significantly earlier than the arms-crossed and hands-on-armrests times, but not significantly different, statistically, from the average time in the arms-free condition.

Average (±SD) actual times were significantly earlier for female participants than for male participants for the rebound event (female participants=1.59±0.28 s, male participants=1.71±0.29 s) and the standing event (female participants= 2.19±0.31 s, male participants= 2.30±0.34 s). No other statistically significant differences occurred for any other actual or normalized event times between male and female participants.


   Discussion
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
Conclusions, Clinical...
 References
 
The primary purpose of this study was to identify discrete invariably occurring events during normal rising from a sitting position by comparing varied arm-use methods. Distinct and invariably occurring sequential events could be identified for every observation of every arm-use condition for every trial performed by every person in this relatively large sample of adults who were healthy. These events were identified by the forces produced (and seat-switch release) as follows: in the vertical dimension—(1) initiation, (2) counter, (3) seat-off, (4) peak, (5) rebound, and (6) standing; in the fore-aft dimension—(1) start, (2) peak, and (3) end; and in the lateral dimension—(1) peak right and (2) peak left.

The STS movement is at least equally important to normal human function as walking because rising from a sitting position is commonly a prerequisite to the initiation of ambulation. The STS movement often is used for clinical assessments51,52 and is considered a major determining factor for independence among elderly people and people with disabilities.53 Although hundreds of studies have been conducted related to the STS movement, no consensus has been established for a common standard method of identifying events during this task.

Most previous studies included relatively small sample sizes, ranging from a single-case study42 to 55 subjects.40 The sample size of 100 participants (50 male, 50 female) in the present study was double the number of the largest previous study with equal numbers of each sex.36 All participants in the present study were healthy, and most participants were young (average age [±SD]= 21.8±4.6), but with a wide range of body types (Tab. 1) and apparently a wide variety of strategies for performing the 4 distinct methods of rising (see ranges in Tabs. 2, 3, 4, 5, 6, and 7).

Although the 6 vertical and 3 fore-aft events each always occurred in sequential order during all 4 arm-use conditions, significant differences were found for some of the event average forces and times among the conditions. Lateral force events also occurred with a regular pattern and were similar for most, but not all, participants. Most previous studies constrained subjects to rising with the arms crossed, and, although some studies compared those constrained kinetic or kinematic results to 1 or 2 of the 3 most commonly used methods of normal STS movements (arms free, hands on knees, and using armrests), no previous study compared these 3 methods with the constrained, and often considered control, condition of rising with the arms crossed (ie, no arm use).

Many of the events cited in the previous studies, with results mostly derived from joint angle displacements or moments, did not consistently occur sequentially either between or within individuals. All events consistently occurred sequentially for all arm-use conditions in the present study.

Previous studies often constrained subject positions before initiation of rising by controlling seat height; trunk, knee, and ankle angles; or foot position, and frequently constrained all of these variables. Many of the studies also constrained movements by regulating the rising speed or restricting use of the arms (most commonly crossed), or both (see previous publications for more extensive reviews and comparisons6,45). By generally constraining STS positions and movements, numerous researchers reported descriptions of movements that do not normally occur. The present study included a standard-height chair and did not constrain rising speed or sitting positions, other than the 3 most common arm-use methods or with the arms crossed.

Comparisons of Standardized Event Forces

The hands-on-armrests condition produced significantly less average vertical forces than the other conditions at the seat-off and vertical peak events. The logically apparent reason for the lesser force during the hands-on-armrests condition was that the force difference was applied through the hands on the armrests during those events.

These findings supported the results of other studies demonstrating that the use of armrests significantly reduced forces compared with other arm-use conditions.54–57 The reduction of vertical peak ground reaction force by using the armrests in the present study, approximately 16% less than for the other 3 arm-use conditions, was similar to the 18.5% tibiofemoral force reduction reported by Ellis and colleagues.56 Seedhom and Terayama57 reported that the averages of 2 subjects' peak knee joint forces were 106.5% of body weight without using armrests and 19.5% of body weight using armrests. Alexander et al,17 in a study comparing armrest-use and arms-free conditions, reported that young subjects applied a mean force of 24% of body weight on the armrests during STS movements, comparable to the approximately 17% difference found between armrest-use and arms-free conditions in the present study.

Although average forces for the seat-off and vertical peak events were significantly reduced in the hands-on-armrests condition compared with the other conditions in the current study, no significant differences occurred among conditions for the fore-aft peak forces. The average fore-aft peak force in the hands-on-armrests condition was only slightly less than those in the other conditions, suggesting that minimal additional average force was used on the armrests to produce horizontal movement. Individual strategies, however, varied as the fore-aft peak force event included as much as 46% of body weight (hands-on-armrests condition) and as little as 1% of body weight among all conditions, including the hands-on-armrests condition (see ranges in Tab. 3), with average fore-aft peak forces for all conditions of approximately 9% to 10% of body weight. It was evident that individual strategies influenced movements during rising among arm-use conditions, with some individuals greatly assisting their movements by using the arms while others mostly ignored potential contributions of arm-use during the STS movement.

The seat-off force was significantly less for the arms-free condition than for the arms-crossed and hands-on-knees conditions, which suggested that swinging of the arms may have contributed to vertical momentum at seat-off for the arms-free condition. Statistically, the average rebound event forces were significantly less for the arms-free and arms-crossed conditions than for the hands-on-knees and hands- on-armrests conditions. These findings suggested that use of the arms to push off from relatively stationary objects (knees or armrests) introduced greater post–peak vertical forces, although all rebound forces were below body weight. None of the fore-aft or lateral average forces were significantly different among the arm-use conditions.

Male participants produced significantly more force during the vertical seat-off event than female participants produced. Although the condition x sex interaction (P=.009) was not significant with the adjusted P value, the male participants contributed considerably more vertical seat-off force (12.8% of body weight more) than female participants contributed for the hands-on-knees condition, compared with differences for the other conditions (1.6%–2.5% of body weight more), suggesting that male participants pushed down on their knees more than female participants at seat-off. Male participants also produced significantly greater average fore-aft peak force than female participants produced, but the differences were relatively constant across conditions (1.9%–2.9% of body weight more). We also found that female participants transferred more average force to the left than male participants transferred during lateral weight shift. The significant main effect for lateral left force showed that the average differences between sexes had greater contributions for the arms-free and hands-on-armrests conditions (1.5% and 1.2% of body weight more, respectively) than the arms-crossed and hands-on-knees conditions (0.9% and 0.3% of body weight more, respectively).

A possible explanation is that, when the arms could contribute asymmetrical forces more readily, as with the arms-free and hands-on-armrests conditions, female participants produced more force on one side (left) than male participants produced. Using a force platform for a seat, one study of 4 female and 3 male subjects demonstrated significantly greater average force on the left buttock during sitting,44 but no explanation was offered for the asymmetrical posture. Few other studies reported comparisons between male and female subjects during STS movements. Average force applied to the armrests was found to be nearly the same for male and female subjects in one previous study.17 In another study, male subjects produced 18.5% less tibiofemoral force, compared with 21% less force for female subjects, with the use of armrests compared with free arm-use, but the difference was not significant between sexes.56 Another study58 showed that center of force was positioned more anteriorly for male subjects than for female subjects during standing, but male subjects exhibited less body sway.

Comparison of Event Forces With Other Studies

Because previous studies did not report all 11 event forces identified in the present study, overall direct comparisons were impossible. However, some studies reported average peak vertical and fore-aft forces. The average of 5 previous studies reporting mean vertical peak force results was 118.7% of body weight (range= 111.0%–125.5%).16,27–29,59 That average was similar to the average of 116.0% of body weight of the vertical peak event force for the arms-crossed condition found in the present study. McGibbon et al28 reported the average peak fore-aft event force as 12.1% of body weight, slightly greater than the 10.1% average for the arms-crossed condition reported here. The differences may have been a result of the constrained lower-extremity positioning in those studies.

Numerous previous publications included graphs with patterns remarkably similar to recordings in the present study (Figs. 2 and 3). The events were identifiable regardless of whether they were from force platform recordings or from joint angle displacement graphs.5,16,25,27–29,36,42–44,50,58,60,61 However, of the 11 events identified in the present study, few were named or described in those studies, and some studies did not identify any of the events observed in this study. Some previous reports identified forces at seat-off, peak vertical or peak fore-aft, or lateral events, but none identified all of the events defined in the present study. Almost all reports ignored the counter and rebound events.

Some of the invariably occurring force event results included in graphed data from other publications could be calculated or interpolated from the graphs. That is, some previous studies published force platform recordings showing the identical events as the present study which could be calculated for comparison. Mostly the calculated force values from the single representative subject's graph in each study were remarkably similar to the results of this study and all were well with in the force ranges of the 100 subjects included here. Mostly the events and forces described in the present study were not reported by those authors, other than appearing in the graphs of force platform recordings.

Three-dimensional force graph data derived from 5 reports of single representative subjects28,42,44,60,61 showed that the average ground reaction forces (as percentage of body weight) for the 6 vertical events were: initiation=22%, counter=16%, seat-off= 81%, peak=128%, rebound=78%, and standing=100%. The averages of the 100 subjects' vertical ground reaction forces for the same events (arms-crossed condition) in the current study were: initiation=20%, counter= 12%, seat-off=72%, peak=117%, rebound=79%, and standing=100%. Among those studies that included fore-aft and lateral data, average force values were: peak fore-aft=14.4%, right=4%, and left=–1%. The same values from the current study were: peak fore-aft=10.1%, right=3.5%, and left=–0.9% for the arms-crossed condition. With the exception of the rebound event, the average values from the single-subject data were slightly greater, but generally similar to the results presented here. The differences possibly were due to the constrained lower-extremity positions required in those studies.

Some controversy exists regarding measurement of the force and time of the compulsory seat-off event, which may be the most critical event for transitioning balance. The vast majority of previous studies calculated seat-off from kinematic recordings using various definitions, but identifying this event without an instrumented seat may not be valid.28 Two studies that included an instrumented seat (force platforms) reported vertical ground reaction forces (from force platforms beneath the feet) at seat-off as 74% of body weight44 and 87% of body weight,28 each for a single subject, similar to the average for the arms-crossed condition and within the range of seat-off forces in the present study (mean=72.2%, range=29%–99%).

Other studies26,62 showed lateral force asymmetry during the STS movement, as was found in this study. However, the average lateral forces in either direction were less than 4% of body weight in adults who were healthy, which did not result in any apparent loss of balance throughout the STS movement and was probably a normal, learned weight shift from the preferred side to the nonpreferred side. Similar to a previous report,62 one side was systematically preferred during most trials in the present study, which was unexplainable by limb preference.

Comparison of Standardized Event Times

The average actual and normalized vertical peak event times for the hands-on-armrests condition of 1.36 seconds and 61.1% were significantly later than the other 3 conditions. This may have been the result of a reliance of the hands on the armrests to continue to produce force during rising. The vertical event times for the hands-on-armrests condition occurred slightly earlier than for the other 3 conditions at initiation, counter, and seat-off events, but were similar to the other arm- use conditions at the rebound and final standing events. This result, combined with results of significantly less vertical force during the seat-off and peak events, suggested that force was applied to the armrests until approximately full standing force was achieved. Following the separation of the hands from the armrests at the time of peak force, the patterns for the hands-on-armrests condition were temporally similar to the patterns for the other 3 arm-use conditions. Alexander et al17 found no difference between total STS times with or without the use of armrests for young and elderly subjects, but they reported a significantly longer average time to maximum anterior head displacement (which may have been the time of vertical peak force) for elderly subjects using the armrests compared with rising without armrests. Other researchers56,58 reported longer times to rise with the aid of armrests than without armrests, but no event times were included.

Of the significant differences among event times for the arm-use condition, the times for the hands-on-knees condition were earlier than those for 1, 2, or all 3 other conditions for actual and normalized times at the vertical peak, fore-aft start, fore-aft peak, and lateral peak right and left events. Both the average actual and normalized times at the lateral peak right and left events for the hands-on-knees condition were significantly earlier than for the other 3 conditions. Because the hands-on-knees condition could produce force directly through the lower leg onto the force platform during the STS movement, these results might be expected. The significantly greater vertical force of the hands-on-knees condition compared with the arms-free condition at seat-off would tend to support this possibility.

The normalized event times for the arms-crossed condition tended to occur later than for the other arm-use conditions. The vertical peak event time was significantly later for the arms-crossed condition than for the hands-on-knees condition. The fore-aft start event time also was significantly later for the arms-crossed condition than for the hands-on-armrests and hands-on-knees conditions and significantly later than the fore-aft peak event times for the arms-free and hands-on-knees conditions. Instructions for the arms-crossed condition were to maintain the crossed arms close to the body during the STS movement, which may have delayed momentum for producing initial and peak forces.

The average actual times over all arm-use conditions were significantly different between male and female participants for the vertical rebound and standing events. Male participants had average times of 1.71 and 2.30 seconds for the rebound and standing events, and female participants' average times were 1.59 and 2.19 seconds, respectively. Male participants had slightly faster average times for the initiation and counter events, and female participants had earlier average times than male participants for the remaining events, but none were significantly different until the rebound event. There were no significant differences for any of the normalized event times between male and female participants. These results together suggested that female participants took less actual time to complete standing, but moved similar to male participants in the proportions of times between events. Because average height and weight were less for female participants than for male participants, it seems reasonable the actual time to complete the STS movement would be less for women than for men, but proportionally similar. A previous study17 showed no significant difference for actual STS total times between sexes for 7 male and 10 female subjects, but times of other events were not reported.

Comparison of Event Times With Other Studies

Because of diverse methods among previous studies for defining STS event times, direct comparisons with those studies were not possible. Most studies did not record when the signal to start was given. The beginning of a trial usually was determined by the first detected movement of a single joint or other anatomical landmark measured using kinematic recordings, rather than a recorded external start signal. However, in a study that previously included the largest number of subjects (N=55), Nuzik et al40 found that no single body segment consistently identified STS initiation among or within individuals. Recording from a force platform in the present study allowed identification of initiation regardless of body segment movements, and the external start signal ensured that no information would be lost prior to initiation. However, measuring STS times from an external start signal rather than a first movement resulted in longer overall event times when compared with previously reported times. Adjusting times reported in other studies by adding the time to initiation recorded in the present study (the average from the external start signal to the initiation event of approximately 0.28 second) showed that STS event times were remarkably similar to times reported in other studies.

The average actual total time from the start signal to steady standing in this study, including all 4 arm-use condition times, was 2.24 seconds. From similar studies reporting an average total time of young subjects who are healthy rising at a preferred speed,5,21,23,48,50 the adjusted overall average total time was 2.14 seconds. The averaged minimum and maximum total times in the present study over all arm-use conditions were 1.51 and 2.97 seconds, respectively. Among other studies reporting the range of total times,13,22,25,32,36,40,44 the adjusted minimum and maximum averages were 1.74 and 2.59 seconds, respectively. Varied methods in previous studies for identifying initiation of movement may have contributed to the difference in average times found in this study. In addition, nearly all of the studies were conducted with the participants' arms crossed, and this study included 4 arm-use conditions. However, no significant differences among arm-use conditions for total STS times were observed in the present study. The greater average range of total times may be related to the larger number of participants and considerably fewer constraints implemented in this study compared with other studies.

Two previous studies19,44 used an external start signal with force platforms beneath the seated surface and the feet for the purpose of measuring seat unloading. The ground reaction force graphs provided in those reports were remarkably similar to Figures 2 and 3 in this article. The graphs showed times of approximately 0.2 to 0.3 second (10% and 17% of total normalized time) after the signal to start, similar to the average initiation actual ({approx}0.28 second) and normalized (12% of total) times in this study.

Only Carr and Gentile20 made reference to the period of reduced force following initiation, termed "counter event" in the present study. They defined the STS start time as when the vertical force decreased 10 N from baseline sitting, which was during the counter event. Although Carr and Gentile did not term it a counter event, they implied that it occurred on every trial and stated that the decrease in the vertical force platform recording was probably related to an early lifting of the thighs from the seat by contracting the hip flexor muscles while holding the trunk stationary. The counter event in the present study also was observed to be related to hip flexion by lifting the thighs, but some individuals used ankle dorsiflexion, which decreased vertical force from baseline sitting. Strategies varied among individuals producing this reduction of vertical force, as some used trunk extension, thigh lifting, dorsiflexion, or a combination of all of these strategies, to produce the counter event force reduction prior to generating momentum upward. Because the counter event occurred regardless of arm-use condition for every trial, it may be programmed into all strategies of STS movements as an anticipatory postural adjustment.44,63

In this study, the average time to seat-off was 0.82 to 0.90 second among all arm-use conditions, which was approximately 37% to 41% of the average total STS times. Other studies21,22,25,27,28,44 used varied configurations of seat switches or force platform seats to measure the seat-off event. The average actual time to seat-off over all of those studies was 0.83 second, with an average percentage of total time of 40.3% (range=34%–45%). Although the findings were similar, differences may be attributed to methods for defining the time of seat-off.

A few previous studies reported time to peak force, and event time could be interpolated from force platform graphs in other studies. The majority of those studies implemented the free arm-use method for STS. The average normalized time of the vertical peak force event in this study for the arms-free condition was 50.5%. The average time to peak force for 11 other studies16,21,24,27,28,42,44,58,60,61,63 was 44.6%. The average normalized time of the rebound event in this study was 72.9% (arms-free condition). None of the studies mentioned the rebound event, but the average time of the rebound event calculated from graphed data in previous studies was 74.8%. Although these findings were remarkably similar, the differences in percent times may be attributed to differences in defining the starting times.


   Conclusions, Clinical Applications, and Suggestions for Future Study
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
 Conclusions, Clinical...
References
 
Numerous studies related to human gait used standardized events and methods, but relatively few studies addressed other fundamental movements of the human body. Coming to a standing position from a seated position is one of the essential functional activities of daily living. Rising from a chair using the conditions included in this study could be functionally described as including: 6 vertical events (initiation of movement, a counter force prior to seat-off, seat-off, peak force, post–peak rebound force, and final steady standing force), 3 horizontal events (start of force, peak force, and end of force), and 2 lateral events (right and left). Identifying these 11 events during STS movements may be useful for establishing common communication among researchers and clinicians.

These events effectively could be used to compare other STS conditions, such as an up-and-go task,64,65 repeated STS,52,64,66 or stand-to-walk.27 Additionally, the means and ranges of values presented here could be used for comparisons with other population sample groups, such as various age groups or pathologies, fallers and nonfallers, or potentially any other group of interest. Gait laboratories commonly have motion analysis systems with one or more force platforms, which should be able to replicate the methods used in the present study.

Considering the widely varying outcomes from the many previous studies that attempted to standardize STS events and phases, the present method for identifying invariably occurring events is offered. Although all these 11 events may not be of interest to all researchers and clinicians, they provide a method for discretely identifying essential elements that occur during common methods of natural STS movements. Identification and measurement of these events in future studies could provide a much-needed commonality of descriptions and terminology for comparing research studies and making clinical assessments.


   Footnotes
 
Dr Etnyre provided concept/idea/research design, writing, data analysis, and project management. Both authors provided data collection and subjects. Dr Thomas provided consultation (including review of manuscript before submission). The authors acknowledge the physical therapists and students who assisted in the progress of this study. Their contributions to the development and to data collection and analysis were essential for its completion. They included: Ashraf Al-Azzazi, Alex Allori, Maria Arenas, Stephanie Buck, Laura Cannistra, Denise Crawford, Eva Crisp, Sarvang Dalal, Gretchen Dietal, Robert Durant,, Henry Dutchover, Christine Ellen Lee, Jonathan Erickson, Martina Eschenberg, Donna Gragg, Delton Justice, Pauline Kay Van Cleave, Maria Loli Valdes, Thomas Maggio, Melanie McNeal, Rochelle Mead, Willard O'Berry, Pamela O'Mara, Enrique Pineda, Hamdy Radwan, Riva Rahl, Vanita Rebelo, Rita Rodermund-Newman, Amy Stephens, Laura Van Duran, Claudia White, and Anna Williams.

The study was approved by the Internal Review Board of Texas Woman's University, Houston, Tex.

* Advanced Mechanical Technology Inc, 176 Waltham St, Watertown, MA 02472. Back

{dagger} Lafayette Instruments Co, 3700 Sagamore Pkwy N, PO Box 5729, Lafayette, IN 47903. Back


   References
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Physical Therapy, December 1, 2007; 87(12): 1667 - 1667.
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