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Effects of Squat Lift Training and Free Weight Muscle Training on Maximum Lifting Load and Isokinetic Peak Torque of Young Adults Without Impairments

SSM Yeung, MPhil, PT, is Assistant Professor, Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
GYF Ng, PT, PhD, is Associate Professor, Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong (rsgng{at}polyu.edu.hk). Address all correspondence to Dr Ng

Submitted February 9, 1999; Accepted February 21, 2000

	Abstract

 
Background and Purpose. Manual lifting is a frequent cause of back injury, and there is no evidence as to which training mode can provide the best training effect for lifting performance and muscle force. The purpose of this study was to examine the effects of a squat lift training and a free weight muscle training program on the maximum lifting load and isokinetic peak torque in subjects without known neuro-muscular or musculoskeletal impairments. Subjects. Thirty-six adults (20 male, 16 female) without known neuromuscular or musculoskeletal impairments participated. The subjects' mean age was 21.25 years (SD=1.16, range=20–24). Methods. Subjects were divided into 3 groups. Subjects in group 1 (n=12) performed squat lift training. Subjects in group 2 (n=12) participated in free weight resistance training of their shoulder abductors, elbow flexors, knee extensors and trunk extensors. Subjects in group 3 (n=12) served as controls. The maximum lifting load and isokinetic peak torques of the trunk extensors, knee extensors, elbow flexors, and shoulder abductors of each subject were measured before and after the study. Training was conducted on alternate days for 4 weeks, with an initial load of 80% of each subject's maximum capacity and with the load increased by 5% weekly. Results. All groups were comparable for all measured variables before the study. After 4 weeks, subjects in groups 1 and 2 demonstrated more improvement in maximum lifting load and isokinetic peak torque of the back extensors compared with the subjects in group 3, but the 2 training groups were not different. Conclusion and Discussion. The findings demonstrate that both squat lift and free weight resistance training are equally effective in improving the lifting load and isokinetic back extension performance of individuals without impairments.

Key Words: Back • Functional training and activities • Muscle performance, general • Specificity of training

	Introduction

Top Abstract Introduction Method Results Discussion and Conclusions References

 
Manual lifting has been identified as a frequent cause of work-related low back injury (LBI).^1–4 Chaffin and coworkers^5–7 were one of the earliest groups of investigators who showed that workers who could lift less than others were at greater risk for workplace injury to their back. This finding was supported by the results of Cady and colleagues' study of a group of firefighters⁸ and Gundewall and colleagues' study of a group of nurses.⁹ There are reports, however, that isometric back extensor torque was not associated with LBI in the aircraft industry¹⁰ and in a 5-year follow-up study of 456 adults.¹¹ Back strengthening programs have been suggested for workers and, indeed, have sometimes been shown to reduce the incidence of work-related LBI.^9,12,13

Exercise training protocols have been reported to improve either the trunk muscle force or the lifting capacity of workers.^13–17 These protocols included general strength training,^14,15 isometric lumbar extensor exercises,¹³ and task-simulating exercises.^16,17

Advocates of specificity in exercise training contend that adaptation of the body is specific to the type of training load used during exercise.^18–20 Some studies^18–20 have shown that improvements in some muscles were specific to the type of contraction and movement used for training. However, the concept of specificity of exercise training is still controversial due to equivocal research findings. For example, it has been shown that different modes of training (isometric versus isokinetic, concentric versus eccentric) resulted in similar improvement of muscle performance (that is, variables measured in each of the training modes were similar after training despite differences in the type of contraction and movement used for training),^21–24 that training with similar muscle activities of stair climbing or combined running and cycling resulted in similar improvement in running performance,^25,26 and that training in one activity (swimming) could lead to an improvement in a different activity (running).²⁷ Therefore, in order to improve performance of functional tasks such as lifting, we do not have evidence as to which training mode, task specific or general strength training, can provide the best training effect.

Recently, Ng et al²⁸ reported 12.8% improvement in maximum lifting load following a 4-week squat lift training program and Yeung et al²⁹ reported 11.03% improvement in maximum lifting load following a 4-week free weight muscle training program. Both studies were experimental in nature on young adults without impairment, and they showed comparable improvement in lifting capacity in both training modes. The mode, however, that would lead to greater improvement in maximum lifting load is not known. Therefore, we compared the effects of a squat lift training program and an individual muscle resistance training program on the maximum lifting load and isokinetic peak torque of trunk and limb muscles (trunk extensors, shoulder abductors, elbow flexors, and knee extensors) in young adults without known neuromuscular or musculoskeletal impairments.

	Method

Top Abstract Introduction Method Results Discussion and Conclusions References

 
Subjects

Twenty men and 16 women in their early twenties (mean age=21.25 years, SD=1.16, range=20–24) volunteered for this study. All subjects were university students who had no known neuromuscular or musculoskeletal impairments and who had never participated in any regular physical training. They were randomly divided into 3 groups by drawing lots. Subjects assigned to group 1 (7 male, 5 female) performed squat lift exercises, subjects assigned to group 2 (5 male, 7 female) performed free weight resistance exercises for back and limb muscles (back extensors, shoulder abductors, elbow flexors, and knee extensors), and subjects assigned to group 3 (8 male, 4 female) did not receive any training and served as a control group. Subjects in all 3 groups were tested before and after the 4-week training program.

Tests

The measurements obtained were maximum lifting load and isokinetic peak torque for the back extensors, shoulder abductors, elbow flexors, and knee extensors. The testing procedures followed those used in the studies of Ng et al²⁸ and Yeung et al²⁹ and will be explained below.

Maximum lifting load.
The lifting capacity, from floor to shoulder level, was determined using a psychophysical approach.³⁰ The psychophysical method that we used is a measure of perceived stress, which requires subjects to adjust the loads they lift according to their perception of physical strain, which we believe could simulate the normal workplace where the subjects assess the loads with their perception before performing the lift. A plastic box (40 x 29 x 16 cm) with handles on both sides was used for holding the weight and standardizing the dimension of the weight being lifted. Our procedures required the subject to assume a half-squat position beside the box so that the box could be kept close to the body throughout the movement. The subject then lifted the box from the floor to the shoulder level by extending the hips and knees, abducting the shoulders, and flexing the elbows. Afterward, the subject would lower the box by reversing the movement.

Throughout the lift, the subject was asked to attempt to keep the speed at a smooth and continuous rhythm without a halt in any part of the action from the beginning of the lift to the final lowering of the box. A metronome set at one beat per second was placed next to the box to allow the subject to follow the pace of the metronome in performing the lifts. One lifting and lowering cycle took approximately 6 seconds to complete. Based on the range moved by each studied body part at the prescribed pace, we estimated the speed of movement for the respective body parts by dividing the range moved over the time taken. In a pilot study that we conducted on ourselves to estimate the speed of movement of these body parts, we estimated that the knee moved at about 100°/s, that the elbow moved at 80° to 90°/s, that the shoulder moved at 60°/s, and that the trunk moved at 30°/s.

The initial load inside the box was set at 5 kg. If a subject felt that load was too light, 2.5 kg of weight was added for the subsequent lift. If the subject perceived the load to be too heavy, 1.25 kg of weight would be removed from the box each time. Therefore, the weight was adjusted according to the ability of each individual, until the subject felt that it was the maximum load that he or she could lift only once. During the testing procedure, the box was covered and subjects were unaware of the weight inside the box. We considered the load as acceptable only when the difference between each 2 consecutive tests was less than 15%. Otherwise, the test was repeated until the percentage of difference for the 2 loads was within 15%. All subjects required no more than 3 tests. The average of the loads was taken as the subject's maximum acceptable lifting load. During the test, each subject was given at least 3 minutes of rest after each lift in order to prevent muscle fatigue. The rest interval varied among the subjects because some subjects were more adherent than other subjects to the test procedure. Subjects who were more adherent to the test procedure assumed the starting position for the next test at close to the 3-minute mark and then performed the lift. Subjects who were less adherent to the test procedure took somewhat longer to assume the starting position for the next test; some subjects took 15 to 20 seconds longer to assume the starting position.

Isokinetic peak torque.
The muscles tested were the back extensors, shoulder abductors, elbow flexors, and knee extensors on both sides. However, we analyzed only the data obtained for the shoulder abductors, elbow flexors, and knee extensors of the left side, as we expected that the changes on the other side would be similar, given the bilateral nature of the type of contraction and movement used for training. These muscles were tested because of their importance in performing squat lifting. We believed that, if improvement was specific to the type of contraction and movement used for training, the subjects in group 1 could have more improvement in the lifting load than in isokinetic peak torque of the muscles, whereas subjects in group 2 might have more improvement in isokinetic testing than in lifting. A MERAC isokinetic machine^* was used for these tests. The setup for these tests was according to the user's manual for the machine. Each subject performed 3 submaximal contractions as warm-up and then 4 maximal contractions. The highest peak torque value of the 4 contractions was used for analysis. The testing speeds used were 60°/s for the shoulder abductors, 85°/s for the elbow flexors, 100°/s for the knee extensors, and 30°/s for the trunk extensors. These speeds were chosen based on the estimated movement speed of each body part during the lift testing and training in order to reflect the training effect at the respective speed.

Reliability

Reliability of the measurements was tested on 5 subjects. Each subject attended 2 testing sessions conducted 1 week apart. The testing protocols were as described for maximum acceptable lifting load and isokinetic peak torque measurements of the trunk and limb muscles. Subjects in the reliability testing were different from those of the main study. Intraclass correlation coefficients (3,1) for the 5 measured variables ranged from .87 to .98 (Tab. 1).

Subjects in group 2 engaged in resistive exercises for their elbow flexors, shoulder abductors, knee extensors, and back extensors at 80% of their maximal force levels. The maximal force for each muscle group was determined by dividing the isokinetic peak torque by the lever arm of the testing machine. The training weight was applied at the same position as the isokinetic test pads to the limbs so that the training stimulus would be equivalent to 80% of the peak torque. The measurement of peak torque was obtained from the isokinetic testing. After obtaining the peak torque measurement, we divided it by the movement arm of the testing machine to obtain the force measurement. We multiplied this force value by a factor of 0.8, so that when the subject was exercising with this weight as applied to the position of the isokinetic test pad, the resultant resistance would be equal to 80% of the peak torque value. In each of the training sessions, the muscle group was trained with 3 sets of 10 repetitions, with 1 minute of rest between sessions. During training, the subjects attempted to follow the pace of the metronome, which was set at 45 beats per minute. Training was conducted 3 times weekly on alternate days for 4 weeks. Similar to the procedure for the subjects in group 1, the weights were progressively increased by 5% per week starting from the second week.

Subjects in the control group did not receive any training throughout the study period. The 3 groups were compared before and after the 4-week period for their maximum lifting load and isokinetic peak torque of the back and limb muscles.

Data Analysis

The data were analyzed using the Statistical Package for the Social Sciences (SPSS) personal computer program (version 7.5). One-way analysis of variance (ANOVA) and chi-square tests were used to determine whether there were differences in age, weight, height, and sex ratio among the 3 groups. A multivariate ANOVA was used to determine whether there were differences in peak torque and maximum lifting load among the 3 groups before training. The training effect was determined by subtracting the pretraining measurements from the posttraining measurements of each subject. These difference scores (ie, differences between the pretraining and posttraining measurements) were normalized as percentages of the pretraining values and analyzed by multivariate ANOVA. Significance for all tests was set at P.05.

	Results

Top Abstract Introduction Method Results Discussion and Conclusions References

 
There was no difference in age, weight, height, and sex distribution among the 3 groups (Tab. 2). The multivariate ANOVA indicated no difference in peak torque and maximum lifting load among the 3 groups before training (Tab. 3).

	Discussion and Conclusions

Top Abstract Introduction Method Results Discussion and Conclusions References

The training program for group 1 involved multiple squat lifts to the shoulder level at more than 80% of the maximum lifting load of the subjects. This training intensity has been reported to be high enough to elicit a training response.³¹ The improvement in lifting capacity (22%) of our subjects is comparable to the findings of Sharp and Legg,³² who showed a 26% increase in maximal repetitive lifting capacity with 4 weeks of repet-itive lift training. Improvement of this type (eg, for task-specific training [training tasks essentially identical to what is tested]) has been shown by different researchers for different muscle groups and tasks.^18–20 Genaidy and colleagues^16,17 further demonstrated that a job-simulated exercise training program could improve lifting capacity. A possibility always exists, however, that with very specific training similar to a test, subjects may just get better on the test.

In the free weight muscle training program, the training tasks were not the same as those of the lifting test. The finding that these subjects improved in their lifting capacity suggests that individual muscle training can also improve performance. The concomitant improvement in isokinetic peak torque of the back extensors in both training groups indicates a role for the back muscles in lifting. According to the National Institute for Occupational Safety and Health (NIOSH) normative figures, the 90th percentile of men performed torso lifts at 77 kg and leg (squat) lifts at 134 kg, which requires good back extensor force.³³ This range is far greater than that for the high-near and high-far arm lifts, defined as lifting strength at shoulder level and close to and away from the body, respectively. Therefore, an increase in the back muscle force would likely improve squat lifting capacity, as shown in our study.

We adopted similar intensities with a task-specific squat lift training program (group 1) and an individual muscle free weight training program (group 2) and found that the magnitude of improvement in lifting was essentially the same in group 1 (21.91%) and group 2 (21.97%). Specificity of training response, therefore, may not apply to all conditions and muscles. Indeed, other researchers^{21–24,34,35} have also reported that subjects who engaged in different modes of training had similar improvements in muscle performance in the same conditions of testing. These studies included comparing the effects of isometric and isokinetic training,²¹ concentric versus eccentric training,^22–24 and concentric isokinetic training on concentric and eccentric performance.³⁴ All of these studies showed that improvement was not specific to the mode of training, as measured by isokinetic testing or isometric muscle testing. Similar responses have also been found in a back extensor strengthening program in which isometric and training resulted in comparable improvements in isometric force.³⁵ Hortobagyi et al³⁶ showed that individuals who had a certain level of performance in one type of test, such as a concentric isokinetic test, were able to achieve the same relative level of performance in other tests, such as eccentric isokinetic tests. Support for the concept of generality is further demonstrated by Foster et al,²⁷ who found that swimming training contributed to improvement in running speed.

In all of the studies discussed, the findings were in agreement with our observation that improvement in muscle force resulting from individual muscle free weight training is also reflected during the task of squat lifting. More importantly, we found that the task-oriented and non–task-oriented training programs had the same improvement in their ability to lift weight during a squat lift. This finding, in our opinion, shows that the non–task-specific training program is comparable to its task-oriented counterpart for improving squat lifting. Because we examined only squat lifting, it is not appropriate for us to generalize our findings to other activities. However, based on our findings, we believe clinicians and exercise scientists should consider being more flexible in planning some training programs, instead of rigidly applying the principle of specificity.

Limitations

Our training program lasted only 4 weeks, and improvements may have been partly due to a learning effect as a result of practice or a combination of learning and physiological changes in muscle.¹⁹ Because of the method we used, we cannot determine which factor was more important. Furthermore, we tested subjects in a very narrow age range, which poses concerns about the generalizability of our findings to different age groups for applied studies. However, the narrow age range should be acceptable in studies in which theoretical constructs are being tested, such as in our study.

Differences in isokinetic peak torque of the elbow flex-ors were found after the training modes were used, but no differences were found when either training group was compared with the control group. Table 4 shows that the percentage differences in the elbow flexors have very large standard deviations. Although the improvement in group 2 (29.7%) was substantially greater than that in group 3 (10.36%), the large standard deviation would have masked the effect. However, we cannot explain why the subjects in group 3 had more than 10% improvement in isokinetic peak torque of the elbow flexors during the 4-week period.

We measured the lifting force but not the lifting posture or other neuromuscular variables. Whether the subjects in both training groups used similar body kinematics during lifting or similar neuromuscular patterns of muscle activity that would directly affect the loading to the body parts is not known.

	Footnotes

 
Both authors provided concept/research design, writing, data collection and analysis, project management, subjects, facilities/equipment, institutional liaisons, clerical support, and consultation (including review of manuscript before submission).

This study was approved by the Ethics Standing Committee for Experimentation With Human Subjects of The Hong Kong Polytechnic University.

^* Universal Gym Equipment Inc, 818 Dows Rd SE, Cedar Rapids, IA 52406.

SPSS Inc, 444 N Michigan Ave, Chicago, IL 60611.

	References

Top Abstract Introduction Method Results Discussion and Conclusions References

 

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This Article Abstract Full Text (PDF) Submit a response Alert me when this article is cited Alert me when Rapid Responses are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Yeung, S. S. Articles by Ng, G. Y. Search for Related Content PubMed PubMed Citation Articles by Yeung, S. S. Articles by Ng, G. Y. Related Collections Therapeutic Exercise Injuries and Conditions: Back Injuries and Conditions: Low Back Social Bookmarking                      What's this?