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Perspectives |
TE Johnston, PT, PhD, MBA, is Research Specialist, Shriners Hospitals for Children, 3551 N Broad St, Philadelphia, PA 19140 (USA)
Address all correspondence to Dr Johnston at: tjohnston{at}shrinenet.org
Submitted July 28, 2006;
Accepted April 9, 2007
 | Abstract |
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| Introduction |
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 TopAbstract Introduction Cycling in People Without...Cycling in Individuals With...ConclusionReferences |
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The purpose of this perspective is to review the relevant literature on cycling biomechanics in order to provide clinicians with information on factors that may affect a cycling intervention for individuals with disabilities. This article will include information on cycling biomechanics in people with and without disability.
| Cycling in People Without Disabilities and Implications for Individuals With Disabilities |
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Likewise, analysis of joint moments during cycling has primarily been isolated to the sagittal plane.40 When cycling at a cadence of 60 rpm, a power output of 120 W, a seat height based on the distance between the ischial tuberosity and the medial malleolus, and the foot positioned with the ball of the foot on the pedal surface, young men who were healthy with an average weight of 75.5 kg had mean (±SD) hip flexion moments of 34.3±9.1 N·m, hip extension moments of 8.9±2.6 N·m, knee flexion moments of 28.8±17.5 N·m, knee extension moments of 11.9±2.6 N·m, and peak dorsiflexion moments of 31.9 N·m (there were no plantar-flexion moments). All of these moments were external moments.41 As with kinematics, 3D analysis has the added benefit of providing information that may assist in understanding the causes of knee pain that may be due to external and internal moments occurring during cycling.40
The knee, however, is the only joint that has been studied using 3D kinetics. One study23 demonstrated that a valgus moment was present at the start of the extension phase (peak of 7 N·m) and a varus moment occurred throughout most of the flexion phase (peak of 7 N·m) when young adults cycled at 225 W and 90 rpm. Overall, the moment in the transverse plane was internal throughout most of the revolution with a peak value of 1 N·m in each direction. The authors did note great variability in these measures. In individuals with disabilities, knowledge of joint moments may provide clinicians with information that could lead to methods designed to minimize joint stress. For example, a patient who has had knee ligament surgery may need to minimize transverse and varus/valgus force around the knee, and cycling may be too stressful at higher resistances and cadences. Further research, however, is needed to determine how these moments decrease at lower resistances and cadences.
Many cycling studies with adults have used EMG to gain a better understanding of how muscles function during cycling.8,25,29,31,33 Figure 1 shows an example of muscle activity patterns during cycling. The general weakness of these EMG studies is that the criteria for determining the onset and offset times for muscle activity are inconsistent across studies, making comparisons difficult. Despite this limitation, adults who are healthy have been shown to co-contract agonist and antagonist groups during specific arcs of the cycling revolution.8,40 It also has been shown that adults who are healthy have predictable patterns of EMG activity in the uniarticulate muscles during cycling.
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Many studies with adults have examined the effects of changing the position of the rider on the cycle and have shown that changes in the length of the crank arm, the height of the bicycle seat, and the position of the foot on the pedal can have significant effects on kinematics, kinetics, muscle activation, and energy expenditure during cycling. The effects of different workloads and cycling cadences also have been studied, and many studies have looked at a combination of position, cadence, or workloads and have attempted to interrelate them.
Positioning on the Cycle
One area of debate has been on the length of the crank arm, which is not typically adjustable in bicycles. The crank arm is the rotating bar to which the pedal is attached, and the length of the crank arm is measured from the point of rotation of the crank arm on the cycle to the point of rotation of the pedal on the crank arm (Fig. 2). Some studies have examined the effects of changing the length of the crank arm of the cycle on adults who are healthy4,7 and children.9 Martin and Spirduso7 found that crank length was optimal for power (product of resistance and cadence) production when the crank arm was set at 20% of leg length (standing height minus sitting height) or 41% of tibia length (lateral knee joint space to lateral malleolus) in 16 trained young adult cyclists. The pedaling cadence optimal for power output was found to decrease with increasing crank length.7 Therefore, an increased crank length increases the lever arm, so the user does not have to pedal as quickly to achieve the desired power output. A longer crank arm may be desirable for power production for a patient who is unable to achieve a high cycling cadence.
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The height of the bicycle seat during standard upright cycling has been another area of investigation, with greater extension reported with increasing seat height.6,39,45 Differences have been reported in excursion at the hip, knee, and ankle with seat heights identified as low (102% of the distance between the ischial tuberosity and the medial malleolus), middle (113% of that distance), and high (120% of that distance). With a low seat height, excursions of movement were from 40 degrees to 80 degrees of flexion at the hip, 65 degrees to 125 degrees of flexion at the knee, and 5 degrees to 25 degrees of plantar flexion at the ankle. As the seat was shifted to the middle height, these excursions changed to 32 degrees to 70 degrees at the hip, 46 degrees to 112 degrees at the knee, and 2 degrees of dorsiflexion to 22 degrees of plantar flexion. As the seat was shifted to the high height, these excursions changed to 20 degrees to 65 degrees at the hip, 25 degrees to 105 degrees at the knee, and 12 degrees of dorsiflexion to 20 degrees of plantar flexion.39
Another study6 showed a significant difference in energy expenditure across different seat heights in adults, finding that a seat height set at 100% of leg length (greater trochanter to floor) was the most energy-conserving compared with one set at 105% of leg length. Finally, it has been shown that young adult cyclists who are healthy could obtain greater ankle joint moments at a higher seat height (120% of the distance from the ischial tuberosity to the medial malleolus) rather than a lower one (102% of that distance).10
Based on the findings of these studies on seat height, it appears that the specific goals for the individual should guide the choice of seat height. For example, if a patient has a knee flexion contracture, a lower seat height may be needed to allow that individual to cycle. A lower seat height also may be desirable in order to minimize energy expenditure for a patient with pulmonary or cardiac concerns. If the desire is to strengthen the calf muscles or to obtain greater extension range of motion, a higher seat height may be the better choice. A higher seat height also may better challenge the cardiorespiratory system during exercise, potentially leading to exercise effects such as an improvement in maximum oxygen consumption. These are just a few examples of how the information from biomechanical studies can be used to affect patient care.
Researchers have reported on the effects of changing the position of the foot on the pedal on ankle moments10,27 and tibiofemoral compressive forces.11 Two reports10,11 were based on the same study of 6 young male recreational cyclists, in which 2 positions of the foot on the pedal were studied: (1) an anterior position where the head of the second metatarsal was placed on the center of the pedal and (2) a posterior position where the instep was placed on the center of the pedal.
Ericson et al reported that ankle internal moments decreased when the foot was in the posterior position on the pedal compared with the anterior position,10 indicating that the ankle muscles exerted less torque in the posterior position. For example, at a seat height based on 113% of the distance between the ischial tuberosity and medial malleolus, a power output of 120 W, and a cadence of 60 rpm, the maximal plantar-flexion moment was reported to be 15.6±3.4 N·m with the foot in the posterior position and 30.9±2.8 N·m in the anterior position in young men. Therefore, there is a greater demand on the calf muscles and greater potential strengthening effects on these muscles when cycling with the foot in the anterior position.10
In this study,10 seat height did have an effect with greater ankle moments seen with higher seat heights. Overall compressive forces at the ankle and knee also have been studied. As the seat assumes a proportion of the body weight, compressive forces at the ankle were found to be approximately 29% of what is encountered during walking based on a 71.3 kg person.10 Tibiofemoral compressive forces during cycling were found to be approximately 0.3 to 2 times body weight depending on workload, cadence, and saddle height compared with tibiofemoral compressive forces of 2 to 4 times body weight during walking.11 At the knee, no differences have been reported in tibiofemoral compressive or strain forces between the anterior and posterior foot positions.11
In addition to the choice of the anterior or posterior foot position, there is the choice to position the foot in inversion or eversion on the pedal, in cases when the pedal allows this manipulation. Placing the foot in 10 degrees of eversion has been shown to decrease the varus external moment by 55% and the peak internal rotation moment by 53% during the extension phase compared with cycling with the foot in neutral inversion/eversion. The moments were increased above values of the neutral positions when the foot was in 10 degrees of inversion (varus by 47%, internal rotation by 88%).27
Another positioning decision that can be made for cycling involves the choice of an upright or a recumbent cycle. In one study,46 lower-extremity internal moments during recumbent cycling in adults who were healthy were compared with values previously published for adults who were healthy during upright cycling. It was found that the knee extensor moment during the extension phase of cycling while in the recumbent position was less and that the hip extensor moment was greater than those seen during upright cycling. In addition, a 90-degree shift in the pattern of the general muscle moments was seen at the hip with both extensor and flexor general muscle moments occurring 90 degrees later in the revolution during recumbent cycling as compared to upright cycling.46 The study by Gregor et al46 suggests that recumbent cycling may promote the use of the hip extensors. An additional study examined differences in knee loads when cycling in an upright or recumbent position and found that anterior and posterior shear forces were greater in the upright position. Therefore, a recumbent cycle may be a better choice for individuals who have had an anterior cruciate ligament reconstruction47 or with patients with anterior or posterior knee instability.
Effects of Cadence and Workload
Cycling cadence has been shown to have a relationship with joint moments,19 power output,19,20 EMG patterns,48,49 and energy expenditure.13,50 A general pattern of increasing moments at the hip, knee, and ankle and decreasing pedal force moments has been reported as cycling cadence increased in an experienced cyclist.19 In the study by Redfield and Hull,19 the hip moment was most significantly affected, which the authors attributed to the involvement of the hip in both acceleration and deceleration of the lower extremity during cycling, with the largest hip moment seen with deceleration during the transition from extension to flexion. This indicates that higher cadences may demand more work from the hip during the transitions between flexion and extension. In contrast, however, another study51 showed that an increased workload led to an increase in the moment at the hip and knee, but that an increased cadence only led to an increase in the moment at the hip.
A significant interaction between power output and cadence was reported in another study20 involving young adult male recreational cyclists. In this study, increasing power output with constant cadence led to an increase in peak pedal force, indicating that the leg was exerting more force into the pedal. However, increasing cadence while keeping power output constant led to a decrease in peak pedal force. Therefore, a lower resistance and increased cadence may be indicated to decrease loading across all joints.
Muscle activity also has been shown to change with changes in cadence. One study48 involving 10 young male competitive cyclists showed that muscles responded differently to increases in cadence, with increased activation in some muscles (gastrocnemius, hamstring, and vastus medialis) but no change in activation in other muscles (tibialis anterior and rectus femoris) when cycling between 45 and 120 rpm at 250 W. Interestingly, the gluteus maximus and soleus muscles showed significant trends with the lowest activation values when cycling at 90 rpm. This study, therefore, showed that the overall patterns of muscle activity during cycling could change with changing cadences, indicating that there may be different outcomes in measures such as strength (force-generating capacity of a muscle) when cycling at different cadences. Another study22 showed that increasing cadence (50, 65, 80, 95, and 110 rpm) led to greater activity of the gastrocnemius muscle, but not the soleus muscle, when cycling at a power output of 200 W.
Finally, the relationship between cadence and energy expenditure also has been studied. One study13 showed that cadence did not significantly contribute to energy expenditure during submaximal cycling. Pedal speed (cadence x crank length x 2
60), however, did significantly contribute, suggesting that energy expenditure is higher with an increased crank arm length when cadence and workload are held constant. In contrast, another study50 showed that a relationship existed between cadence and energy expenditure with the trough of the curve at 60 or 75 rpm in athletic young adult noncyclists when cycling to exhaustion. Some of the differences seen between these 2 studies may have been due to the differences in testing, because one test was submaximal13 and the other test was to exhaustion.50 A submaximal test may be more energy conserving than a test designed to achieve physical exhaustion.
Workload has been shown to affect compressive and strain forces at the knee11 as well as the freely chosen cycling cadence,12 the gross efficiency of cycling,12 the ability to reach peak power during an all-out cycling test,16 and EMG patterns.49 An increasing workload has been shown to increase the compressive forces at the knee during cycling.11 For example, when cycling at 60 rpm, a mid-range seat position as described earlier, and an anterior foot position, the compressive force increased from approximately 0.3 times body weight at 0 W to approximately 2 times body weight at 240 W.11 This finding has implications for treatment of patients in whom high compressive forces may be contraindicated.
In addition, freely chosen cycling cadence has been shown to increase with increasing load, and gross efficiency (power output divided by metabolic energy input) has been shown to be lower at higher loads in 7 young men who were healthy.12 The low cycling loads studied were 9 to 36 kg/m2 and the high loads ranged from 56 to 182 kg/m,2 which were set by using the high and low gears of the cycle, respectively. Cycling cadence and gross efficiency, therefore, are inversely related, with a higher cadence decreasing efficiency.
In a pediatric study,16 prepubescent boys also increased their cycling cadence in response to increasing loads. However, the highest loads prevented the children from being able to reach the peak power obtained with lower loads during an all-out cycling test because the children became fatigued prior to reaching a comparable peak power.16 These findings suggest that lower loads may be more desirable to obtain optimal measures of power, at least in children. The authors reported that a workload of 0.05 kg per kg of body weight appeared optimal for their young subjects. If the goal is to maximize power, therefore, a lower load may be desirable in children.
Workload also has been shown to affect EMG activity by altering the onset and offset of muscle activity during the revolution. Several muscles have been shown to have earlier onsets (gluteus maximus, rectus femoris, biceps femoris, vastus lateralis, anterior tibialis, and soleus) and earlier offsets (gluteus maximus, rectus femoris, biceps femoris, and vastus lateralis) when the resistance demand (in kilograms) was increased.49 In this study, the magnitude of muscle activity also was shown to change in the rectus femoris, vastus lateralis, anterior tibialis, and gastrocnemius muscles. Clinically, this finding indicates potential for a greater effect on these muscles when cycling at greater loads. Another study,32 however, showed no change in muscle activity levels when workload increased by changing gears.
| Cycling in Individuals With Disability |
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TopAbstractIntroductionCycling in People Without...Cycling in Individuals With...ConclusionReferences |
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Similar findings were shown in an earlier study53 in which subjects who had a cerebrovascular accident (CVA) also preferentially generated power at the hip or the knee on the involved side but not at both joints while cycling. The involved limb of these subjects also showed earlier activation of ankle plantar flexor and hip extensor moments. Overall, these subjects displayed less power generation at the ankle and knee and increased power at the hip on the involved side.
Although the subjects with diabetes or who had a CVA in these studies52,53 do not represent all individuals with disabilities, some of the impairments that might contribute to the difference in cycling moments may be similar. Perell et al52 suggested that decreased ankle mobility in the subjects with diabetes possibly led to some of the changes in the extension phase, causing the subjects to rely more on the hip. In addition, the subjects with diabetes52 or who had a CVA53 tended to display decreased ankle power during walking, which might affect ankle power during cycling as well. These impairments also may be present in individuals with other disorders.
Differences have been reported in muscle activation patterns for individuals with chronic CVA. Coordination deficits have been shown in the involved lower extremity, including prolonged activity in the vastus medialis muscle, increased activity in the rectus femoris muscle during flexion, and decreased activity in the hamstring muscles during flexion.54 Increasing the workload for subjects with chronic CVA did not increase inappropriate muscle activity55 or movement asymmetry between sides.56
Finally, altered muscle activation patterns during cycling have been reported in children with cerebral palsy (CP) and include prolonged periods of muscle activity and increased co-contraction compared with children with typical development.57,58 Children with CP also have increased movement in the transverse and frontal planes during cycling and decreased cycling efficiency.58 These altered movement patterns for children with CP as well as for adults who have had a CVA are likely related to strength and motor control deficits, impairments that often are seen in patients with neurological disorders.
| Conclusion |
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TopAbstractIntroductionCycling in People Without...Cycling in Individuals With...ConclusionReferences |
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Further research is needed to examine the effects of manipulating the positioning, cadence, and workload during cycling for individuals with disabilities to gain a better understanding of manipulations that may lead to improvements in impairments specific to the needs of each person. Based on the literature from other populations, a recumbent position may encourage the use of the hip extensors, and a higher seat height and placing the ball of the foot on the pedal may encourage the use of the plantar flexors. Moments around the hip, knee, and ankle may increase with increases in cadence, and moments at the hip and knee may increase with increasing workloads. These increases in load moments and thus forces through the lower extremities may be desirable to increase bone density but may not be desirable for individuals with joint pain. Finally, the cardiovascular system may be challenged to a greater extent by increasing the seat height and increasing the pedal speed. All of these possible effects warrant further study in order to optimize the potential benefits of cycling for individuals with disabilities. However, the extensive literature on cycling in adults who are healthy can serve as a guide for choosing more appropriate cycling interventions for individuals with disabilities.
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