Effects of exercise on hip range of motion, trunk muscle performance, and gait economy.

Key Words: Exercise, Gait, Hip range of motion, Static stretching 

Passive stretching exercises are often prescribed by clinicians and coaches for individuals with the goal of improving flexibility. The beneficial effects of passive stretching on improving hip flexion range of motion (ROM) and the associated ability to perform a straight leg raise have been well documented.[1-9] These studies have utilized stretching procedures to increase the length of the hamstring muscles. The length of the hamstring muscles is considered to play an important role in the efficiency of walking.[3] The length of the hip flexor muscles and the extensibility of the anterior ligaments of the hip are also considered to play an important role in the efficiency of walking.[10,11]

During walking, the leg is brought into the trailing position by extension of the hip, anterior tilting of the pelvis, and extension of the lumbar spine. If excessive tightness of the hip ligaments, muscles, or fasciae prevents complete extension of the hip joints, an individual commonly compensates for this limitation by accentuating the anterior tilt of the pelvis and lumbar lordosis.[10-13] Hip extension ROM and the length of the one-joint (psoas major, iliacus, pectineus, adductor longus and brevis) and two-joint (rectus femoris, tensor fascia lata, sartorius) hip flexor muscles are tested using a modified Thomas test position.[11,14] This testing procedure places an individual's pelvis in approximately 10 degrees of posterior tilt by flexion of one hip and knee toward the chest while lying supine. The other leg is positioned to extend off the edge of a treatment table far enough to allow the knee to bend freely. An individual is considered to have normal hip ROM if, while in the modified Thomas test position, he or she has 10 degrees of femoral extension relative to the pelvis (testing the length of the one-joint muscles and periarticular structures) as well as 80 degrees of knee flexion (testing the length of the two-joint muscles).[11,14]

In contrast to the numerous reports in the literature documenting improvements in hip flexion ROM following passive stretching, we are aware of only three published reports on the effects of passive stretching on hip extension ROM and the associated ability of subjects to perform a Thomas test. Kottke and associates[15] reported the ROM data of 10 patients of varying ages and pathologies with improved hip extension ROM, presumably due to receiving treatments of sustained passive stretching of 20 minutes' duration. The frequency of treatment was not reported, the length of treatment varied from 4 weeks to 8 months.[15] Moller and associates[16] reported improvements in hip extension ROM of between 2 and 6 degrees after a single treatment session of five contract-relax stretches, which utilized 4- to 6-second contractions of the iliopsoas and rectus femoris muscles followed by end-range passive stretches of 8 seconds. They obtained hip ROM measurements from eight healthy male subjects before and after a 15-minute stretching program designed to stretch six muscle groups of the lower extremities.[16]

In another study that utilized passive hip extension stretching, Godges and associates[4] reported improvements in hip extension ROM in seven recreational athletes immediately following a single treatment session, which included both passive hip flexion and extension stretching. The effects of a long-term passive stretching protocol for improving hip extension ROM, using protocols similar to those used in previous hip flexion ROM studies, have not been reported. One purpose of this study was to analyze the effects of a passive hip extension stretching regimen on hip extension ROM. The stretching regimen utilized in the study was based on the positive preliminary results of our previous study[4] in which ROM improvements (average of 4 [degrees]) were measured following 10 minutes of end-range static stretching (three, 2-minute stretch periods and two, 2-minute rest periods). The stretching regimen in the study is also based on clinical observations of one of the authors (JJG) that gains in hip extension ROM are readily noticeable using clinical goniometric measures after four to six treatment sessions spread out over a 3-week period. We hypothesized that six treatments of passive hip extension stretching (two times per week for 3 weeks) would result in a measurable increase in hip extension ROM.

Hip stretching exercises are also performed by recreational and elite athletes in order to improve athletic performance. Full hip extension and flexion ROM is thought to optimize stride length, thereby improving gait economy.[17,18] Gait economy is assessed by measuring the steady-state oxygen consumption per unit of body weight required to walk or run at a specified velocity.[19] The reliability of gait economy measurements taken during treadmill running has been reported to be high.[17,20-23] Gait economy specifically running economy at high velocities) is highly correlated with running performance in highly trained distance runners.[21]

A recent review of gait economy by Daniels[19] specified the importance of gait economy as a predictor of performance in distance running and the need to determine what factors can affect gait economy. Godges and associates4 reported an improvement in gait economy immediately following one session of sustained passive stretching applied at the end ranges of hip extension and hip flexion. The results of their study support the rationale for preperformance static stretching for hip extension and flexion mobility for those individuals or athletes who want to walk or run more efficiently. The specific effect of which stretching procedure, either hip flexion or hip extension, most significantly affects gait economy, however, remains undetermined. We hypothesized that normal hip extension ROM is of primary importance for achieving optimal gait economy. This hypothesis is based on clinical observations reported in the literature of the deleterious effects on the efficiency of walking noted in patients who have hip flexion contractures.[10,13,15] The second purpose of this study was to analyze the effects of hip extension stretching on walking and running economy in recreational athletes who were measured to have less-than-normal hip extension ROM. We hypothesized that an improvement in hip extension ROM would be associated with corresponding improvements in gait economy in this subject population.

Limited hip extension ROM and lack of extensibility of the one-joint hip flexors are reported to predispose individuals to have an excessive anterior pelvic tilt.[10-15] Another biomechanical factor purportedly associated with excessive anterior pelvic tilt is excessive muscle length and weakness of the abdominal muscles.[11,14] In our experience, only on rare occasions do recreational athletes meet the criteria of Kendall and McCreary[14] and others[24-27] for optimal functioning of the abdominal muscles as determined by performance of the double leg-lowering test (DLLT). This test is performed in the supine position and is described as the ability of the trunk flexors, or the abdominal muscles, to maintain the pelvis in a position of posterior pelvic tilt as the fully extended legs are slowly lowered to the table from a position of 90 degrees of hip flexion.[14] We suspected that a group of recreational athletes who served as subjects were typically deficient in this task and would therefore benefit from a program designed to train the trunk flexors to stabilize the pelvis in a posterior pelvic tilt position during leg-lowering exercises. The third purpose of this study was to assess the effectiveness of training the trunk flexors in the ability to perform the DLLT. In previous studies, Hemborg et al[28] and Smidt et al[29] documented that the trunk flexors exhibit a significant positive gain in strength following progressive resistive exercises. We hypothesized that a measurable improvement in the subjects' ability to perform DLLT would be achieved by performing 5 to 10 minutes of daily trunk flexor exercises for 3 weeks.

Our study also examined whether changes in trunk flexor muscle performance, as measured by the DLLT, would result in a change in walking and running economy. Kottke and Kubicek[10] described how the tilt of the pelvis affects the position of the center of gravity of the superincumbent body mass--the head, arms, and trunk. They specifically described that excessive forward or anterior tilting of the pelvis during walking may result in the displacement of the center of mass of the head, arms, and trunk anterior to the axis of motion at the hip. Consequently, this displacement will cause the hip to move into flexion if the extensor musculature of the trunk and hip do not contract to maintain the center of mass of the head, arms, and trunk posterior to the axis of motion of the hip. Perhaps training the trunk flexors to stabilize the pelvis in a posterior pelvic tilt would allow the center of mass of the head, arms, and trunk to remain in a position that would lessen the muscular work load of the trunk and hip extensors during walking and running. If the muscular work load is reduced, this would be reflected in a decrease in oxygen consumption required to walk or run at a specified velocity, thereby improving gait economy. Thus, the final purpose of this study was to examine the effects of changes in trunk flexor muscle performance on gait economy.

Method

Subjects

Twenty-five healthy, athletic, male college students (X [bar] age=21 years, SD = 1, r-ange = 19-24; X [bar] weight = 75 kg, SD=6, range=63-88; X [bar] height= 178 cm, SD=5, range=169-185) participated in the study. The testing and intervention protocols used in this study closely matched the protocols used in our previous investigation.[4] The submaximal treadmill gait velocities used in both of these studies required the subjects to be in the upper 20% of cardiorespiratory fitness for a gender- and age-matched population.[30] To enhance the possibility of fulfilling this criterion, the subjects were recruited from the population of students taking physical education and sports medicine courses at Pepperdine University (Malibu, Calif). All subjects actively participated in some form of physical activity as recreational athletes. This participation typically involved activities such as intramural and recreational basketball, volleyball, and racquetball or tennis, as well as regular running, bicycling, and weight training for maintenance of physical fitness. All subjects were free of known orthopedic dysfunction that might influence their tolerance to (1) walking and running on a treadmill and (2) connective tissue stretching procedures.

Another criterion for admission to the study was the presence of limited hip extension ROM as measured by a modified Thomas test. This test required the subject to lie in a supine position with the thighs extended past the end of the table. The subject brought both knees to the chest, then released one leg, while tightly holding the other knee to the chest to maintain the lumbar spine flat against the table. This clinical examination procedure is described by both Jull and Jandal[11] and Kendall and McCreary.[14] If the released leg was short of full hip extension (< 0 [degrees] ), the subject was confirmed as having limited hip extension ROM and included in the study. Both legs were tested. if either leg had full hip extension ROM ([is greater than or equal to] > 0 [degrees] ), the subject was excluded from the study. With this screening procedure, 25 of the 30 individuals tested met the criteria to be admitted to the study.

The modified Thomas test was used as an easily administered, initial test for screening-potential subjects to determine the presence of hip extension ROM limitations. This testing procedure, however, did not control for any possible compensatory movements of anterior pelvic tilt, hip abduction, or knee extension that may be used, albeit unwittingly, by the subject to enable the released leg to reach the table. Therefore, during the goniometric measurement of hip extension ROM, which controlled for these compensatory motions, any subject who demonstrated full hip extension ROM ([is greater than or equal to] 0 [degrees]) was excluded from the study. All of the subjects who tested positive for limited hip extension ROM according to the modified Thomas test procedure were also determined, on the basis of goniometric measurement, to have less than 0 degrees of hip extension ROM for both legs.

Prior to participation in the study, all subjects reviewed and signed an approved informed consent statement. The subjects were informed that their participation in the study required one training session on a treadmill as well as attendance at two other laboratory sessions scheduled 3 weeks apart, but at the same time of day. Subjects were instructed to maintain their physical activity level over the course of the study and to refrain from exercise on the days of testing.

Procedure

During both the preintervention and postintervention testing sessions, all measurements were taken in the same order, with the same investigators performing the same measurements throughout the study. The measurements were performed in the following order: walking economy, running economy, trunk flexor muscle performance, right hip extension ROM, and left hip extension ROM. Thus, the 16-minute treadmill walking and running measurement protocol was performed first to control for possible exercise-induced warm-up effects on flexibility[5] or strength.[31] Following the gait economy measures, trunk flexor muscle performance, right hip extension ROM, and left hip extension ROM were measured (trial 1). After a 1-minute rest period, trunk flexor muscle performance, right hip extension ROM, and left hip extension ROM measurements were repeated (trial 2). After the preintervention measurements, the subjects were randomly assigned to one of three intervention groups: a trunk flexor strengthening group (n=9), a passive hip extension stretching group (n=9), or a control group (n=7). All data, were collected in the Motor Behavior and Human Performance Laboratories at Pepperdine University (Malibu, Calif).

Measurement of gait economy.

Steady-state submaximal oxygen consumption was determined via open-circuit spirometry(*) using a treadmill exercise test. Expired gas analyzers were calibrated with gases of known concentrations prior to each individual submaximal test. The subjects walked for 8 minutes at 108 m/min (4 mph). Following the initial 8 minutes of walking, the speed was increased from 108 m/min to 200 m/min (7.5 mph) over a 2-minute time period. The subjects then ran for 6 minutes at 200 m/min. Oxygen consumption was measured during the last 4 minutes of the walking and the running periods in order to obtain measurements of steady-state oxygen consumption. The walking and running speeds used in this study correspond to speeds that closely approximated 40% and 80% of maximal oxygen consumption (Vo.sub.2]max) in a sample of male, recreational athletes (N=7, X [bar] age=20 years, X [bar] weight=75 kg) who served as subjects in a previous study of the authors.[4] The subjects in the previous study viewed the 40% of [Vo.sub.2]max speed (108 m/min) as being a familiar brisk walking speed and the 80% of [Vo.sub.2]max speed (188-228 m/min) as being a familiar running speed. Also, in the previous study, there was an improvement in gait economy at the 40% and 80% of [Vo.sub.2]max speeds immediately following one session of passive hip stretching.[4] To provide testing consistency and eliminate the need for performing a [Vo.sub.2]max graded exercise test, the 200-m/min speed was chosen to closely approximate the 80% of [Vo.sub.2]max speed for the subjects in this study. Reliability for [Vo.sub.2]max measurements obtained during treadmill graded exercise testing, which involves running at progressively greater physiologically demanding speeds and incline grades to the limit of exhaustion, has been reported to be high (r=.95).[22] In a study examining the reliability of gait economy measurements at submaximal work loads across 4 days, no significant differences in gait economy were found and the authors reported a variation in oxygen consumption measurements of less than 1% across the three test sessions.[20] Several investigators,[17,20,23] however, have suggested that an "accommodation" or "familiarization" training session on the treadmill should be performed at the pace that the gait economy measurements are to be determined in order to maximize the reliability of the gait economy measurements. Therefore, 1 week prior to the preintervention testing session, all subjects in this study participated in a familiarization treadmill exercise session using the same protocol as outlined previously.

Measurement of trunk flexor muscle performance. The leg-lowering test, described by Kendall and McCreary[14] and by Walker et al,[27] was used. Each subject removed his shoes prior to this test. The subject lay supine on an unpadded wooden table while an examiner elevated the fully extended legs to the point at which the sacrum began to lift off the table, but not past 90 degrees of hip flexion. A second examiner read standardized instructions to the subjects. These instructions emphasized the importance of keeping the lower back pressed against the table as the subjects lowered their fully extended legs to the table over a 10-second period. The same examiner read the standardized instructions to each of the subjects immediately prior to their performance of each test. A third examiner (JJG) positioned the axis of a standard goniometer over the subject's left hip. His right hand kept the stationary arm of the goniometer parallel to the tabletop, and a light manual contact of his left hand kept the movable arm of the goniometer aligned with the lowering femur. This examiner kept his line of sight at the level of the interface between the tabletop and the subject's lower back to detect the instant that the lower back began to lift off the table (Fig. 1).

The end point of the test was determined by the examiner visually sighting when the lower back began to lift off the table. The subject was not made aware of this instant and continued to lower his legs to the table as instructed. The third examiner then handed the goniometer to a fourth examiner. The fourth examiner then read the goniometer and recorded the angle. To reduce any potential examiner bias by the third examiner while performing the goniometric measurement and to eliminate any performance feedback for the subject, the fourth examiner did not communicate the results of the goniometric reading to the other examiners or to the subject. The measurement dial of the goniometer was not blinded in this study, in contrast to the study by Walker and associates.[27] We believed that this was an unnecessary step for reducing experimenter bias because (1) it was necessary for the third examiner to keep his line of vision entirely focused on the subject's lower back and (2) the third examiner was unaware of the measurements taken during the previous tests.

Measurement of hip extension range of motion. Hip extension ROM was evaluated with a modified Thomas test as outlined by Godges et al.[4] Leg braces, locked at 90 degrees of knee flexion, were placed on both extremities. One investigator flexed the subjects' hips so that the lumbar spine was flat against the tabletop. For consistency, the flexion angle of the contralateral hip was measured initially by a second experimenter and remained constant throughout all subsequent measurements. The contralateral thigh was maintained in this flexed position while the ipsilateral thigh was lowered toward the table (an investigator prevented hip abduction or adduction of the thigh). The subject was instructed to relax and allow gravity to lower the leg toward the floor. The third investigator performed the goniometric measurements. The stationary arm of the goniometer was aligned parallel to the tabletop, and the movable arm of the goniometer was aligned with the lowering femur. The measurements were taken at the instant the thigh stopped moving into extension in response to the gravitational force Fig. 2). We estimate that each goniometric measurement of hip extension ROM was completed within 5 seconds following the onset of the gravitational pull of the thigh toward the floor.

Intervention protocol. The trunk flexor exercise group followed a leg-lowering exercise progression. These exercises are one form of supine leg-lowering exercises commonly used to improve the "strength" of the abdominal muscles. They require the subject to maintain a posteriorly tilted pelvic orientation, thus reducing the lumbar lordosis and flattening of the lumbar curve against the supporting surface. The initial exercises are performed with the hips and knees in a flexed position (> 90 [degrees]) to mechanically lessen the muscular force required of the trunk flexors to maintain the pelvis in the posteriorly tilted position. The exercises progress in difficulty by lessening the flexion angles at the hips and knees of one or both legs. During this progression, strict maintenance of the pelvis in the posteriorly tilted position provides a training stimulus for improving muscle performance of the trunk flexors.[29] The initial instructions to the subjects, to perform the exercise routine for 5 minutes, twice daily for 3 weeks, were given by the same experimenter. Specific emphasis was placed on proper performance of the exercises and on maintaining abdominal control of the posterior pelvic tilt during the leg-lowering sequence. The subjects were monitored two times per week, and their performance and progression of the exercises was evaluated and, if necessary, modified to ensure correct execution of the exercises. To obtain a compliance measure, the exact frequency of each subject's participation was also recorded.

The passive hip extension stretching exercise group received the static stretching applied at the end of available hip extension ROM as outlined by Godges et al.[4] The stretching procedure used the same subject positioning that was used for the hip extension ROM measurements. A stretching force of 15% of the subject's body weight was applied using weights suspended through a strap around the subject's midthigh. The stretching force was maintained for 2 minutes. The 2-minute stretching period was followed by a 2-minute period in which the stretching force was removed and the leg was placed in a position of hip flexion while the contralateral thigh received the stretch. This cycle was repeated three times, resulting in a total stretching time of 6 minutes for each thigh, interspersed by two, 2-minute periods of no stretch. The subjects underwent this stretching routine two times per week for a total of six stretching sessions. There was at least 48 hours between stretching periods.

The intensity (15% of the subject's body weight applied to the midthigh), duration (three, 2-minute stretching periods), and type (static) of stretching force outlined were used for two reasons. First, in a previous investigation,[4] these variables produced a strong, but tolerable, stretching sensation to the anterior hip and thigh. Second, utilization of this stretching method resulted in improvement in hip extension ROM when measurements were obtained immediately following the treatment session. The frequency of the stretching routine, two times per week, was chosen based on the clinical observations made by one of the investigators (JJG). We note that patients with joint ROM restrictions are able to demonstrate progressive improvements in joint ROM when passive stretching treatments are administered two times per week.

Subjects in the control group, as well as those in both experimental groups, were instructed to continue their current activity levels throughout the 3 weeks of the study. They were specifically instructed not to increase their exercise beyond their current levels and not to begin a new strengthening or stretching program in addition to the intervention programs outlined.

Postintervention measurements. Twenty-four to forty-eight hours following the last passive hip extension stretching procedure or trunk flexor exercise monitoring period, postintervention measurements were taken. The control group was measured 3 weeks following the initial measurement. The order of the measurements and all procedures were the same as during the preintervention data-collection periods.

Data Analysis

To examine intratester reliability of the measurements of trunk flexor muscle performance, right hip extension ROM, and left hip extension ROM, intraclass correlation coefficients (ICC[1,21])[32] were calculated between trial 1 and trial 2 on all preintervention and postintervention measures. The ICCs between trials 1 and 2 for preintervention and postintervention measures were .86 and .95 for trunk flexor muscle performance, .80 and .88 for right hip extension ROM, and .95 and .71 for left hip extension ROM. These high intraclass correlations indicated good intratester reliability from trial 1 to trial 2.[33] All further analyses were performed on the means of the trial 1 and trial 2 measurements.

Descriptive statistics were performed on all dependent measures. A three X two-way (groups X test sessions) analysis of variance (ANOVA) for repeated measures for unequal subject numbers was performed on each of the five dependent variables. These dependent measures were right hip extension ROM, left hip extension ROM, trunk flexor muscle performance, walking economy, and running economy. A significance level of .05 was chosen, but a Bonferroni test33 was used to prevent inflated Type I errors. According to this adjustment, a probability level of .01 would indicate significant differences. Therefore, if the ANOVA indicated that the group X test session interaction was significant (P<.01), a simple main effects test was performed to determine which group or groups differed significantly.[33]

Results

Knowledge of the level of compliance with an intervention program is essential in determining its effectiveness. In this investigation, all the subjects in the passive hip extension stretching exercise group completed the six stretching sessions as scheduled. The subjects in the trunk flexor exercise group were instructed to perform the specific exercise regimen for 5 minutes, two times per day for 21 days, resulting in a total of 42 exercise sessions. The average number of trunk flexor sessions completed per subject was 32 (76%), with a range of 23 to 39 sessions.

The means, standard deviations, and ranges for each of the five dependent measures are shown in Table 1. A significant groups X test sessions interaction was found for hip extension ROM of the right leg (F[1,22] = 12.69, P<.01) (Tab. 2) and the left leg F[1,22] 12.80, P<.01) (Tab. 3). The analysis of simple main effects revealed that the hip extension stretching group significantly increased right hip extension ROM (F[1,22]=26.89, P<.01) and left hip extension ROM F[1,221 =35.43, P<.01) from pretest to posttest. The hip extension stretching group increased their right hip extension ROM by 12.1 degrees and their left hip extension ROM by 9.8 degrees (Tab. 1). The other two groups demonstrated no significant change in hip extension ROM from pretest to posttest. [TABULAR DATA 1, 2 and 3 OMITTED]

Another significant groups X test sessions interaction was found for trunk flexor muscle performance (F[1,221=5.28, P<.01) (Tab. 4). The analysis of simple main effects revealed that the trunk flexor exercise group significantly improved their trunk flexor muscle performance (F[1,22] = 19.18, P <.01) over the 3-week exercise program. The trunk flexor exercise group improved their trunk flexor muscle performance by 18.9 degrees (41.5 [degrees]-60.4 degrees) from the pretest to the posttest, whereas the other two groups showed no significant changes in trunk flexor muscle performance (Tab. 1). The ANOVA performed on the dependent measures of walking and running economy revealed no significant changes for any of the three groups from preintervention to postintervention (Tabs. 1, 5, 6). [TABULAR DATA 4, 5 and 6 OMITTED]

Discussion

Significant improvements in hip ROM following a repeated series of passive stretching exercises are consistent with the findings of previous studies.[1,3,6,8] These studies, however, improved the range of hip flexion as measured with passive straight-leg-raising tests. This improvement in straight-leg-raising ROM is presumably related to an increase in the length of the hamstring musculature.[1,3,6,8] In contrast, our investigation demonstrated the ability to improve hip extension ROM. It can be hypothesized that this improvement in hip extension ROM is related to an elongation of the hip flexor muscles. Improved mobility of the hip capsules and ligaments, sacroiliac periarticular structures, or femoral perineural structures, however, may be equal, if not more important, factors related to the limitation and improvement of hip extension ROM. The data collected in this study do not provide information related to which anatomical structure underwent a change. We therefore were not able to determine which tissue was affected by the passive stretching. Achieving normal hip extension ROM is often a focus of physical therapy intervention. Of immediate clinical importance is that full hip extension is required during the terminal stance phase of gait.[12,13] A common clinical observation is that limitations in hip extension ROM result in compensations at segments above and below the hip during gait. For example, individuals with limited hip extension ROM will commonly compensate for the lack of hip extension mobility with excessive lumbar extension or excessive ankle plantar flexion (early heel rise) during the terminal stance phase of gait.

The frequency and duration of the stretch were quite different in this study compared with previous studies. Substantial gains in hip extension ROM were found with six treatment sessions spread across a 21-day period using the intensity and duration of sustained passive stretch outlined. We believe these results suggest that, when stretching for improving ROM, the intensity, duration, and frequency utilized in this investigation were sufficient to obtain improvements in hip ROM. Further research is needed to provide more precise information on the variables of frequency, intensity, duration, and type of stretching necessary to obtain optimal improvements in ROM.

Another significant result of this study was the demonstrated improvement in trunk flexor muscle performance, as measured by the DLLT, by the group of subjects who performed the trunk flexor exercises. This finding is consistent with previous reports in the literature documenting improvement in trunk flexor muscle performance following resistive training in asymptomatic subjects.[28,29] Supine leg-lowering exercises, performed with an emphasis on stabilization of the pelvis by the trunk flexors, are commonly used in the clinical treatment of patients with low back pain who are determined to have less-than-optimal trunk flexor musculature strength and control. Further research is needed, however, to ascertain whether these trunk flexor exercises are (1) effective in improving trunk flexor muscle performance in a sample of patients with low back pain and (2) beneficial in treating the symptomatology and disability in patients with low back pain syndromes.

The measurable improvements in hip extension ROM in the stretching group were not associated with corresponding improvements in walking or running gait economy. One possible explanation for this finding is that not enough change in hip ROM was elicited. In the previous study by Godges et al,[4] 40 minutes of sustained stretching, which improved both hip flexion and hip extension ROMs, was associated with an improved economy of walking and running. Perhaps changes in gait economy would be found if the intervention program involved a longer duration (ie, 20 minutes) of stretching for each leg, more frequent (ie, daily) stretching, or a longer time period (ie, 12 weeks). In addition, emphasis may need to be placed on hamstring muscle stretching, or a combination or hamstring muscle stretching and hip extension stretching, to achieve the improvements in gait economy previously reported.[4]

The measurable improvement in trunk flexor muscle performance also was not associated with improved gait economy. As with hip ROM, the efficacy of a more intensive or prolonged exercise protocol, or a combined trunk flexor and extensor strengthening exercise protocol on functional tasks such as walking, are future research questions. The efficacy of combining stretching regimens with strengthening exercises on functional tasks is another area for future research.

Another possible explanation for this lack of change in gait economy may be that the isolated and static measures of flexibility and muscle performance in the supine position are not highly correlated with the multi-segmental, dynamic activities of walking and running. It can be argued that an element of coaching (ie, verbal or kinesthetic cueing) may be required to assist the subjects in integrating the isolated improvements in strength or flexibility into a more efficient and coordinated motor program. It may be erroneous for physical therapists and athletic coaches to assume that the training effects achieved in the supine position will have carryover (transfer) to associated motor tasks performed in upright, weight-bearing positions. For example, it cannot be assumed that the improved hip flexibility or trunk strength achieved through exercises performed in a supine position will effect a beneficial change in the performance of the hip and trunk muscles during walking and running.

Conclusions

Six treatment sessions of passive stretching performed over a 3-week period were sufficient to improve hip extension ROM in healthy, athletic, male subjects. These improvements in hip extension ROM were achieved with a relatively infrequent stretching regimen (two sessions per week), which consisted of 6 minutes of end-range stretching per session.

Three weeks of daily (5-10 minutes per day) performance of supine leg-lowering exercises by healthy, athletic, male subjects improved the performance of their trunk flexor muscles, as measured by the DLLT.

Improvements in hip extension ROM or trunk flexor muscle performance were not associated with improvements in walking and running economy. The clinical implication of these results is that the training of an isolated task, such as hip flexibility or trunk strengthening, in a horizontal, non-weight-bearing position may not produce the desired outcome in an associated task performed in a vertical, weight-bearing position. Achieving carryover to walking and running may require (1) a greater change in hip ROM or trunk muscle performance than what was achieved in this 3-week intervention program or (2) additional cueing or specific training to integrate improvements in performance of isolated tasks into a more complex motor program.

Acknowledgments

We acknowledge the assistance of Carrie Faehling Nelson, Christie L Powers, and Christine M Tinberg during the intervention and data-collection phases of this project. (*)Vista metabolic measurements cart, Vacumetrics Inc, 5770 Nicolle St, Ventura, CA 930003.

References

[1] Bohannon RW. Effect of repeated eight-minute muscle loading on the angle of straight-leg raising. Phys Ther 1984;64:491-497. [2] Borms J, Roy PV, Santens JP, Haentjens A. Optimal duration of static stretching exercises for improvement of coxo-femoral flexibility. J Sports Sci. 1987;5:39-47. [3] Gajdosik RL. Effects of static stretching on the length and resistance to passive stretch of short hamstring muscles. Journal of Orthopaedic and Sports Physical Therapy. 1991; 14: 250-255. [4] Godges JJ, MacRae H, Longdon C, et al. The effects of two stretching procedures on hip range of motion and gait economy. Journal of Orthopaedic and Sports Physical Therapy 1989;10:350-357. [5] Henricson AS, Fredriksson K, Persson I, et al. The effect of heat and stretching on the range of hip motion. Journal of Orthopaedic and Sports Physical Therapy. 1984;6:110-115. [6] Medeiros JM, Smidt GL, Burmeister LF, Soderberg GL. The influence of isometric exercise and passive stretch on hip joint motion. Phys Ther. 1977;57:518-523. [7] Moore MA, Hutton RS. Electromyographic investigation of muscle stretching techniques. Med Sci Sports Exerc. 1980;12:322-329. [8] Starring DT, Gossman MR, Nicholson GG, Lemons J. Comparison of cyclic and sustained passive stretching using a mechanical device to increase resting length of hamstring muscles. Phys Ther. 1988;68:314-320. [9] Tanigawa MC. Comparison of the hold-relax procedure and passive mobilization on increasing muscle length. Phys Ther. 1972;52: 725-735. [10] Kottke FJ, Kubicek WG. Relationship of the tilt of the pelvis to stable posture. Arch Phys Med Rehabil. 1956;37:81-89. [11] Jull GA, Janda V. Muscles and motor control in low back pain: assessment and management. In: Twomey LT, Taylor JR, eds. Physical Therapy of the Low Back New York, NY: Churchill Livingstone Inc; 1987:260-272. [12] Cerny K. Pathomechanics of stance: clinical concepts for analysis. Phys Ther. 1984;64:1851-1859. [13] Perry J. Gait Analysis: Normal and Pathological Function. Thorofare, NJ: SLACK Inc; 1992:245-254. [14] Kendall FP, McCreary EK. Muscles: Testing and Function. Baltimore, Md: Williams & Wilkins; 1983:160-163, 213-219. [15] Kottke FJ, Pauley DL, Ptak RA. The rationale for prolonged stretching for correction of connective tissure. Arch Phys Med Rehabil. 1966; 47:345-352. [16] Moller M, Ekstrand J, Oberg B, Gillquist J. Duration of stretching effects on range of motion in lower extremities. Arch Phys Med Rehabil. 1985;66:171-173. [17] Cavanagh PR, Williams KR. The effect of stride length variation on oxygen uptake during distance running. Med Sci Sports Exerc. 1982;14:30-35. [18] Shields SL. The effect of varying lengths of stride on performance during submaximal treadmill stress testing. J Sports Med Phys Fitness. 1982;22:66-72. [19] Daniels JT. A physiologist's view of running economy. Med Sci Sports Exerc. 1985;17:332-338. [20] Morgan DW, Martin PE, Baldini FD, Krahenbuhl GS. Effects of a prolonged maximal run on running economy and running mechanics. Med Sci Sports Exerc. 1990;22:834-840. [21] Conley DL, Krahenbuhl GS. Running economy and distance running performance of highly trained athletes. Med Sci Sports Exerc. 1980;12:357-360. [22] MacDougall JD, Wenger HA, Green HJ. Physiological Testing of the High-Performance Athlete. Champaign, Ill: Human Kinetics Publishers Inc; 1991:146. [23] Schieb D. Kinematic accommodation of novice treadmill runners. Res Q Exerc Sport. 1986;57:1-7. [24] Biering-Sorensen F. Physical measure as risk indicators for low back trouble over a one-year period. Spine. 1984;9:106-119. [25] Heino JG, Godges JJ, Carter CL. Relationship between hip extension range of motion and postural alignment. Journal of Orthopaedic and Sports Physical Therapy. 1990; 12: 257-261. [26] Smidt GL, Blanpied PR, Anderson MA, White RW. Comparison of clinical and objective methods of assessing trunk muscle strength: an experimental approach. Spine. 1987;12:1020-1024. [27] Walker ML, Rothstein JM, Finucane SD, Lamb RL. Relationships between lumbar lordosis, pelvic tilt, and abdominal performance. Phys Ther. 1987;67:512-516. [28] Hemborg B, Moritz U, Hamborg J, et al. Intra-abdominal pressure and trunk muscle activity during lifting: effect of abdominal muscle training in healthy subjects. Scand J Rehabil Med 1986;15:183-196. [29] Smidt GL, O'Dwyer KD, Lin SY, Blanpied PR. The effect of trunk resistive exercise on muscle strength in postmenopausal women. Journal of Orthopaedic and Sports Physical Therapy. 1991;13:300-309. [30] Astrand I. Work capacity in men and women with special reference to age. Acta Physiol Scand 1960;49(suppl 169). [31] Bergh U, Ekblom B. Physical performance and peak aerobic power at different body temperatures. J Appl Physiol. 1979;46:885-892. [32] Shrout PE, Fleiss J. Intraclass correlations: uses in assessing rater reliability. Psychol Bull. 1979;86:420-428, [33] Keppel G. Design and Analysis: A. Researcher's Handbook. Englewood Cliffs, NJ: Prentice-Hall; 1982:146-149, 165, 302-333.