***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The purpose of this study was to investigate the influence of a 6-wk ballistic stretching training program on various parameters of the human gastrocnemius medialis muscle and the Achilles tendon . It is known that ballistic stretching is an appropriate means of increasing the range of motion ( RoM ) , but information in the literature about the mechanical adaptation of the muscle-tendon unit ( MTU ) is scarce . Therefore , in this study , a total of 48 volunteers were r and omly assigned into ballistic stretching and control groups . Before and following the stretching intervention , we determined the maximum dorsiflexion RoM with the corresponding fascicle length and pennation angle . Passive resistive torque ( PRT ) and maximum voluntary contraction ( MVC ) were measured with a dynamometer . Muscle-tendon junction ( MTJ ) displacement allowed us to determine the length changes in tendon and muscle , and hence to calculate stiffness . Mean RoM increased significantly from 33.8 ± 6.3 ° to 37.8 ± 7.2 ° only in the intervention group , but other functional ( PRT , MVC ) and structural ( fascicle length , pennation angle , muscle stiffness , tendon stiffness ) parameters were unaltered . Thus the increased RoM could not be explained by structural changes in the MTU and was likely due to increased stretch tolerance INTRODUCTION Previous studies have reported inhomogeneous changes in quadriceps femoris ( QF ) cross-sectional area ( CSA ) in response to strength training . It is assumed that these differential changes in muscle shape influence the muscle 's functional capacity during high-force and high-power movements . The purpose of the current study was to compare intermuscular and intramuscular QF adaptations to high-load strength training and fast-speed power training . METHODS Thirty-six non-strength-trained men were r and omly assigned to four groups and completed 8 wk of parallel-depth heavy squat-lift training ( HS-P ) , parallel-depth jump squat training ( JS-P ) , volitional-depth jump squat training ( JS-V ) , or no training ( C ) . Quadriceps femoris , vastus lateralis ( VL ) , intermedius ( VI ) , medialis ( VM ) , and rectus femoris ( RF ) CSA were measured in distal- , mid- , and proximal-thigh regions using extended field-of-view ultrasonography and compared using a 3 × 2 mixed-model MANOVA with Bonferroni post hoc tests ( P < 0.05 ) . RESULTS Parallel-depth heavy squat-lift training and JS-P elicited similar changes in mid-CSA(QF ) as well as summed CSA of the QF , VL , VI , and VM . Cross-sectional area of the VL ( CSA(VL ) ) and CSA(VI ) increased in both HS-P and JS-P at mid-thigh , but only JS-P significantly increased CSA proximally , and only HS-P significantly increased CSA distally . Cross-sectional area of the VM ( CSA(VM ) ) increased in HS-P and JS-P distally , but only HS-P increased at mid-thigh . No hypertrophy was observed in RF at any location and no significant differences were observed between JS-P and JS-V. Parallel-depth heavy squat-lift training elicited greater proximal hypertrophy in each of the vasti muscles , whereas only JS-P elicited distal VL and VI hypertrophy . CONCLUSIONS These observed inhomogeneous changes in CSA may alter the thigh 's moment of inertia and moment arms of muscle " compartments , " and the influence of elastic component force transmission on the muscular force expression . Such selective hypertrophy is speculated to be biomechanically beneficial to high-force or high-power movements used in training Proprioceptive neuromuscular facilitation ( PNF ) stretching programs have been shown to be the most effective stretching technique to increase the range of motion ( ROM ) . The objective of this study was to examine the mechanism of effect of PNF stretching on changes in the ROM .  Sixty-two healthy subjects were r and omized into two groups : a PNF stretching group and a control group . The PNF group performed a 6-week stretching program for the calf muscles . Before and after this period , all subjects were evaluated for dorsiflexion ROM , passive resistive torque ( PRT ) of the plantar flexors and stiffness of the Achilles tendon . The results of the study revealed that the dorsiflexion ROM was significantly increased in the PNF group ( DeltaROMext : 5.97+/-0.671 degrees ; DeltaROMflex : 5.697+/-0.788 degrees ) . The PRT of the plantar flexors and the stiffness of the Achilles tendon did not change significantly after 6 weeks of PNF stretching . These findings provide evidence that PNF stretching results in an increased ankle dorsiflexion . However , this increase in ROM could not be explained by a decrease of the PRT or by a change in stiffness of the Achilles tendon , and therefore can be explained by an increase in stretch tolerance Objective : To evaluate the chronic effects of a static stretching program on the muscle architecture of biceps femoris ( BF ) and vastus lateralis ( VL ) muscles in ultrasound ( US ) images . Design : R and omized controlled longitudinal trial . Setting : Biomechanics Laboratory of Physical Education School of the Army , Rio de Janeiro , Brazil . Participants : The study included 24 healthy and physically active male volunteers ( 19.05 ± 1.40 years , 1.73 ± 0.07 m , and 73.15 ± 8.33 kg ) , r and omly allocated to 1 of 2 groups : stretching group ( SG , n = 12 ) and control group ( n = 12 ) . Interventions : The SG was su bmi tted to 3 sets of 30 seconds of static stretching 3 times a week during 8 weeks . Main Outcome Measures : Ultrasound equipment ( 7.5 MHz ) was used for the evaluation of BF and VL muscle architecture variables ( pennation angle , fiber length , muscle thickness , and fascicle displacement ) before and after training . Knee range of motion ( ROM ) and isometric flexion and extension torque ( TQ ) were also measured . Results : There were no significant changes in muscle architecture , TQ , and maximum knee flexion angle ( P > 0.05 ) . However , maximum knee extension angle ( MEA ) increased significantly in the SG ( pretraining : 159.37 ± 7.27 degrees and posttraining : 168.9 ± 3.7 degrees ; P < 0.05 ) . Conclusions : Volume or intensity ( or both ) of the stretching protocol was insufficient to cause structural changes in the VL and BF muscles . The increase in MEA could not be explained by muscle architecture changes . Clinical Relevance : To describe changes in the VL and BF muscle tendon unit using US after a long-term stretching program to identify which structures are responsible for ROM increase BACKGROUND It is known that static stretching is an appropriate means of increasing the range of motion , but information in the literature about the mechanical adaptation of the muscle-tendon unit is scarce . Therefore , the purpose of this study was to investigate the influence of a six-week static stretching training program on the structural and functional parameters of the human gastrocnemius medialis muscle and the Achilles tendon . METHODS A total of 49 volunteers were r and omly assigned into static stretching and control groups . Before and following the stretching intervention , we determined the maximum dorsiflexion range of motion with the corresponding fascicle length and pennation angle . Passive  resistive torque and maximum voluntary contraction were measured with a dynamometer . Muscle-tendon junction displacement allowed us to determine the length changes in tendon and muscle , and hence to calculate stiffness . Fascicle length , pennation angle , and muscle tendon junction displacement were measured with ultrasound . FINDINGS Mean range of motion increased significantly from 30.9 ( 5.3 ) to 36.3 ( 6.1 ) in the intervention group , but other functional ( passive resistive torque , maximum voluntary contraction ) and structural ( fascicle length , pennation angle , muscle stiffness , tendon stiffness ) parameters were unaltered . INTERPRETATION The increased range of motion could not be explained by the structural changes in the muscle-tendon unit , and was likely due to increased stretch tolerance possibly due to adaptations of nociceptive nerve endings 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 r and omly 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 st and ardised 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 This investigation determined the effects of a static stretching program with different stretching protocol s on the flexibility and passive resistance of the hamstrings of young adults . Forty healthy subjects ( 24 males and 16 females ) aged 18 to 30 years were r and omly assigned to one of four groups . The two training groups underwent static stretch training of the hamstrings either with a four-week protocol or with an eight-week protocol . The other two groups acted as control groups . A significant increase in flexibility of hamstrings was found in both of the two training groups ( P<0.05 ) . No difference was found in the range of motion gained between the two training groups . An increase in passive resistance at the corresponding maximal joint angle was only demonstrated in the four-week training group ( P<0.05 ) . Both protocol s are effective in terms of improving flexibility of hamstrings . However , if injury is reduced when there is relatively lower passive resistance at the end-of-range , then the eight-week training regimen would be recommended The aim of this assessor-blind r and omised 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 r and omly 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 This study examined the effects of a 6-week stretching program on the dynamic passive elastic properties of the calf muscle-tendon unit ( MTU ) of unconditioned younger women . After r and om assignment of 12 women ( age 18–31 years ) to a  stretching group (  SG ) or to a  control group ( CG ) , six subjects in the SG and four subjects in the CG completed the study . For the initial tests , a Kin-Com ® dynamometer moved the ankle from plantarflexion to maximal dorsiflexion ( DF ) with negligible surface EMG activity in the soleus , gastrocnemius and tibialis anterior muscles . Angular displacement , passive resistive torque , area under the curve ( passive elastic energy ) and stiffness variables were reduced from the passive DF torque curves . The SG then completed ten static wall stretches held 15 s each , five times a week for 6 weeks , the CG did not . The tests were repeated and the changes between the tests and retests were examined for group differences ( Mann – Whitney U ) . The SG had significant increases in the maximal passive DF angle ( 7 ° ± 4 ° ) , maximal passive DF torque ( 11.2 ± 8.3 N m ) , full stretch range of motion ( 23 ° ± 24 ° ) , full stretch mean torque ( 3.4 ± 2.1 N m ) , and area under the full stretch curve ( 22.7 ± 23.5 ° N m ) compared to the CG ( P ≤ 0.019 ) . The passive stiffness did not change significantly . The results showed that a stretching program for unconditioned calf MTUs increased the maximal DF angle and length extensibility , as well as the passive resistive properties throughout the full stretch range of motion . The adaptations within the calf MTU provide evidence that stretching enhances the dynamic passive length and passive resistive properties in unconditioned younger women This study investigated 
    Output:
    
Small effects were seen for maximal tolerated passive torque , but trivial effects were seen for joint resistance to stretch , muscle architecture , muscle stiffness , and tendon stiffness .
Stretching interventions with 3- to 8-week duration do not seem to change either the muscle or the tendon properties , although it increases the extensibility and tolerance to a greater tensile force .
Adaptations to chronic stretching protocol s shorter than 8 weeks seem to mostly occur at a sensory level