***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: In the present experiment we obtained the tensile properties of the human gastrocnemius tendon , a high-stressed tendon suitable for spring-like action during locomotion . Measurements were taken in vivo in six men . The gastrocnemius tendon elongation during tendon loading-unloading induced by muscle contraction-relaxation was measured using real-time ultrasonography .  Tendon forces were calculated from the moment generated during isometric plantarflexion contraction , using tendon moment arm length data obtained in vivo with the tendon travel method . Tendon stiffness data were calculated from the slope of the tendon force-elongation curve , and were then normalized to the tendon 's original dimensions , obtained from morphometric analysis of sonographs , to estimate the tendon Young 's modulus . Mechanical hysteresis values were obtained from area calculations by numerical integration . The elongation of the tendon increased curvilinearly with the force acting upon it , from 1.7+/-1 mm ( 0.8+/-0.3 % strain ) at 87.5+/-8.5 N to 11.1+/-3.1 mm ( 4.9+/-1 % strain ) at 875+/-85 N. The tendon Young 's modulus and mechanical hysteresis were 1.16+/-0.15 GPa and 18+/-3 % , respectively . These values fall within the range of values obtained from in vitro experiments and are very similar to the respective values recently obtained from in vivo measurements in the less highly stressed human tibialis anterior tendon ( 1.2 GPa and 19 % ) , thus indicating that the material properties of tendon are independent of physiological loading and function . Combining the present tendon force-elongation data with previously reported Achilles tendon force data recorded during walking indicates that the gastrocnemius tendon would provide approximately 6 % of the total external work produced by the locomotor system . This estimate illustrates the contribution of passive elastic mechanisms on the economy and efficiency of walking . The contributions would be greater in more active exercise such as running The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions . Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions  at five angular velocities . Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography . During maximal isometric contraction , fascicle length was shorter and pennation angle larger compared with values at rest ( P < 0.01 ) . During isokinetic concentric contractions from 0 to 4.36 rad/s , fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm ( 41 % ; P < 0.01 ) , whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees ( 34 % ; P < 0.01 ) . During eccentric muscle actions , fascicles contracted quasi-isometrically , independent of angular velocity . The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component , which decreases with increasing contraction velocity . The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening The aim of this study was to determine the effects of 14 wk of plyometric training on mechanical properties of the Achilles tendon .  Nineteen subjects were r and omly assigned to trained or  control group .  Cross-sectional area ( CSA ) , stiffness , and dissipation coefficient of the Achilles tendon were measured before and after the training period . In the trained group , a decrease in dissipation coefficient ( -35.0 % ; P<0.05 ) and an upward trend in stiffness ( + 24.1 % ) of the Achilles tendon was found , without any changes in Achilles tendon CSA ( P>0.05 ) . Plyometric training enhances the muscular tension transmission mainly through a reduction in energy dissipated by the tendon . The lack of changes in the Achilles tendon CSA indicates that changes in mechanical properties would mainly result from a qualitative change in tendinous tissues rather than from changes in the geometry of the Achilles tendon While microgravity exposure is known to cause deterioration of skeletal muscle performance , little is known regarding its effect on tendon structure and function . Hence , the aims of this study were to investigate the effects of simulated microgravity on the mechanical properties of human tendon and to assess the effectiveness of resistive counter measures in preventing any detrimental effects . Eighteen men ( aged 25 - 45 yr ) underwent 90 days of bed rest :  nine performed resistive exercise during this period ( BREx group ) , and nine underwent bed rest only ( BR group ) . Calf-raise and leg-press exercises were performed every third day using a gravity-independent flywheel device . Isometric plantar flexion contractions were performed by using a custom-built dynamometer , and ultrasound imaging was used to determine the tensile deformation of the gastrocnemius tendon during contraction . In the BR group , tendon stiffness estimated from the gradient of the tendon force-deformation relation decreased by 58 % ( preintervention : 124 + /- 67 N/mm ; postintervention : 52 + /- 28 N/mm ; P < 0.01 ) , and the tendon Young 's modulus decreased by 57 % postintervention ( P < 0.01 ) . In the BREx group , tendon stiffness decreased by 37 % ( preintervention : 136 + /- 66 N/mm ; postintervention : 86 + /- 47 N/mm ; P < 0.01 ) , and the tendon Young 's modulus decreased by 38 % postintervention ( P < 0.01 ) . The relative decline in tendon stiffness and Young 's modulus was significantly ( P < 0.01 ) greater in the BR group compared with the BREx group . Unloading decreased gastrocnemius tendon stiffness due to a change in tendon material properties , and , although the exercise counter measures did attenuate these effects , they did not completely prevent them . It is suggested that the total loading volume was not sufficient to completely prevent alterations in tendon mechanical properties Architectural properties of the triceps surae muscles were determined in vivo for six men . The ankle was positioned at 15 degrees dorsiflexion ( -15 degrees ) and 0 , 15 , and 30 degrees plantar flexion , with the knee set at 0 , 45 , and 90 degrees . At each position , longitudinal ultrasonic images of the medial ( MG ) and lateral ( LG ) gastrocnemius and soleus ( Sol ) muscles were obtained while the subject was relaxed ( passive ) and performed maximal isometric plantar flexion ( active ) , from which fascicle lengths and angles with respect to the aponeuroses were determined . In the passive condition , fascicle lengths changed from 59 , 65 , and 43 mm ( knee , 0 degrees ; ankle , -15 degrees ) to 32 , 41 , and 30 mm ( knee , 90 degrees ankle , 30 degrees ) for MG , LG , and Sol , respectively . Fascicle shortening by contraction was more pronounced at longer fascicle lengths . MG had greatest fascicle angles , ranging from 22 to 67 degrees , and was in a very disadvantageous condition when the knee was flexed at 90 degrees , irrespective of ankle positions . Different lengths and angles of fascicles , and their changes by contraction , might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses Plyometric training is commonly used to improve athletic performance ; however , it is unclear how each component of the muscle – tendon complex ( MTC ) is affected by this intervention . The effects of 14 weeks of plyometric training on the passive stiffness of the gastrocnemii muscles and Achilles tendon was determined simultaneously to assess possible local adaptations of elastic properties . The passive force – length relationship of the gastrocnemii MTC and elongation of the gastrocnemii muscles were determined using ultrasonography during passive cyclic stretching in 19 subjects divided into trained ( n = 9 ) and control ( n = 10 ) groups . An upward trend in stiffness of the gastrocnemii MTC ( P = 0.09 ) and a significant increase in the intrinsic gastrocnemii muscle stiffness were found ( P < 0.05 ) . In contrast , no significant change in gastrocnemii tendon stiffness , or in muscle and tendon geometry , was determined ( P > 0.05 ) . Considering the lack of change in gastrocnemii muscle geometry , the change in the gastrocnemii muscle stiffness may be mainly due to a change in the intrinsic mechanical properties of the muscular tissues Objective : To prospect ively investigate tendon thickness and tendon structure by ultrasonography in patients treated with eccentric calf muscle training for painful chronic Achilles tendinosis located at the 2–6 cm level in the tendon . Methods : The patients were examined with grey scale ultrasonography before and 3.8 years ( mean ) after the 12 week eccentric training regimen . At follow up , a question naire assessed present activity level and satisfaction with treatment . Results : Twenty six tendons in twenty five patients ( 19 men and six women ) with a mean age of 50 years were followed for a mean of 3.8 years ( range 1.6–7.75 ) . All patients had a long duration of painful symptoms ( mean 17.1 months ) from chronic Achilles tendinosis before treatment . At follow up , 22 of 25 patients were satisfied with treatment and active in Achilles tendon loading activities at the desired level . Ultrasonography showed that tendon thickness ( at the widest part ) had decreased significantly ( p<0.005 ) after treatment ( 7.6 ( 2.3 ) v 8.8 ( 3 ) mm ; mean ( SD ) ) . In untreated normal tendons , there was no significant difference in thickness after treatment ( 5.3 ( 1.3 ) mm before and 5.9 ( 0.8 ) mm after ) . All tendons with tendinosis had structural abnormalities ( hypoechoic areas and irregular structure ) before the start of treatment . After treatment , the structure was normal in 19 of the 26 tendons . Six of the seven patients with remaining structural abnormalities experienced pain in the tendon during loading . Conclusions : Ultrasonographic follow up of patients with mid-portion painful chronic Achilles tendinosis treated with  eccentric calf muscle training showed a localised decrease in tendon thickness and a normalised tendon structure in most patients . Remaining structural tendon abnormalities seemed to be associated with residual pain in the tendon Background : Controversy exists on stretching and warm-up in injury prevention . We hypothesized that warm up has a greater effect on Achilles tendon biomechanics than static stretching . This study investigated static stretching and warm-up on Achilles tendon biomechanics in recreational athletes , in vivo . Material s and Methods : Ten active , healthy subjects , 5 males , 5 females , With a mean age of 22.9 years with no previous Achilles tendon injuries were recruited . Typical stretching and warm-up routines were created . Testing was performed in a r and omized cross-over design . A custom-built dynamometer was utilized to perform controlled isometric plantarflexion . A low profile ultrasound probe was utilized to visualize the musculotendinous junction of the medial gastrocnemius . An eight-camera motion capture system was used to capture ankle motion . Custom software calculated Achilles tendon biomechanics . Results : Achilles tendon force production was consistent . No statistically significant differences were detected in stretch , stiffness , and strain between pre- , post-stretching , and post-warm-up interventions . Conclusion : Stretching or warm-up alone , and combined did not demonstrate statistically significant differences . Stretching and warm-up may have an equivalent effect on Achilles tendon biomechanics . Prolonged and more intense protocol s may be required for changes to occur . Clinical Relevance : Stretching and warm-up of the Achilles before exercise are commonly practice d. Investigating the effect of stretching and warm-up may shed light on potential injury prevention PURPOSE Many studies have been undertaken to define the effects of static and ballistic stretching . However , most research ers have focused their attention on joint range-of-motion measures . The objective of the present study was to investigate whether static- and ballistic-stretching programs had different effects on passive resistive torque measured during isokinetic passive motion of the ankle joint and tendon stiffness measured by ultrasound imaging . METHODS Eighty-one healthy subjects were r and omized into three groups : a static-stretch group , a ballistic-stretch group , and a control group .  Both stretching groups performed a 6-wk stretching program for the calf muscles . Before and after this period , all subjects were evaluated for ankle range of motion , passive resistive torque of the plantar flexors , and the stiffness of the Achilles tendon . RESULTS The results of the study reveal that the dorsiflexion range of motion was increased significantly in all groups . Static stretching result ed in a significant decrease of the passive resistive torque , but there was no change in Achilles tendon stiffness . In contrast , ballistic stretching had no significant effect on the passive resistive torque of the plantar flexors . However , a significant decrease in stiffness of the Achilles tendon was observed in the ballistic-stretch group . CONCLUSION These findings provide evidence that static and ballistic stretching have different effects on passive resistive torque and tendon stiffness , and both types of stretching should be considered for training and rehabilitation programs We prospect ively studied the effect of heavy-load eccentric calf muscle training in 15 recreational athletes ( 12 men and 3 women ; mean age , 44.3 7.0 years ) 
    Output:
    
There is evidence that maximal isometric contractions and prolonged static stretching ( > 5 min ) of the triceps surae complex cause an immediate decrease in AT stiffness , whereas prolonged running and hopping have minimal effect .
Limited but consistent evidence exists , indicating that AT hysteresis is reduced after prolonged static stretching .
Consistent evidence supports a reduction in free AT diameter ( anterior-posterior ) after dynamic ankle exercise , and this change appears most pronounced in the healthy tendon and after eccentric exercise .
The mechanical and morphological properties of the AT in vivo are affected by acute exercise in a mode- and dose-dependent manner .
Transient changes in AT stiffness , hysteresis , and diameter after unaccustomed exercise modes and doses may expose the tendon to increased risk of strain injury and impact on the mechanical function of the triceps surae muscle-tendon unit