Can apparent increases in muscle extensibility with regular stretch be explained by changes in tolerance to stretch?

The aim of this study was to determine whether an intensive stretch program increases muscle extensibility or subjects' tolerance to an uncomfortable stretch sensation. Twenty healthy able-bodied individuals with limited hamstring muscle extensibility were recruited. A within-subjects design was used whereby one leg of each subject was randomly allocated to the experimental condition and the other leg was allocated to the control condition. The hamstring muscles of each subject's experimental leg were stretched for 20 minutes each weekday for four weeks. Hamstring muscle extensibility (angle of hip flexion corresponding with a standardised torque) and stretch tolerance (angle of hip flexion corresponding with maximal torque tolerated) were assessed on both legs at the beginning and end of the study. The intervention did not increase the extensibility of the hamstring muscles (mean change in hip flexion was -1 degree, 95% CI -4 to 3 degrees) but did increase subjects' tolerance to an uncomfortable stretch sensation (mean change in hip flexion was 8 degrees, 95% CI 5 to 12 degrees). These results highlight the importance of distinguishing between real and apparent increases in muscle extensibility when assessing the effectiveness of stretch, and indicate that whilst a four-week stretch program increases subjects' tolerance to an uncomfortable stretch sensation it does not increase hamstring muscle extensibility.     

Randomised trial of the effects of four weeks of daily stretch on extensibility of hamstring muscles in people with spinal cord injuries

The aim of this assessor-blind randomised controlled trial was to determine the effect of four weeks of 30 minute stretches each weekday on extensibility of the hamstring muscles in people with recent spinal cord injuries. A consecutive sample of 16 spinal cord-injured patients with no or minimal voluntary motor power in the lower limbs and insufficient hamstring muscle extensibility to enable optimal long sitting were recruited. Subjects' legs were randomly allocated to experimental and control conditions. The hamstring muscles of the experimental leg of each subject were stretched with a 30 Nm torque at the hip for 30 minutes each weekday for four weeks. The hamstring muscles of the contralateral leg were not stretched during this period. Extensibility of the hamstring muscles (hip flexion range of motion with knee extended, measured with a 48 Nm torque at the hip) of both legs was measured by a blinded assessor at the commencement of the study and one day after the completion of the four-week stretch period. Changes in hamstring muscle extensibility from initial to final measurements were calculated. The effect of stretching was expressed as the mean difference in these changes between stretched and non-stretched legs. The mean effect of stretching was 1 degree (95% CI -2 to 5 degrees). Four weeks of 30 minute stretches each weekday does not affect the extensibility of the hamstring muscle in people with spinal cord injuries.     

Immediate effect of passive and active stretching on hamstrings flexibility: a single-blinded randomized control trial

[Purpose] This study compared the efficacy of passive and active stretching techniques on hamstring flexibility. [Subjects] Fifty-four healthy young subjects were randomly assigned to one of three groups (2 treatment groups and 1 control group). [Methods] Subjects in the passive stretching group had their knees extended by an examiner while lying supine 90° of hip flexion. In the same position, subjects in the active stretching group extended their knees. The groups performed 3 sets of the assigned stretch, with each stretch held for 10 seconds at the point where tightness in the hamstring muscles was felt. Subjects in the control group did not perform stretching. Before and immediately after stretching, hamstring flexibility was assessed by a blinded assessor, using the active knee-extension test. [Results] After stretching, there was a significant improvement in the hamstring flexibilities of the active and passive stretching groups compared with the control group. Furthermore, the passive stretching group showed significantly greater improvement in hamstring flexibility than the active stretching group. [Conclusion] Improvement in hamstring flexibility measured by the active knee-extension test was achieved by both stretching techniques; however, passive stretching was more effective than active stretching at achieving an immediate increase in hamstring flexibility.Key words: Hamstring flexibility, Active stretching, Passive stretching     

Effect of modified hold-relax stretching and static stretching on hamstring muscle flexibility

[Purpose] The aim of present study was to compare the effectiveness of modified hold-relax stretching and static stretching in improving the hamstring muscle flexibility. [Subjects and Methods] Forty-five male subjects with hamstring tightness were included in this study. The subjects were randomly placed into three groups: the modified hold-relax stretching, static stretching and control groups. The modified hold-relax stretching group performed 7 seconds of isometric contraction and then relaxed for 5 seconds, and this was repeated five times daily for five consecutive days. The static stretching group received 10 minutes of static stretching with the help of a pulley and weight system for five consecutive days. The control group received only moist heat for 20 minutes for five consecutive days. A baseline reading of passive knee extension (PKE) was taken prior to the intervention; rest measurements were taken immediate post intervention on day 1, day 3, day 5, and after a 1 week follow-up, i.e., at the 12th day. [Results] On comparing the baseline readings of passive knee extension (PKE), there was no difference noted between the three groups. On comparing the posttest readings on day 5 between the 3 groups, a significant difference was noted. However, post hoc analysis revealed an insignificant difference between the modified hold-relax stretching and static stretching groups. There was a significant difference between the static stretching and control groups and between the modified hold-relax stretching and control groups. [Conclusion] The results of this study indicate that both the modified hold-relax stretching technique and static stretching are equally effective, as there was no significant difference in improving the hamstring muscle flexibility between the two groups.Key words: Modified hold-relax stretching technique, Static stretching, Flexibility     

The acute effects of different hamstring muscle stretching techniques on reducing muscle resistance to movement by ultrasound elastography

ObjectiveMany stretching methods are applied on the hamstring muscles for reducing muscle resistance on movement. The aim of our study to investigate the acute effects of static stretching and the Mulligan traction straight leg raise (TSLR) technique administered to the hamstring muscles on reducing its resistance to movement.MethodsThe study included 22 healthy male volunteers (19.5 ± 0.98 years). After the individuals were randomly divided into two groups, static stretching was administered to the hamstring muscles in the first group and the Mulligan TSLR technique was administered on the same muscle in the second group. Active knee extension angles of the individuals were evaluated by a digital goniometer and muscle strain was assessed by ultrasound elastography.ResultsWhile there was a significant difference in all parameters in both groups after the treatment, the changes in the active knee extension angle and the strain index value of the biceps femoris musculotendinous junction were higher in the Mulligan TSLR group compared to the static stretching group (p < 0.05).ConclusionIt was concluded that, static stretching and the Mulligan TSLR technique administered to the hamstring muscles are effective ways to increase the range of motion. The Mulligan TSLR technique, however, can be opted in healthy individuals and patients since it is painless and more effective in reducing muscle resistance to movement and range of motion.     

Hamstring muscle strain treated by mobilizing the sacroiliac joint

The purpose of this study was to compare the effectiveness of two types of treatment of hamstring muscle strains. Twenty patients with hamstring muscle strains were assigned randomly to an Experimental Group (n = 10) or a Control Group (n = 10). Peak torque production of the quadriceps femoris and hamstring muscles and hamstring muscle length were measured before and after treatment. The hamstring muscles of the Experimental and Control groups were treated with moist heat followed by passive stretching. The Experimental Group also received manipulation of the sacroiliac joint. The change in hamstring muscle peak torque was significantly greater for the Experimental Group than for the Control Group (p less than .005). No significant differences existed between the two groups in either quadriceps femoris muscle peak torque or hamstring muscle length. The results of this study suggest a relationship between sacroiliac joint dysfunction and hamstring muscle strain.     

Effect of superficial heat, deep heat, and active exercise warm-up on the extensibility of the plantar flexors

BACKGROUND AND PURPOSE: Warm-up prior to static stretching enhances muscle extensibility. The relative effectiveness of different modes of warm-up, however, is unknown. The purpose of this study was to evaluate the effectiveness of superficial heat, deep heat, and active exercise warm-up prior to stretching compared with stretching alone on the extensibility of the plantar-flexor muscles. SUBJECTS: Ninety-seven subjects (59 women, 38 men) with limited dorsiflexion range of motion (ROM) were randomly assigned to 1 of 5 groups. Female subjects had a mean age of 27.6 years (SD=7.68, range=17-50), and male subjects had a mean age of 26.8 years (SD=6.87, range=18-48). METHODS: The first group (group 1) was a control group and did not perform the stretching protocol. The 4 experimental groups (groups 2-5) performed a stretching protocol 3 days per week for 6 weeks. Group 2 performed the static stretching protocol only; group 3 performed active heel raises before stretching; group 4 received 15 minutes of superficial, moist heat to the plantar-flexor muscles before stretching; and group 5 received continuous ultrasound for 7 minutes before stretching. Dorsiflexion ROM measurements were taken initially and after 2, 4, and 6 weeks. RESULTS: All experimental groups increased active and passive range of motion (AROM and PROM). The mean AROM/PROM differences at 6 weeks were 1.11/1.39 degrees for group 1, 4.10/6.11 degrees for group 2, 4.16/4.21 degrees for group 3, 4.38/4.90 degrees for group 4, and 6.20/7.35 degrees for group 5. The group receiving ultrasound before performing the stretching protocol (group 5) displayed the greatest increase in both AROM (6.20() and PROM (7.35(). DISCUSSION AND CONCLUSION: Among the modalities tested, the use of ultrasound for 7 minutes prior to stretching may be the most effective for increasing ankle dorsiflexion ROM.     

Mechanisms of increased passive compliance of hamstring muscle-tendon units after spinal cord injury

Background

People with spinal cord injury sometimes develop abnormally compliant hamstring muscle-tendon units. This study investigated whether the increased muscle-tendon compliance is due to a change in the passive properties of the muscle fascicles or tendons, or to muscle tears.

Methods

Semimembranosus muscle fascicle lengths were measured from ultrasound images obtained from 15 spinal cord injured subjects and 20 control subjects while the hip was passively flexed with the knee extended. Semimembranosus muscles of spinal cord injured subjects were inspected for tears using ultrasound imaging.

Findings

The mean (SD) hip angle at 30 Nm was 97 (SD 24) degrees in spinal cord injured subjects and 70 (SD 11) degrees in control subjects, indicating that spinal cord injured subjects had very compliant hamstring muscle-tendon units. The ratio of change in fascicle length to change in muscle-tendon length was not statistically different between spinal cord injured subjects and control subjects: muscle fascicles lengthened by 0.30 (SD 0.24) mm/mm in spinal cord injured subjects and 0.42 (SD 0.29) mm/mm in control subjects. These data were used to show that there was evidence of increased tendon compliance of spinal cord injured subjects compared to control subjects, but no evidence of increased muscle fascicle compliance. No tears were observed in semimembranosus muscles of spinal cord injured subjects.

Interpretation

The increased hamstring muscle-tendon compliance apparent in some spinal cord injured subjects is due, at least in part, to increased tendon compliance. There was no evidence that the increased muscle-tendon compliance was due to muscle tears.     

Effect of stretching program in an industrial workplace on hamstring flexibility and sagittal spinal posture of adult women workers: A randomized controlled trial

Objectives: To determine the effect of a stretching program performed in the workplace on the hamstring muscle extensibility and sagittal spinal posture of adult women. Material and methods: Fifty-eight adult women volunteers (mean age of 44.23 ± 8.87 years) from a private fruit and vegetable company were randomly assigned to experimental (n=27) or control (n=31) groups. The experimental group performed three exercises of hamstrings stretching of 20 seconds per exercise, three sessions a week for a period of 12 weeks. The control group did not participate in any hamstring stretching program. Hamstring flexibility was evaluated through the passive straight leg raise test and toe-touch test, performed both before and after the stretching program. Thoracic and lumbar curvatures and pelvic inclination were measured in relaxed standing and toe-touch test with a Spinal Mouse. Results: Significant increases (p < 0.01) in toe-touch score and straight leg raise angle (in both legs) were found in the experimental group during post-test, while the control group showed a non-significant decrease for both toe-touch score and straight leg raise test. A significant decrease in thoracic curve and significant increase in pelvic inclination were found in the toe-touch test for the experimental group (p <0.05). However, no significant changes were found in standing posture for any group. Conclusions: Hamstring stretching exercises performed in the working place are effective for increasing hamstring muscle extensibility. This increase generates a more aligned thoracic curve and more anterior pelvic inclination when maximal trunk flexion is performed.     

Muscle activation during proprioceptive neuromuscular facilitation (PNF) stretching techniques

Proprioceptive neuromuscular facilitation (PNF) techniques are often used to induce muscle relaxation and increase joint range of motion (ROM). However, the relationship between muscle activation and ROM with PNF is not well understood. The purpose of this study was to investigate the effect of three common PNF stretching techniques on hamstring muscle activation and knee extension. Three PNF techniques: stretch-relax (SR), contract-relax (CR) and agonist contract-relax (ACR) were applied to ten male and female subjects aged 23-36 years who were stabilized to isolate knee extension measurements. Knee joint position and EMG activity from quadriceps and hamstring muscles were computer processed throughout technique application. The results revealed mean hamstring EMG activity increased 8-43% within a given trial of ACR and CR respectively, and did not diminish across trials. SR produced a 11% decrease in mean hamstring EMG activity. ACR produced 3-6% greater knee extension values than CR and SR respectively, in spite of 71-155% greater hamstring EMG activity during ACR. The data suggest that CR and ACR do not evoke sufficient relaxation in muscles opposing knee extension to overcome tension facilitation generated by stretch. Thus, increases in ROM are achieved while the hamstrings are under considerable tension. Such tension increases muscle vulnerability to soreness and strain if stretching continues. The degree of knee extension produced via SR, although 3-6% less than CR and ACR, was achieved during simultaneous reduction in hamstring activity and may be the safer stretching technique.     

Effects on Hamstring Muscle Extensibility,Muscle Activity,and Balance of Different Stretching Techniques

[Purpose] The purpose of this study was to investigate the effects of two different stretching techniques on range of motion (ROM), muscle activation, and balance. [Subjects] For the present study, 48 adults with hamstring muscle tightness were recruited and randomly divided into three groups: a static stretching group (n=16), a PNF stretching group (n=16), a control group (n=16). [Methods] Both of the stretching techniques were applied to the hamstring once. Active knee extension angle, muscle activation during maximum voluntary isometric contraction (MVC), and static balance were measured before and after the application of each stretching technique. [Results] Both the static stretching and the PNF stretching groups showed significant increases in knee extension angle compared to the control group. However, there were no significant differences in muscle activation or balance between the groups. [Conclusion] Static stretching and PNF stretching techniques improved ROM without decrease in muscle activation, but neither of them exerted statistically significant effects on balance.Key words: Balance, Muscle activation, Stretching     

The effect of duration of stretching of the hamstring muscle group for increasing range of motion in people aged 65 years or older

BACKGROUND AND PURPOSE: Stretching protocols for elderly people (> or = 65 years of age) have not been studied to determine the effectiveness of increasing range of motion (ROM). The purpose of this study was to determine which of 3 durations of stretches would produce and maintain the greatest gains in knee extension ROM with the femur held at 90 degrees of hip flexion in a group of elderly individuals. SUBJECTS: Sixty-two subjects (mean age = 84.7 years, SD = 5.6, range = 65-97) with tight hamstring muscles (defined as the inability to extend the knee to less than 20 degrees of knee flexion) participated. Subjects were recruited from a retirement housing complex and were independent in activities of daily living. METHODS: Subjects were randomly assigned to 1 of 4 groups and completed a physical activity questionnaire. The subjects in group 1 (n = 13, mean age = 85.1 years, SD = 6.4, range = 70-97), a control group, performed no stretching. The randomly selected right or left limb of subjects in group 2 (n = 17, mean age = 85.5 years, SD = 4.5, range = 80-93), group 3 (n = 15, mean age = 85.2 years, SD = 6.5, range = 65-92), and group 4 (n = 17, mean age = 83.2 years, SD = 4.6, range = 68-90) was stretched 5 times per week for 6 weeks for 15, 30, and 60 seconds, respectively. Range of motion was measured once a week for 10 weeks to determine the treatment and residual effects. Data were analyzed using a growth curve model. RESULTS: A 60-second stretch produced a greater rate of gains in ROM (60-second stretch = 2.4 degrees per week, 30-second stretch = 1.3 degrees per week, 15-second stretch = 0.6 degrees per week), which persisted longer than the gains in any other group (group 4 still had 5.4 degrees more ROM 4 weeks after treatment than at pretest as compared with 0.7 degrees and 0.8 degrees for groups 2 and 3, respectively). DISCUSSION AND CONCLUSION: Longer hold times during stretching of the hamstring muscles resulted in a greater rate of gains in ROM and a more sustained increase in ROM in elderly subjects. These results may differ from those of studies performed with younger populations because of age-related physiologic changes.     

Passive versus active stretching of hip flexor muscles in subjects with limited hip extension: a randomized clinical trial

BACKGROUND AND PURPOSE: Active stretching is purported to stretch the shortened muscle and simultaneously strengthen the antagonist muscle. The purpose of this study was to determine whether active and passive stretching results in a difference between groups at improving hip extension range of motion in patients with hip flexor muscle tightness. SUBJECTS AND METHODS: Thirty-three patients with low back pain and lower-extremity injuries who showed decreased range of motion, presumably due to hip flexor muscle tightness, completed the study. The subjects, who had a mean age of 23.6 years (SD = 5.3, range = 18-25), were randomly assigned to either an active home stretching group or a passive home stretching group. Hip extension range of motion was measured with the subjects in the modified Thomas test position at baseline and 3 and 6 weeks after the start of the study. RESULTS: Range of motion in both groups improved over time, but there were no differences between groups. DISCUSSION AND CONCLUSION: The results indicate that passive and active stretching are equally effective for increasing range of motion, presumably due to increased flexibility of tight hip flexor muscles. Whether the 2 methods equally improve flexibility of other muscle groups or whether active stretching improves the function of the antagonist muscles is not known. Active and passive stretching both appeared to increase the flexibility of tight hip flexor muscles in patients with musculoskeletal impairments.     

Effects of an eight-week stretching program on the passive-elastic properties and function of the calf muscles of older women

BACKGROUND: Calf muscle stretching programs are used to increase dorsiflexion range of motion at the ankle, yet the effects of the stretching programs on the passive properties of aged calf muscles and on standing and ambulatory function have not been studied. This initial study examined the effects of an eight-week stretching program on the passive-elastic properties of the calf muscles of older women and on selected functional activities. METHODS: Nineteen women aged 65-89 years with limited dorsiflexion range of motion first completed a timed agility course, a timed 10-m walk and a standing functional reach test. A dynamometer then moved the right ankle from plantarflexion to maximal dorsiflexion and back to plantarflexion at 5 deg s(-1) to measure calf muscle passive properties. The women were randomly assigned to a group that stretched three-times a week for eight-weeks (n=10) or to a control group (n=9) that did not. The tests were repeated after the stretching program. FINDINGS: The stretching group showed increased maximal dorsiflexion range of motion, passive resistive forces (Newtons [N]), and the absorbed and retained passive-elastic energy (deg N) (P<0.05). They also had decreased times for the agility course and the 10-m walk (P<0.05). The functional reach test did not change for either group. INTERPRETATION: The eight-week stretching program most likely increased the maximal length, length extensibility and passive resistive forces of the calf muscles. Adaptations of other ankle and leg structures also may have contributed. The passive adaptations were associated with enhanced performances of ambulatory functional activities.     

Effects of Active Individual Muscle Stretching on Muscle Function

[Purpose] We investigated the effect of active individual muscle stretching (AID) on muscle function. [Subjects] We used the right legs of 40 healthy male students. [Methods] Subjects were divided into an AID group, which performed stretching, and a control group, which did not. We examined and compared muscle function before and after stretching in the AID and control groups using a goniometer and Cybex equipment. [Results] A significant increase in flexibility and a significant decrease in muscle strength output were observed in the AID group after the intervention. [Conclusion] These results suggest that AID induces an increase in flexibility and a temporary decrease in muscle output strength.Key words: Active individual muscle stretching, Muscle function, Flexibility     

Effects of the hold and relax-agonist contraction technique on recovery from delayed onset muscle soreness after exercise in healthy adults

[Purpose] This study was conducted to verify the effects of the hold relax-agonist contraction and passive straight leg raising techniques on muscle activity, fatigue, and range of motion of the hip joint after the induction of delayed onset muscle soreness in the hamstring muscle. [Subjects] Sixty subjects were randomly assigned to a hold relax-agonist contraction group and a passive straight leg raising group. [Methods] Subjects in the experimental group underwent hold relax-agonist contraction at the hamstring muscle, while subjects in the control group underwent passive straight leg raising at the hamstring muscle. [Results] Subjects in the hold relax-agonist contraction group showed a significant increase in hamstring muscle activity and hip joint angle and a significant decrease in muscle fatigue. In the passive straight leg raising group, the hip joint angle increased significantly after the intervention. In the hold relax-agonist contraction group, hamstring muscle activity increased significantly and muscle fatigue decreased significantly. [Conclusion] We conclude that the hold relax-agonist contraction technique may be beneficial for improving muscle activation and decreasing muscle fatigue.Key words: Delayed onset muscle soreness, Hold relax-agonist contraction, Passive straight leg raising     

A randomized clinical trial for the effect of static stretching and strengthening exercise on pelvic tilt angle in LBP patients

BackgroundStretching and strengthening exercises are commonly used to improve muscle shortness of the hamstring as any tension in this muscle can have an effect on the pelvic posture. Thus, the aim of this study was to evaluate the effects of two methods of improving short hamstring on the angle of pelvic tilt in LBP sufferers.MethodsForty-five low back pain patients aged 19–59 years with hamstring tightness participated in this clinical trial. The patients were categorized randomly into three groups: 1- static stretching, 2-strengthening exercise and 3-control group. The two intervention groups received physical therapy and special exercise program thrice a week in a total of 12 sessions, while the control group received only conventional physical therapy. Before and after the treatment implementation, the pelvic tilt and straight leg raising (SLR) degree were assessed for each group.ResultAfter 12 sessions of treatment, the ANCOVA models indicated non-significant differences in pelvic tilt angle and SLR score changes (p > 0.05), among the three groups. In addition, no statistically significant correlation was observed between the pelvic tilt and SLR test [except for the strengthening exercise group (Pearson correlation coefficient = −0.54, P < 0.05)].ConclusionsIn LBP sufferers, both static stretching and strengthening of hamstring muscle in its lengthened position caused elongation and extensibility in the hamstring muscle and increased SLR test score, but did not change pelvic tilt angle.     

Comparison of cyclic and sustained passive stretching using a mechanical device to increase resting length of hamstring muscles

The purpose of this study was to compare the effects of cyclic versus sustained passive stretching with a mechanical device on resting hamstring muscles' length. Group 1 subjects (5 men, 17 women) underwent cyclic stretching of their right hamstring musculature, and Group 2 subjects (5 men, 16 women) underwent sustained stretching of their right hamstring musculature. The stretching procedures were performed for 15 minutes on 5 consecutive days. A follow-up examination of the subjects' relative knee flexion range of motion was made one week posttreatment. The Group 1 subjects had a mean ROM increase of 15.4 +/- 5.0 degrees after the five stretching treatments and maintained a mean ROM increase of 10.4 +/- 5.5 degrees on the follow-up examination (p less than .001). In Group 2, the five stretching treatments resulted in a mean ROM increase of 13.4 +/- 4.4 degrees, and a mean increase of 7.9 +/- 4.0 degrees was maintained on the follow-up examination (p less than .001). Linear regression analysis revealed that initial ROM, sex, and treatment method significantly contributed to increases in ROM from Day 1 of treatment to the follow-up examination (F = 6.04; df = 4,36; p less than .0008). The cyclic stretching method resulted in a greater gain in ROM when the other variables were considered. Predicted values of ROM increases were also examined and discussed.     

Repeated passive stretching: acute effect on the passive muscle moment and extensibility of short hamstrings

OBJECTIVE: To examine the response of short hamstring muscles to repeated passive stretching. DESIGN: A repeated measures design. SETTING: A university laboratory for human movement analysis in a department of rehabilitation. SUBJECTS: Students (7 men, 10 women) from the Department of Human Movement Sciences. MAIN OUTCOME MEASURES: The lift force, range of motion, pelvic-femoral angle, first sensation of pain, and electromyogram of the hamstrings were measured. RESULTS: Comparison of the data of the test group (n = 17) after five successive passive stretch tests by means of an instrumental straight-leg raising test showed no significant change of the variables passive muscle stiffness and extensibility (p>.05). CONCLUSION: The acute effect of repeated passive stretching of short hamstring muscles is negligible. With an instrumental straight-leg raising test, the relevant muscle variables can be examined noninvasively.     

Effects of repeated ankle stretching on calf muscle-tendon and ankle biomechanical properties in stroke survivors

Background

The objective of this study was to investigate changes in active and passive biomechanical properties of the calf muscle–tendon unit induced by controlled ankle stretching in stroke survivors.

Methods

Ten stroke survivors with ankle spasticity/contracture and ten healthy control subjects received intervention of 60-min ankle stretching. Joint biomechanical properties including resistance torque, stiffness and index of hysteresis were evaluated pre- and post-intervention. Achilles tendon length was measured using ultrasonography. The force output of the triceps surae muscles was characterized via the torque–angle relationship, by stimulating the calf muscles at a controlled intensity across different ankle positions.

Findings

Compared to healthy controls, the ankle position corresponding to the peak torque of the stroke survivors was shifted towards plantar flexion (P < 0.001). Stroke survivors showed significantly higher resistance torques and joint stiffness (P < 0.05), and these higher resistances were reduced significantly after the stretching intervention, especially in dorsiflexion (P = 0.013). Stretching significantly improved the force output of the impaired calf muscles in stroke survivors under matched stimulations (P < 0.05). Ankle range of motion was also increased by stretching (P < 0.001).

Interpretation

At the joint level, repeated stretching loosened the ankle joint with increased passive joint range of motion and decreased joint stiffness. At the muscle–tendon level, repeated stretching improved calf muscle force output, which might be associated with decreased muscle fascicle stiffness, increased fascicle length and shortening of the Achilles tendon. The study provided evidence of improvement in muscle tendon properties through stretching intervention.