Stretch Tolerance Research Articles

Extensibility of the Hamstrings Is Best Explained by Mechanical Components of Muscle Contraction, Not Behavioral Measures in Individuals With Chronic Low Back Pain

To examine the relationship between hamstring extensibility by use of the instrumented straight leg raise; mechanical components of muscle contraction, including muscle recruitment, passive torque measures of tissue stiffness, and eccentric strength; and self-reported measures of pain and disability. Cross-sectional study. University laboratory. Twenty-one individuals with chronic nonspecific axial lower back pain and 15 healthy control subjects. Instrumented straight leg raise, concentric and eccentric hamstring strength, self-reported measures of pain, disability, fear avoidance, general health and well-being Objective measures included hamstring extensibility, hamstring muscle stiffness, absolute and relative concentric/eccentric strength, concentric/eccentric strength ratios. Self-reported measures included Oswestry disability index, visual analog pain scale, fear avoidance beliefs, and general health and well being. Patients with lower back pain had lower range of motion, greater changes in muscle stiffness, and impaired concentric-to-eccentric strength levels. Stepwise regression identified measures of stiffness as significantly predicting hamstring extensibility (adjusted r(2) = 0.58, F = 23.76, P < .001). Self-reported measures were not associated with extensibility. Gender differences were noted for passive stiffness and absolute strength. For women, later onset of the medial hamstrings also was associated with greater hamstring extensibility. Decreased extensibility of the hamstrings was associated with increased passive stiffness during the common range of motion (20 to 50 degrees ). Impaired stretch tolerance is associated with actual mechanical restriction, not behavioral measures indicating increased pain or fear-avoidant behavior. With no relationship to actual disability and contradictory findings in the literature for the relationship of the hamstrings to the mechanics of the low back, it is unclear whether decreased hamstring extensibility should be targeted in rehabilitation programs for axial lower back pain.

The Role Of Neural Tension In Hamstring Flexibility

PURPOSE: The Slump Test is used to assess adverse neural tension in patients with low back and hamstring injuries, and involves tensioning the neural tissues by flexing the cervical and thoracic spine during hamstring stretch. It is not known if this neural tension increases resistance to stretch. Therefore, the purpose of this study was to determine if neural tension, via the Slump Test, during passive hamstring stretch, affects maximum range of motion (ROM), stretch discomfort, resistance to stretch, or the electromyographic (EMG) response to stretch in healthy subjects. METHODS: Resistance to stretch, EMG response, ROM and stretch discomfort were measured bilaterally during passive hamstring stretches with and without neural tension (n=8, age 39±13 yr, weight 154±24 lb). Stretches were performed on an isokinetic dynamometer (5°/s) with subjects seated, the test thigh flexed 45° above the horizontal, and the seat back at 90° to the horizontal. Knees were passively extended from 90° flexion to maximum stretch tolerance. Starting leg (right or left) and test sequence (stretch with or without neural tension) were alternated between subjects (each stretch separated by 4 min). Stretch-induced EMG signals were normalized to maximum contractions performed prior to stretches. For neural tension stretches the cervical and upper thoracic spine were manually flexed by a physical therapist (Slump Test). RESULTS: Maximum ROM was 8±5° less for the stretches with neural tension versus stretches without neural tension (P<0.01). Stretch discomfort and EMG response were not different between stretches with and without neural tension (discomfort: 7±1 vs. 6±2, P=0.29; EMG: 3±5% vs. 5±5%, P=0.1). Resistance to stretch was progressively greater with increasing ROM for stretches with neural tension versus without neural tension (P<0.001). During the neural tension stretches resistance to stretch at maximum ROM was 12% greater than resistance at the same angle during stretches without neural tension (P<0.05). CONCLUSIONS: Maximum ROM was decreased and passive resistance to stretch was increased by increasing neural tension during passive hamstring stretch. This effect was not due to an increased EMG response to stretch. These data indicate that passive extensibility of neural tissues can limit hamstring flexibility.

Effect of Eccentric Training on the Plantar Flexor Muscle-Tendon Tissue Properties

It has been shown that eccentric training can be effective in the rehabilitation of patients with Achilles tendonopathy. The mechanism behind these results is not clear. However, there is evidence that tendons are able to respond to repeated forces by altering their structure and composition, and, thus, their mechanical properties change. In this regard, the objective of the present study was to investigate whether eccentric training affects the mechanical properties of the plantar flexor's muscle-tendon tissue properties. Seventy-four healthy subjects were randomized into two groups: an eccentric training group and a control group. The eccentric training group performed a 6-wk eccentric training program for the calf muscles. Before and after this period, all subjects were evaluated for dorsiflexion range of motion using universal goniometry, passive resistive torque of the plantar flexors, and stiffness of the Achilles tendon. Passive resistive torque was measured during ankle dorsiflexion on an isokinetic dynamometer. Stiffness of the Achilles tendon was assessed using a dynamometer, in combination with ultrasonography. The results of the study reveal that the dorsiflexion range of motion was significantly increased only in the eccentric training group. The eccentric heel drop program also resulted in a significant decrease of the passive resistive torque of the plantar flexors (from 16.423 +/- 0.827 to 12.651 +/- 0.617 N.m). The stiffness of the Achilles tendon did not change significantly as a result of training. These findings provide evidence that an eccentric training program results in changes to some of the mechanical properties of the plantar flexor muscles. These changes were thought to be associated with modifications to structure rather than to stretch tolerance.

Acute Stretch Perception Alteration Contributes to the Success of the PNF “Contract-Relax” Stretch

Some researchers have suggested that an alteration of stretch perception could be responsible for the success of the contract-relax (CR) stretch, a stretch technique derived from proprioceptive neuromuscular facilitation (PNF). This study was conducted to determine if the alteration of the stretch perception is a possible explanation for the range of motion (ROM) gains of the CR stretch. Eighteen subjects performed two stretches in randomized order: the slow stretch and the CR stretch. The stretch intensity was controlled. The stretch force was measured and compared between the slow stretch and CR stretch. There was a significant difference between the stretch force that could be applied in the PNF stretch (126.0 N) and the slow stretch (108.4 N); P = 0.00086. The average stretch tolerance progressively increased with successive trials from 120.6 N in the first trial to 132.4 N in the fourth trial. The alteration of stretch perception plays a role in the success of the CR form of PNF stretching. At least four repetitions of the CR stretch are recommended to get the greatest ROM gain.

Effects of Stretching on Passive Muscle Tension and Response to Eccentric Exercise

Background Stretching is used in an attempt to improve performance and reduce the risk of muscle injury, with little evidence to support its effectiveness. Hypothesis Four weeks of static or ballistic stretching can attenuate the increased soreness and decreased flexibility seen after eccentric exercise. Study Design Controlled laboratory study. Methods Twenty-nine male subjects were randomly assigned to a static stretching, ballistic stretching, or control group. On each of 4 consecutive days, they completed 4 maximal range of motion stretches using a Cybex isokinetic dynamometer to passively stretch the hamstrings at 0.087 rad · s–1 (5 deg · s–1). Stiffness from 0.87 to 1.48 rad (50°-85°), peak range of motion, work absorption, peak resistive torque, and soreness were measured. Participants then completed 4 weeks of either static or ballistic stretching for a total stretching duration of 3600 seconds. After training, the 4 days of testing were repeated with an eccentric exercise task added after day 1. Results Stretching groups had an increase in range of motion and stretch tolerance after 4 weeks of stretching, with no change in muscle stiffness, work absorption, or delayed onset muscle soreness. After eccentric exercise, they also had greater range of motion and stretch tolerance than did controls. Conclusion Both static stretching and ballistic stretching increase range of motion, most likely as a result of enhanced stretch tolerance rather than changes in muscle elasticity. Four weeks of stretching maintain range of motion and stretch tolerance in the days after eccentric exercise.

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.

Passive Force, Angle, and Stiffness Changes after Stretching of Hamstring Muscles

Periodic or long-term stretching programs have demonstrated changes in joint range of motion. The suggested mechanisms for these increases in range of motion are changes in the tissue properties of the muscle and more recently stretch tolerance. However, few studies have examined changes in passive resistive forces and related these changes to increases in range of motion. The purpose of this study was to investigate the influence of a 6-wk hamstring-stretching program on knee extension range of motion, passive resistive forces, and muscle stiffness. A randomized control trial with repeated measures was undertaken with 43 school-age subjects. Hamstring extensibility was assessed by a passive knee extension test using a Kincom isokinetic dynamometer. The intervention group participated in a 6-wk hamstring-stretching program. Stretches were performed 5 d x wk(-1), once per day, held for 30 s, and for 3 repetitions. The control group did not stretch over the 6-wk intervention period. Measurements of hamstring extensibility were repeated at the end of the 6-wk intervention. After the intervention period, significant (P < 0.05) increases in knee extension range of motion, passive resistive force, and stiffness were observed in the experimental group. No significant differences were observed in the control group's findings for the same variables. The findings of this study are consistent with other literature that shows periodic stretching programs over a 6-wk time frame produce significant changes in knee extension range of motion. The finding of increased stiffness in the new range of motion, gained as a result of the stretching program, provides some evidence that structural changes had occurred in the stretched muscles.

Modification of stretch tolerance in a stooping position.

A stooping (slump) position is believed to add tension to the nerve tissue complex. This study was designed to determine whether this position would have an effect on the stretch tolerance in a passive knee extension. Thirteen healthy individuals were tested. The knee extension was stopped by the subjects at "onset of pain". Joint range of motion and passive resistance to the extension were recorded in four test situations: upright sitting and stooping position, with the ankle joint in either the neutral or maximal dorsi-flexed position. A significant decrease in range of motion was seen when shifting from upright to stooping position: Delta angle -2.4 degrees (P<0.01). According to this, the passive tissue tension was accepted at significantly lower values in stooping position: Delta torque -1.2 N m (P<0.01). Testing with maximal dorsi-flexion of the ankle showed more pronounced changes: Delta angle -3.4 degrees (P<0.001); Delta torque -2.3 N m (P<0.001), but the effect of foot position was not significant. Knee joint range of motion was acutely diminished in a stooping position. Thus, stretch tolerance was affected by manipulation of structures, which were not directly mechanically related to this joint. An influence from the nerve tissue complex must be considered to be a factor when describing the mechanisms behind altered stretch tolerance.

Extensibility and stiffness of the hamstrings in patients with nonspecific low back pain

Halbertsma JPK, Göeken LNH, Hof AL, Groothoff JW, Eisma WH. Extensibility and stiffness of the hamstrings in patients with nonspecific low back pain. Arch Phys Med Rehabil 2001;82:232-8. Objective: To investigate the extensibility and stiffness of the hamstrings in patients with nonspecific low back pain (LBP). Design: An experimental design. Setting: A university laboratory for human movement analysis in a department of rehabilitation medicine. Participants: Forty subjects, a patient group (20) and a healthy control group (20). Interventions: Subjects laid supine on an examination table with a lift frame, with left leg placed in a sling at the ankle. Straight leg raising, pulling force, and activity of hamstring and back muscles were recorded with electrodes. Patients indicated when they experienced tension or pain. Main Outcome Measures: The lift force, leg excursion, pelvic-femoral angle, first sensation of pain, and the electromyogram of the hamstrings and back muscles measured in an experimental straight-leg raising set-up. Results: The patient group showed a significant restriction in range of motion (ROM) and extensibility of the hamstrings compared with the control group. No significant difference in hamstring muscle stiffness can be assessed between both groups. Conclusion: The restricted ROM and the decreased extensibility of the hamstrings in patients with nonspecific LBP is not caused by increased muscle stiffness of the hamstrings, but determined by the stretch tolerance of the patients.

Effects of the Mulligan Traction Straight Leg Raise Technique on Range of Movement

The Mulligan traction straight leg raise (SLR) technique is used therapeutically to increase range of SLR when it is limited due to low-back dysfunction or hamstring tightness. The effects of this technique were investigated in 26 normal subjects (mean age 26 years; 13 male). As the movement of SLR comprises hip flexion and posterior pelvic rotation, ranges of SLR and posterior pelvic rotation were measured pre- and post-intervention using two bubble inclinometers. Prior to data collection, SLR was performed four times to minimize increased movement due to repetitive stretch. Following the intervention, the mean range of SLR significantly increased by 13.3° or 27%, from 49.9° to 63.2°. Mean range pelvic rotation also significantly increased but by only 2.7°. Hip flexion was the major reason for increased range of SLR following the intervention, indicating an increase in hamstring muscle stretch tolerance. The possible mechanisms for improvement in range are discussed.

A biomechanical evaluation of cyclic and static stretch in human skeletal muscle.

The short term effect of static and cyclic stretch paradigms on stiffness and maximal joint range of motion was examined in 12 recreational athletes. To assess flexibility, joint range of motion and resistance to stretch were measured using a dynamometer during a passive stretch of the hamstring muscle group to the point of pain. The recorded torque-angle curve allowed for identification of maximal joint range of motion and calculation of passive muscle-tendon stiffness and energy. Three flexibility assessments (stretch 1 - 3), each 10 min apart, were administered to each leg. A 90 s static stretch and 10 cyclic stretches were performed after the second stretch on the left and right side, respectively. Stiffness in a common range for stretch 1 - 3 was unchanged on both the left and right side. However, on the left side (static stretch) there was a significant effect of flexibility assessment (stretch 1 - 3) (p < 0.0001) with an increased maximal joint angle (p < 0.01) and maximal stiffness (p < 0.05) between all three stretches. Similarly, on the right side (cyclic stretches) there was a significant effect of flexibility assessment (p < 0.0001) with an increased maximal joint angle between stretch 1 and 3 (p < 0.01) and maximal stiffness (p < 0.05) between all stretches. During the static stretch passive torque declined 35+/-4% (p < 0.001). During the cyclic stretches passive energy and hysteresis both declined 17% (p < 0.05) while stiffness increased 12% (p < 0.05). The results of the present study demonstrate that static and cyclic stretching, as it is commonly performed by athletes, increases joint range of motion by increasing stretch tolerance while the viscoelastic characteristics of the muscle remain unaltered.

TIME COURSE OF STRESS RELAXATION WITH REPETITIVE STRETCHING OF HUMAN PLANTARFLEXORS

1445 The purpose of this study was to monitor the stress relaxation response to repeated maximum passive stretching (PSmax) of the ankle plantarflexors. Ten university students (6 men, 4 women) underwent 25 min of cyclical, PSmax (10 stretches [S1-S10] over 25 min, 15 s rest between stretches) to a maximum dorsiflexion position (Dmax) each stretch. Soleus EMG confirmed that each stretch was passive. Dmax joint angle, peak passive torque (PTpeak), and passive torque in Dmax was observed during each 2 min stretch, and passive torque decay (PTd) for the intervals from 0 to 15 s (I1), 15 to 30 s (I2), 30 to 45 s (I3), and 105 to 120 s (IE) was calculated. Dmax increased significantly by S2 (P<0.005) and from S2 to S3 (P<0.05) so that 59%(3.4 °) of a total 5.8 ° increase in Dmax over ten stretches(P<0.005), had occured by S3 (P<0.005). PTpeak increased 12% from S1 to S8 (P=0.06). Total PTd was greater in S1 (P<0.005) than S2-S10. Greater PTd was observed in I1 (P<0.005) than I2, I3, and IE of any single stretch so that 45% of PTd in a 2 min stretch, occurs in the first 15s. Greater PTd was observed in I1 of S1 compared to I1 of all other stretches (P<0.005). These data indicate that a significant viscoelastic stress relaxation response occurs within the first 15s of a single stretch and within the first two stretches of a ten stretch protocol. Tissue creep, and increased stretch tolerance, likely allow for further responses beyond the early viscoelastic stress relaxation.

Resistance of the human posterior lens capsule and zonules to disruption

Purpose: To determine the resistance of the human posterior lens capsule and zonules to disruption during cataract surgery.Setting: Wet lab at Walkergate Hospital, Newcastle upon Tyne, England.Method: Human donor eyes were obtained after removal of a corneoscleral disc. Lens extraction was performed by an extracapsular technique leaving an intact capsular bag. The resistance of the zonules and posterior capsule to disruption by aspiration was measured in each eye, and the resistance of the zonules to damage from direct mechanical stretch was determined.Results: Posterior capsules tolerated a median suction pressure of 200 mm Hg (range 150 to 300 mm Hg) before disruption. Values for female eyes were slightly greater than for male; no significant correlation with age was shown. The zonular fibers tolerated a median suction pressure of 200 mm Hg (range 150 to 300 mm Hg). The median mechanical zonular stretch tolerance was 3.00 mm (range 2.75 to 4.00 mm). No significant correlation was found between zonular suction tolerance or stretch tolerance and sex or between suction tolerance and age, but a negative relationship was noted between maximum tolerated stretch and age.Conclusion: Setting the maximum aspiration pressure during irrigation/aspiration to 150 mm Hg should help prevent capsular or zonular rupture. If more powerful aspiration is required, it should be used with great care so as not to engage the capsule.

Flexibility and its effects on sports injury and performance.

Flexibility measures can be static [end of ROM (range of motion)], dynamic-passive (stiffness/compliance) or dynamic-active (muscle contracted, stiffness/compliance). Dynamic measures of flexibility are less dependent on patient discomfort and are more objective. Acute and chronic changes in flexibility are likely to occur with stretching exercises, but it is difficult to distinguish between changes in stretch tolerance as opposed to changes in muscle stiffness. How flexibility is measured impacts these findings. There is no scientifically based prescription for flexibility training and no conclusive statements can be made about the relationship of flexibility to athletic injury. The literature reports opposing findings from different samples, frequently does not distinguish between strain, sprain and overuse injury, and rarely uses the proper denominator of exposure. There is basic scientific evidence to suggest that active warm-up may be protective against muscle strain injury but clinical research is equivocal on this point. Typically, specific flexibility patterns are associated with specific sports and even positions within sports. The relationship of flexibility to athletic performance is likely to be sport-dependent. Decreased flexibility has been associated with increased in-line running and walking economy. Increased stiffness may be associated with increased isometric and concentric force generation, and muscle energy storage may be best manifested by closely matching muscle stiffness to the frequency of movement in stretch-shorten type contractions.

Determinants of musculoskeletal flexibility: viscoelastic properties, cross-sectional area, EMG and stretch tolerance.

Cross-sectional area, stiffness, viscoelastic stress relaxation, stretch tolerance and EMG activity of the human hamstring muscle group were examined in endurance-trained athletes with varying flexibility. Subjects were defined as tight (n = 10) or normal (n = 8) based on a clinical toe-touch test. Cross-sectional area was computed from magnetic resonance imagining (MRI) images. Torque (Nm) offered by the hamstring muscle group, electromyographic (EMG) activity, knee joint angle and velocity were continuously monitored during two standardized stretch protocols. Protocol 1 consisted of a slow stretch at 0.087 rad/s (dynamic phase) to a pre-determined final angle followed by a 90-s static phase. In the dynamic phase final angle and stiffness was lower in tight (28.0+/-2.9 Nm/rad) than normal subjects (54.9+/-6.5 Nm/rad), P<0.01. In the static phase tight subjects had lower peak (15.4+/-1.8 Nm) and final torque (10.8+/-1.6 Nm) than normal subjects (31.6+/-4.1 Nm, 24.1+/-3.7 Nm, respectively)(P<0.01), but torque decline was similar. Protocol 2 consisted of a slow stretch to the point of pain and here tight subjects reached a lower maximal angle, torque, stiffness and energy than normal subjects (P<0.01). On the other hand, stiffness was greater in tight subjects in the common range (P<0.01). Cross-sectional area of the hamstring muscles and EMG activity during the stretch did not differ between the groups. However, lateral hamstring cross-sectional area was positively related to mid-range stiffness (P<0.05), but inversely related to final stiffness, peak torque and the toe-touch test (P<0.01). Final angle and peak torque in protocol 1 combined to improve the predictability of the toe-touch test (R2=0.77, P<0.001). These data show that the toe-touch test is largely a measure of hamstring flexibility. Further, subjects with a restricted joint range of movement on a clinical toe-touch test have stiffer hamstring muscles and a lower stretch tolerance.

A mechanism for altered flexibility in human skeletal muscle.

1. We investigated the effect of a long-term stretching regimen on the tissue properties and stretch tolerance of human skeletal muscle. 2. Resistance to stretch was measured as torque (in N m) offered by the hamstring muscle group during passive knee extension while electromyographic (EMG) activity, knee joint angle and velocity were continuously monitored during a standardized stretch manoeuvre. Seven healthy subjects were tested before and after a 3 week training period using two separate protocols. Protocol 1 consisted of a slow stretch at 0.087 rad s-1 to a predetermined angle followed by a 90 s holding phase. Subjects were brought to the same angle before and after the training period. Protocol 2 was a similar stretch, but continued to the point of pain. 3. During protocol 1 the torque rose during the stretch and then declined during the holding phase. EMG activity was small and did not change significantly during the protocol. No significant differences in stiffness, energy and peak torque about the knee joint were seen as a result of the training. During protocol 2 the angle to which the knee could be extended was significantly increased as a result of the training. This was accompanied by a comparable increase in peak torque and energy. EMG activity was small and not affected by training. 4. It is concluded that reflex EMG activity does not limit the range of movement during slow stretches and that the increased range of motion achieved from training is a consequence of increased stretch tolerance on the part of the subject rather than a change in the mechanical or viscoelastic properties of the muscle.

Stretching exercises: Effect on passive extensibility and stiffness in short hamstrings of healthy subjects

Passive muscle stretch tests are common practice in physical therapy and rehabilitation medicine. However, the effects of stretching exercises are not well known. With an instrumental straight-leg-raising set-up the extensibility, stiffness, and electromyographic activity of the hamstring muscles have been experimentally determined and the effects of stretching exercises have been evaluated. Fourteen volunteers, aged 20 to 38 years (mean 27.3) were selected from a young healthy population with the toe-touch test (finger-ground distance greater than 0cm), and a straight-leg-raising angle about 80°. According to usual standards the diagnosis was short hamstrings. One group of seven subjects was treated during 4 weeks with a daily home exercise program aimed at stretching the hamstrings, whereas the untreated group was used as a control. Instrumental straight-leg-raising was performed in the subjects of both groups. The significance of the differences between the mean values was determined with the Student's t-test. Comparison of the data obtained before and after the muscle stretching program showed a slight but significant increase in the extensibility of the hamstrings accompanied with a significant increase of the stretching moment tolerated by the passive hamstring muscles. However, the elasticity remained the same. It is concluded that stretching exercises do not make short hamstrings any longer or less stiff, but only influence the stretch tolerance.