Exercise-induced muscle soreness after concentric and eccentric isokinetic contractions.

GK Fitzgerald, MS, Pr, is Assistant Professor, Department of Orthopedic Surgery and Rehabilitation, Program in Physical Therapy, Hahnemann University, MS 502, 201 N 15th St, Philadelphia, PA 19102 (USA). This study was completed in partial fulfillment of the requirements for Mr Fitzgerald's Master of Science degree, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA. Address all correspondence to Mr Fitzgerald.

JM Rothstein, PhD, PT, is Professor and Head, Department of Physical Therapy, College of Associated Health Professions, University of Illinois at Chicago, Chicago, IL, and Chief of Physical Therapy Services, University of Illinois Hospital, 1919 W Taylor St, Chicago, IL 60612.

TP Mayhew, MS, PT, is Assistant Professor, Department of Physical Therapy, School of Allied Health Professions, Medical College of Virginia, Virginia Commonwealth University, PO Box 224, MCV Station, Richmond, VA 23298-0024,

RL Lamb, PhD, PT, is Associate Professor and Chairman, Department of Physical Therapy, School of Allied Health Professions, Medical College of Virginia, Virginia Commonwealth University. This study was approved by the Committee on Human Research, Virginia Commonwealth University. This article was submitted September 4, 1990, and was accepted March 5, 1991.

The purpose of this two-part study was to determine whether the amount of exercise-induced muscle soreness differs between subjects who perform concentric and eccentric isokinetic contractions of their quadriceps femoris muscles. In experiment 1, subjects were randomly assigned to either an eccentric or a concentric exercise group and both groups exercised at the same power level In experiment 2, subjects were randomly assigned to either an eccentric or a concentric exercise group and both groups exercised with maximal effort. Muscle soreness ratings, obtained by using a visual analogue scale, were taken immediately before exercise and at 24 and 48 hours postexercise. Changes in muscle soreness ratings between exercise groups from preexercise to postexercise periods were compared in both experiments, using a one-way between-subjects analysis of variance. There was no difference in the change in muscle soreness from preexercise to postexercise periods between groups exercising at equal power levels Subjects who exercised using eccentric contractions with maximal effort demonstrated greater increases in muscle soreness than those who performed concentric contractions The results suggest that exercise intensity, rather than contraction type, may be the dependent factor in producing exercise-induced muscle soreness. [Fitzgerald Gx Rothstein JM, Maybew TP, Lamb RL. Exercise-induced muscle soreness after concentric and eccentric isokinetic contractions. Phys Ther. 1991;71..505-5-13.] Key Words: Exercise, general; Muscle contraction; Muscle performance, lower extremity.

Participants in exercise programs may, after exercise, experience muscle soreness. Exercises that induce this soreness are thought to require greater use of muscles than an individual's regular level of physical activity. Because the soreness is usually not felt until 8 to 12 hours after cessation of an exercise session, some authors(1-4) refer to exercise-induced muscle soreness as "delayed onset muscle soreness." The intensity of exercise- induced muscle soreness increases during the first 24 to 48 hours after exercise and gradually subsides over the next several days.(1)

The soreness is most likely to be felt when the muscle contracts, but there are reports of pain when the muscle belly is palpated and also of "muscular stiffness" (eg, a feeling of increased resistance to passive lengthening of the muscle).(1,3,5)

Many investigators2-,6,7 have concluded that exercises requiring the use of eccentric muscle contractions will result in greater muscle soreness than exercises that require mostly concentric contractions. Although isokinetic exercises are commonly used in clinical practice, the effect of these types of exercises, which involve concentric and eccentric contractions, has not been studied.

Some researchers have stated that, when the same amount of torque is produced by a muscle during concentric and eccentric contractions, fewer motor units are recruited during the eccentric contraction.(2,8) Armstrong(1) and Asmussen(2) believe that the risk of damage to muscle and its associated connective tissue is greater during eccentric contractions than during concentric contractions, because less muscle tissue is contracting when producing a given torque. These authors believe that exercise-induced muscle soreness associated with eccentric muscle contractions is probably due to damage to muscle or connective tissue caused by overload of these tissues.

Studies comparing the magnitudes of exercise-induced muscle soreness produced after concentric and eccentric muscle contractions have usually lacked adequate controls of exercise intensity.(2 6,7) Although some researchers have concluded that eccentric muscle contractions cause greater muscle soreness than concentric muscle contractions, methodological problems in their studies cast doubt on these conclusions. Subjects in some studies,(4,6) for example, were asked to perform concentric and eccentric contractions against different loads of resistance and for varying numbers of repetitions. Observed differences in the magnitude of the soreness may have been due to differences in the intensity of the exercises rather than to the type of muscle contraction.

The intensity of exercise can be controlled by training subjects to perform concentric and eccentric muscle contractions at the same power level. Some dynamometers allow limb segments to move at constant velocities during concentric and eccentric contractions. Subjects who produce equal amounts of torque during concentric and eccentric contractions at the same velocity of movement are exercising at equal power levels. Isokinetic exercise devices provide the means for appropriate comparisons of exercise-induced muscle soreness produced by concentric and eccentric muscle contractions.

We conducted two experiments to examine the variables influencing the development of soreness. in the first experiment, we wanted to determine whether there was a contraction-type-dependent difference in the amount of exercise-induced muscle soreness in subjects who exercised at equal levels of power. The purpose of the second experiment was to determine whether there was a contraction-type-dependent difference in the amount of exercise-induced muscle soreness in the quadriceps femoris muscles of subjects who exercise at maximal effort.

We believed that when concentric and eccentric contractions were performed at 90% of the maximum power produced during concentric quadriceps femoris muscle contraction, the torque produced during the contractions would probably not be great enough to cause tissue damage. if tissue damage did occur, we believed there would probably be no difference in the amount of tissue damage between exercise groups. We therefore hypothesized that there would be no difference in the magnitude of quadriceps femoris muscle soreness after exercise in subjects who performed concentric and eccentric isokinetic contractions at equal power levels.

We believed that, with maximal effort, greater levels of torque would be produced during eccentric contractions than during concentric contractions and that tissue damage would thus be more likely to occur from the eccentric contractions than from the concentric contractions. We therefore hypothesized that, when subjects exercised at maximal effort, there would be greater quadriceps femoris muscle soreness in subjects who performed eccentric isokinetic contractions than in those who performed concentric isokinetic contractions.

Method

This study consisted of two experiments. In experiment 1, comparisons were made between groups exercising at equal levels of power. In experiment 2, comparisons were made between groups exercising with maximal effort. The methods used in both experiments were similar. Different subjects were used in each experiment.

Subjects

Subjects in both experiments were nondisabled volunteers with no complaints of pain in their right lower extremities and no history of knee or quadriceps femoris muscle pathology. Subjects did not regularly participate in activities requiring intense use of lower-extremity muscles. Analgesic, anti- inflammatory, or any other type of medication that would affect neuromuscular function was not taken by subjects for a minimum of 1 week prior to the study. All subjects signed an informed consent form before participating in the study.

Twenty subjects participated in experiment

1. These subjects were randomly assigned to groups that performed either concentric or eccentric muscle contractions at a target power level equivalent to 90% of the maximum power produced during concentric contraction of their quadriceps femoris muscles. Six subjects were eliminated from the study because they were unable to maintain the target power level during exercise.

These subjects were replaced by subjects who were able to maintain the target power level, so that each group would consist of 10 subjects. The concentric contraction group consisted of 9 women and 1 man, with an age range of 21 to 34 years -R=24.6, SD=3.9). The eccentric contraction group consisted of 7 women and 3 men, with an age range of 21 to 36 years X=27.5, SD=5.7).

An additional 20 subjects participated in experiment 2. These subjects were randomly assigned to groups of 10 subjects each who performed either concentric or eccentric muscle contractions with maximal effort. The ability to maintain a specific power level was not a factor in experiment

2. Therefore, no subjects were eliminated in this experiment. The concentric contraction group consisted of 6 women and 4 men, with an age range of 21 to 34 years X=27.1, SD=5.3). The eccentric contraction group consisted of 8 women and 2 men, with an age range of 21 to 39 years X=26.4, SD=5.9).

Instrumentation

The instrumentation was the same in both experiments. A Kin-Com(R) dynamometer* was used to measure the power produced during contractions and to provide resistance during the exercise portion of the study. The voltage output from the force, angle, and velocity transducers (from the Kin-Com(R)'s strain gauge, potentiometer, and tachometer) was processed through an AMMI Analog-to-Digital Board in a Keithley DAS Measurement and Control System (series 500).(+) Data acquisition was controlled using DADISP I software (version 1.01).(++) The frequency of analog-to-digital conversion was 250 Hz.

The calibration of the Kin-Com(R) force and angle recordings was tested before data collection began. Calibration was retested at the completion of data collection. The results of the prestudy and poststudy calibration testing indicated that the Kin-Come did not lose calibration during testing.

A visual analogue scale VAS) was used to measure muscle soreness (Fig. 1). The VAS has been shown to be reliable and valid as a ratio-scale measure of both clinical and experimental pain.9 Muscle soreness was always measured during maximal isometric contractions of the right quadriceps femoris muscle.

Procedure

The experimental procedure in experiments 1 and 2 consisted of five sessions. These sessions were conducted on five consecutive days. A summary of the procedures used in each session is outlined in Figure 2.

Initial Evaluation

The subject sat on the Kin-Com(R) table with the right hip flexed to 90 degrees and the right leg next to the Kin-Com(R)'s lever arm. The right hip position was determined by visual inspection. After the subject was positioned, a back support was placed behind the subject. The location of the back support was recorded so that the support could be placed in the same position during subsequent testing sessions. The subject's right knee joint was aligned with the axis of rotation of the dynamometer's lever arm.

The most prominent aspect of the right lateral femoral epicondyle was used as an anatomical landmark to represent the axis of rotation of the knee joint.

The pad of the dynamometer's lever arm was attached to the subject's right leg, 1 cm proximal to the medial malleolus. The distance from the center of the pad to the lever arm's axis of rotation was recorded and was later used to calculate torque values. After the subject's limb length was determined, the back support was removed. The subject was then brought to a supine position on the Kin-Com(R) table in preparation for determination of a gravity-correction value. The supine position was selected for two reasons. Preliminary observations indicated that, if the subject's knee was fully extended while sitting, tension in the hamstring muscles increased the value of the force exerted by the limb on the lever arm (Sheryl Finucane, PT, personal communication).

Preliminary studies also indicated that limb weight measurements were most accurate when the lever arm was close to the horizontal position. The supine position, therefore, allowed measurements of limb weights and eliminated the effects of hamstring muscle tension on the measurements.

The "gravity-correction" option on the Kin-Com(R) computer was used to determine limb weight with the limb fully extended. After the gravity-correction procedure was completed, the subject was repositioned in sitting as previously described. Straps were placed firmly across the subject's pelvis and also across the middle portion of the subject's right thigh. The subject's knee was positioned in 90 degrees of flexion.

A goniometer was used to determine the angle of flexion. A value of 90 degrees was then entered into the computer, using the Kin-Com(R)'s "anatomical joint reference" option, This ensured that the computer's lever arm angle recording was equivalent to the subject's knee joint angle.

After the anatomical joint reference procedure was completed, preexercise baseline muscle soreness ratings were taken. The subject's knee was positioned at 60 degrees of flexion. The subject was then instructed to perform a maximal isometric contraction of the quadriceps femoris muscle and to maintain this contraction for 3 seconds. Immediately following the isometric contraction, the subject was asked to use the VAS to rate the magnitude of muscle soreness perceived in the quadriceps femoris muscle during the contraction. The subject placed a mark on the VAS that indicated the magnitude of muscle soreness experienced. The distance from the extreme left of the VAS to the subject's mark was measured to the nearest millimeter.

This value was used as the subject's soreness rating. The subject rested for 5 minutes. The soreness rating procedure was then repeated. Two soreness ratings were taken during each soreness rating session so that test-retest reliability of soreness measurements could be determined.

After the muscle soreness ratings were obtained, maximum concentric quadriceps femoris muscle power was determined using the Kin-Com(R)'s evaluation program. A start angle of 90 degrees and a stop angle of 15 degrees for lever arm motion were entered into the computer. The velocity of lever arm motion was set at 30[deg]/s. Lever arm acceleration and deceleration were set at "high" to maximize the time period in which constant velocity occurred. The lever arm initial force forth" was set at 150% of the limb weight value determined during the gravity-correction procedure. The lever arm initial force "back" was set at zero to allow for passive return of the limb to the start position.

Subjects were instructed to perform 10 maximal concentric quadriceps femoris muscle contractions. Preliminary studies, performed by one of the authors (GKF), indicated that subjects would produce their maximal concentric contraction within 10 contractions. A 45-second rest period was given between each contraction.

Peak torque, average torque, and average speed from the 10 maximal concentric quadriceps femoris muscle contractions were calculated using DADISP II software.* The power produced during each of these contractions was calculated by multiplying the average torque by the average speed of the contraction. After the power produced by each contraction was calculated, the contraction producing the greatest amount of power was identified. The training target power level was 90% of the power produced during this contraction.

Preliminary studies performed by the authors were used to develop the training protocol. We found that subjects could maintain the training target power level when force markers that could be used as targets to control torque output were displayed on the Kin-Com(R)'s computer monitor. A diagram of these force markers is provided in Figure 3. We also found that different force marker values had to be used for concentric and eccentric contraction exercise groups so that the same training target power level could be maintained. Marker values for subjects performing concentric contractions were set at 85% and 95% of the peak torque from the concentric contraction that produced maximum power. Marker values for subjects performing eccentric contractions were set at 65% and 75% of the peak torque from the concentric contraction that produced maximum power. Because the Kin-Com(R) computer cannot display markers in units of torque, marker values were converted to units of force (newtons).

Practice Session

Subjects were seated on the Kin-COM(R) table as described for the initial evaluation. Program settings in the Kin-Com(R)'s evaluation mode were the same as those previously described, except that subjects performing eccentric contractions had a value of zero for the initial force "forth" setting. Their initial force "back" setting was 150% of their limb weight. Values for the force markers were entered into the Kin-Com(R) computer.

For practice, depending on group assignment, subjects were instructed to perform either 10 concentric or 10 eccentric quadriceps femoris muscle contractions. Subjects were asked to push against the lever arm in such a way as to maintain the torque tracing for as long as possible within the marker boundaries displayed on the computer monitor. No time limit was used during the practice period. A 2-minute rest period followed the 10 practice contractions.

After the rest period, subjects performed 10 quadriceps femoris muscle contractions, while attempting to maintain their torque tracings within the marker boundaries. A 20-second rest period was provided between contractions. This was the amount of time needed for the computer to process force, speed, and angle data. Force, speed, and angle voltage signals were processed for each of these 10 contractions for subjects in experiment 1. In experiment 2, these signals were not recorded for the 10 contractions performed by subjects.

In experiment 1, immediately following the practice session, the power produced during each contraction was calculated as previously described for the initial evaluation. The power produced by each contraction was expressed as a percentage of the target power. if the mean percentage of the target power was not within -20% of the target power, the subject was eliminated from the study. The subject was eliminated from the study because it was unlikely that he or she would maintain the target power level during exercise.

Power was not calculated for the contractions performed by subjects in experiment 2. The ability to maintain a specific power level was not a factor in this experiment. These subjects participated in the practice session so that they would be performing amounts of physical activity similar to those performed by the subjects in experiment 1.

Exercise Session

Subjects were seated on the Kin-Com(R) as described for the initial evaluation. Two postpractice soreness ratings were then obtained, using the method described for the initial evaluation. After the postpractice soreness ratings were taken, the settings for the Kin-Com(R)'s evaluation mode were entered into the computer as described for the practice session. Depending on group assignment, subjects were asked to perform seven sets of 10 concentric or eccentric quadriceps femoris muscle contractions. In preliminary studies, we determined that subjects who performed concentric contractions were unable to maintain the desired level of power for more than seven sets of 10 contractions. Subjects in experiment 1 were asked to maintain force levels for each contraction within the marker boundaries displayed on the computer monitor.

Subjects in experiment 2 were asked to perform each contraction with maximal effort. A 2-minute rest period was provided between each set of 10 contractions in both experiments.

To ensure that subjects in experiment 1 were exercising at 90% of maximum concentric power, 2 contractions from each set of 10 contractions were randomly selected for calculations of power production. Therefore, a total of 14 contractions per subject were sampled for power calculations. Subjects in experiment I were eliminated from the study if a minimum of 10 sampled contractions did not fall within [+ or -] 10% of the target power.

Postexercise Soreness Rating Sessions

The 24-hour and 48-hour postexercise soreness ratings were taken during sessions 4 and 5, respectively. The method for measuring soreness in these sessions was the same as that described for the initial evaluation.

Dependent Measures

The dependent measure used for statistical analysis was the difference in soreness ratings from the postpractice soreness rating session to the postexercise rating sessions. The postpractice ratings were used as baseline ratings, because any change in soreness ratings that may have occurred from participation in the evaluation and practice sessions would be reflected in these soreness ratings. The preexercise soreness ratings, therefore, were not used in the statistical analysis.

Statistical Analysis

The same statistical analyses were used in both experiments. The VAS has been shown to be reliable and valid as a ratio-scaled measure of pain.9 Parametric statistical tests are therefore appropriate for analyzing VAS data.

An intraclass correlation coefficient (ICC[1,1]) was used to determine test-retest reliability of VAS soreness ratings for the postpractice rating session.10 Test-retest reliability of the postpractice ratings was determined by comparing the first rating with the second rating taken during session 3. The ICC for this comparison was .74 in experiment 1 and .67 in experiment 2.

An ICC (1,1) was also used to determine test-retest reliability of the differences in soreness ratings between postpractice and postexercise soreness rating sessions. The difference between the first postpractice rating and the first 24-hour postexercise rating was compared with the difference between the second postpractice rating and the second 24-hour postexercise rating. The ICC for this comparison was .35 in experiment 1 and .89 in experiment 2. Test-retest reliability for the difference between postpractice and 48-hour postexercise ratings was determined in the same manner as that described for the 24hour postexercise ratings. The ICC for this comparison was .57 in experiment 1 and .94 in experiment 2.

The ICCs indicate that the differences in soreness ratings from postpractice to postexercise rating sessions demonstrated poor reliability in experiment 1. These same comparisons in experiment 2 demonstrated relatively good reliability. This finding will need to be considered when interpreting the results of experiment 1.

A one-way between-subjects analysis of variance (ANOVA)(11) was used to analyze the VAS soreness data. Three separate comparisons were made between groups. The first comparison was made to determine whether there was a difference between the groups' soreness ratings taken during the postpractice rating session (immediately before the exercise session). The second and third comparisons were made to determine whether changes in VAS soreness ratings occurred at 24 and 48 hours postexercise, respectively.

Results

Experiment 1

A summary of raw VAS muscle soreness data is presented in Table 1. The difference in muscle soreness ratings from the postpractice to the postexercise rating sessions was calculated from these data for each subject. The differences in muscle soreness ratings from the postpractice to the postexercise sessions were then compared between groups. We believed these comparisons would be more meaningful than comparing raw VAS data between groups. Between-subject comparisons are problematic using our design because of differences in baseline data and because of individual differences.

The results of a one-way between-subjects ANOVA for postpractice muscle soreness ratings are summarized in Table 2. No significant difference was found between groups. This finding suggests that the groups were similar in rating muscle soreness of their quadriceps femoris muscles prior to the exercise portion of test session 3.

The results of one-way between-subjects ANOVAs for comparisons between groups of the difference in soreness ratings from the postpractice rating session to the 24-hour and 48hour postexercise rating sessions are summarized in Tables 3 and 4, respectively. The difference in soreness ratings at 24 and 48 hours postexercise was similar for both groups.

Experiment 2

The results of a one-way between-subjects ANOVA for postpractice muscle soreness ratings are summarized in Table 5. No significant difference was found between groups for this comparison. This finding suggests that the groups were similar in rating muscle soreness of their quadriceps femoris muscles before the exercise portion of test session 3.

The results of one-way beuween-subjects ANOVAs for comparisons between groups of the difference in soreness ratings from the postpractice rating session to the 24-hour and 48-hour postexercise rating sessions are summarized in Tables 6 and 7, respectively. The subjects who performed eccentric quadriceps femoris muscle contractions with maximal effort reported a greater increase in muscle soreness after exercise than those who performed concentric contractions at 24 hours postexercise. The difference in soreness ratings at 48 hours postexercise was similar for both groups.

When the same amount of torque is produced by a muscle during concentric and eccentric contractions, fewer motor units are recruited during the eccentric contraction.(2,11) Armstrong(1) and Asmussen(2) believe that exercise-induced muscle soreness associated with eccentric contractions is probably due to damage to muscle or connective tissue caused by overloading of these tissues. They believe that the risk of damage to muscle and its associated connective tissue is greater during eccentric contractions than during concentric contractions, because less muscle tissue is contracting when resisting a given force.

Experiment I of this study examined the change in muscle soreness ratings from preexercise to postexercise conditions in subjects who performed concentric or eccentric muscle contractions at equal levels of power. Because the velocity of movement was constant, subjects were actually producing the same levels of torque. If, at the same levels of torque production, there is greater risk of tissue damage during eccentric contractions than during concentric contractions, as hypothesized by Armstrong(1) and Asmussen,(2) subjects who performed eccentric contractions should have had greater increases in postexercise muscle soreness than those who performed concentric contractions in experiment 1. No significant difference was found in the change in quadriceps femoris muscle soreness at 24 and 48 hours postexercise between groups. This finding does not support Armstrong's' and Asmussen's(2) hypothesis.

The reliability of our muscle soreness measurements in experiment I appeared to be poor. No significant difference may have been found between groups because the reliability of the soreness measurements was questionable. When subjects were asked whether they were experiencing muscle soreness during functional activities (eg, walking, stooping, stair climbing) at 24 and 48 hours postexercise, there was no report of muscle soreness from subjects in either group. We believe, therefore, that there probably was no difference in muscle soreness between groups postexercise.

The reliability of muscle soreness measurements performed by other researchers who studied exercise-induced muscle soreness after concentric and eccentric contractions is not known. Various investigators(2 6,7) did not indicate the reliability of their soreness measurements, yet they concluded that eccentric contractions produce greater postexercise muscle soreness than do concentric contractions. Because the reliability of their soreness measurements is unknown, the extent to which their results may be attributed to measurement error cannot be determined. Our results, although they should be considered in the context of our reliability estimates, represent the only data for which we can estimate the reliability of measurement in the study of exercise-induced muscle soreness. Several investigators(2,6,7) concluded that exercises primarily requiring the use of eccentric muscle contractions produce greater muscle soreness than do exercises that require mostly concentric muscle contractions. A variety of exercise methods were used in these studies. Asmussen(2) and Newham et al(3) used a platform-stepping exercise method. Talag(4) and Clarkson et al6 used an isotonic exercise method. In these studies, the power produced during muscle contractions was not equal for different contraction types. Subjects who performed eccentric muscle contractions in these studies probably produced greater levels of power than did those who performed concentric contractions. Subjects who performed eccentric contractions probably experienced greater muscle soreness because they exercised at greater intensities than did those who exercised with concentric contractions.

There are no reports that compare the difference in the magnitudes of muscle soreness produced after concentric and eccentric isokinetic muscle contractions. When our subjects exercised with maximal effort in experiment 2, there was greater muscle soreness at 24 hours postexercise in subjects who performed eccentric muscle contractions than in those who performed concentric contractions. The muscle soreness measurements in experiment 2 appeared to be reliable. When subjects in experiment 2 were asked whether they experienced muscle soreness during functional activities 24 and 48 hours after session 3, only subjects who performed eccentric contractions reported soreness.

The results of experiment 2 indicate that, when subjects are asked to perform muscle contractions with maximal effort, greater muscle soreness occurs after eccentric contractions than after concentric contractions. This finding is consistent with the observations of other researchers who had subjects exercise maximally.

Other observations have indicated that subjects produce greater levels of torque during maximal eccentric isokinetic contractions than during maximal concentric isokinetic contractions Canet Kues, MS, PT, personal communication). Muscle soreness is probably greater after maximal isokinetic eccentric contractions because the muscle must resist greater levels of torque during these contractions than during maximal concentric contractions.

The velocity of limb movement in this study was constant at 30[deg]/s. This velocity was selected because our preliminary studies indicated that muscle soreness could not be induced when higher constant velocities of movement were used (eg, 60[deg] and 90[deg]/s). The results of experiment 2 may not apply to isokinetic exercise at higher limb velocities. Further research is needed to examine this issue. The results of our study appear to indicate that the intensity of muscle contraction, rather than the type of contraction, is the dependent factor in producing exercise-induced muscle soreness. This finding would imply that individuals could perform concentric and eccentric isokinetic contractions without experiencing muscle soreness if the intensity of exercise were controlled and consisted of less than maximal effort.

Conclusions

There appears to be no difference in the degree of exercise-induced muscle soreness between subjects who perform concentric isokinetic contractions and those who perform eccentric isokinetic contractions of their quadriceps femoris muscles at equal levels of power. The change in exercise-induced muscle soreness is greater 24 hours after exercise for subjects who perform eccentric isokinetic contractions than for those who perform concentric isokinetic contractions of their quadriceps femoris muscles at maximal effort. This observation is probably explained by the greater torque production that occurs during eccentric isokinetic contractions than during concentric isokinetic contractions performed with maximal effort.

Acknowledgments

We would like to thank Sheryl Finucane, PT, and Janet Kues, MS, PT, for their comments and assistance in the development of our experimental procedures. We would also like to thank Phil McClure, MS, PT, Neal Pratt, PhD, PT, and Mary Watkins, MS, PT, for their editorial assistance.

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