***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 relations between the velocity of prestretch and the mechanical energy liberated during the subsequent isovelocity release were studied in contractions of frog single fibers and human muscles . During isometric contractions of frog single fibers , a ramp stretch of varied velocity ( amplitude , 0.02 fiber length ; velocity , 0.08 - 1.0 fiber length/s ) followed by a release ( amplitude , 0.02 fiber length ; velocity , 1.0 fiber length/s ) was given , and the amount of work liberated during the release was measured . For human muscles , elbow flexions were performed with a prestretch of varied velocity ( range , 40 degrees ; velocity , 30 - 180 degrees/s ) followed by an isokinetic shortening ( velocity , 90 degrees/s ) . In both frog single fibers and human muscles , the work production increased with both the velocity of stretch and the peak of force attained before the release up to a certain level ; thereafter it declined with the further increases of these variables . In human muscles , the enhancement of work production was not associated with a significant increase in integrated electromyogram . This suggests that changes in intrinsic mechanical properties of muscle fibers play an important role in the stretch-induced enhancement of work production Stretch of an activated muscle causes a transient increase in force during the stretch and a sustained , residual force enhancement ( RFE ) after the stretch . The purpose of this study was to determine whether RFE is present in human muscles under physiologically relevant conditions ( i.e. , when stretches were applied within the working range of large postural leg muscles and under submaximal voluntary activation ) . Submaximal voluntary plantar flexion ( PF(v ) ) and dorsiflexion ( DF(v ) ) activation was maintained by providing direct visual feedback of the EMG from soleus or tibialis anterior , respectively . RFE was also examined during electrical stimulation of the plantar flexion muscles ( PF(s ) ) . Constant-velocity stretches ( 15 degrees /s ) were applied through a range of motion of 15 degrees using a custom-built ankle torque motor . The muscles remained active throughout the stretch and for at least 10 s after the stretch . In all three activation conditions , the stable joint torque measured 9 - 10 s after the stretch was greater than the isometric joint torque at the final joint angle . When expressed as a percentage of the isometric torque , RFE values were 7 , 13 , and 12 % for PF(v ) , PF(s ) , DF(v ) , respectively . These findings indicate that RFE is a characteristic of human skeletal muscle and can be observed during submaximal ( 25 % ) voluntary activation when stretches are applied on the ascending limb of the force-length curve . Although the underlying mechanisms are unclear , it appears that sarcomere popping and passive force enhancement are insufficient to explain the presence of RFE in these experiments The purpose of this study was to investigate whether the isometric muscle force , redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle , is smaller than the purely isometric muscle force at the corresponding length .  Isometric knee extensor moments , surface electromyographic ( EMG ) signals of quadriceps femoris , and interpolated twitch moments ( ITMs )  were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles .  It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole , whereas the corresponding EMG and ITM values were the same . This study is the first to demonstrate force depression following muscle shortening for voluntary contractions . We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM Although residual force enhancement ( RFE ) , i.e. enhanced force after active muscle stretch , is shown to be present in voluntarily activated human muscles , its relevance for everyday human movement is still elusive . Natural human motion is mainly composed of voluntarily submaximally activated muscle contractions driving coordinated multi-joint movements . Up to now there has been no study that directly investigated the presence of RFE following stretch when performing a submaximal multi-joint movement . For this purpose , n=13 subjects performed feedback controlled bilateral leg extensions at the level of 30 % maximum voluntary activation in a motor-driven leg press dynamometer . Isometric-eccentric-isometric and purely isometric contractions were arranged in a r and omized experimental protocol . Kinematics , forces and muscular activity were measured using optical motion tracking , 3d force plates and EMG of 9 lower extremity muscles . ANOVA identified significant RFE of external reaction force , and knee extension and plantar flexion torque ( calculated by inverse dynamics ) . Enhanced force and torque ranged between 3 % and 22 % and was present for up to 22s post-stretch . In spite of motor redundancy for solving a given task , no differences between contraction conditions were observed for any of the analyzed muscles , except for tibialis anterior . On the basis of our results , RFE is present in everyday alike human movement and might be an evolutionary optimization mechanism to enhance muscular performance at a given amount of energetic effort Muscle force is potentiated by countermovement ; this phenomenon is called stretch-shortening cycle ( SSC ) effect . In this study , we examined the factors strongly related to SSC effect in vivo , focusing on tendon elongation , preactivation , and residual force enhancement . Twelve healthy men participated in this study . Ankle joint angle was passively moved by a dynamometer , with a range of motion from 15 ° dorsiflexion ( DF ) to 15 ° plantarflexion ( PF ) . Muscle contraction was evoked by electrical stimulation , with stimulation timing adjusted to elicit three types of contraction : ( 1 ) concentric contraction without preliminary contraction ( CON ) , ( 2 ) concentric contraction after preliminary eccentric contraction ( ECC ) , and ( 3 ) concentric contraction after preliminary isometric contraction ( ISO ) . Joint torque was recorded at DF5 ° , PF0 ° , and PF5 ° , respectively . SSC effect was calculated as the ratio of joint torque obtained in ECC or ISO with respect to that obtained in CON at the aforementioned three joint angles . SSC effect was prominent in the first half of movement in both ECC ( DF5 ° , 329.3 ± 101.2 % ; PF0 ° , 159.2 ± 29.4 % ; PF5 ° , 125.5 ± 20.8 % ) and ISO ( DF5 ° , 276.4 ± 87.0 % ; PF0 ° , 134.5 ± 24.5 % ; PF5 ° , 106.8 ± 18.0 % ) conditions . SSC effect was significantly larger in ECC than in ISO at all joint angles ( P < 0.001 ) . Even without preliminary eccentric contraction ( i.e. , ISO condition ) , SSC effect was clearly large , indicating that a significant part of SSC effect is derived from preactivation . However , the active lengthening-induced force potentiation mechanism ( residual force enhancement ) also contributes to SSC effect For electrically stimulated muscles , it has been observed that maximal muscle force during and after stretch is substantially greater than the corresponding isometric force . However , this observation has not been made for human voluntary contractions . We investigated the effects of active muscle stretch on muscle force production for in vivo human adductor pollicis ( n = 12 ) during maximal voluntary contractions and electrically induced contractions . Peak forces during stretch , steady-state isometric forces following stretch , and passive forces following muscle deactivation were compared to the corresponding isometric forces obtained at optimal muscle length . Contractions with different stretch magnitudes ( 10 , 20 , and 30 deg at a constant speed of 10 deg s(-1 ) ) and different speeds ( 10 , 20 , and 60 deg s(-1 ) over a range of 30 deg ) were performed in triplicate in a r and om order , balanced design . We found three novel results : ( i ) there was steady-state force enhancement following stretch in voluntarily contracted muscles ; ( ii ) some force enhancement persisted following relaxation of the muscle and ( iii ) force enhancement , for some stretch conditions , exceeded the maximum isometric force at optimal muscle length . We conclude from these results that voluntary muscle contraction produces similar force enhancement to that observed in the past with electrically stimulated preparations . Therefore , steady-state force enhancement may play a role in everyday movements . Furthermore , these results suggest that non-uniformities in sarcomere length do not , at least not exclusively , account for the force enhancement following active muscle stretch , and that the stretch magnitude-dependent passive force enhancement observed here may be responsible for the enhancement of force above the isometric reference force at optimal muscle length OBJECTIVE : To test the feasibility of creating a valid and reliable checklist with the following features : appropriate for assessing both r and omised and non-r and omised studies ; provision of both an overall score for study quality and a profile of scores not only for the quality of reporting , internal validity ( bias and confounding ) and power , but also for external validity . DESIGN : A pilot version was first developed , based on epidemiological principles , review s , and existing checklists for r and omised studies . Face and content validity were assessed by three experienced review ers and reliability was determined using two raters assessing 10 r and omised and 10 non-r and omised studies . Using different raters , the checklist was revised and tested for internal consistency ( Kuder-Richardson 20 ) , test-retest and inter-rater reliability ( Spearman correlation coefficient and sign rank test ; kappa statistics ) , criterion validity , and respondent burden . MAIN RESULTS : The performance of the checklist improved considerably after revision of a pilot version . The Quality Index had high internal consistency ( KR-20 : 0.89 ) as did the subscales apart from external validity ( KR-20 : 0.54 ) . Test-retest ( r 0.88 ) and inter-rater ( r 0.75 ) reliability of the Quality Index were good . Reliability of the subscales varied from good ( bias ) to poor ( external validity ) . The Quality Index correlated highly with an existing , established instrument for assessing r and omised studies ( r 0.90 ) . There was little difference between its performance with non-r and omised and with r and omised studies . Raters took about 20 minutes to assess each paper ( range 10 to 45 minutes ) . CONCLUSIONS : This study has shown that it is feasible to develop a checklist that can be used to assess the method ological quality not only of r and omised controlled trials but also non-r and omised studies . It has also shown that it is possible to produce a checklist that provides a profile of the paper , alerting review ers to its particular method ological strengths and weaknesses . Further work is required to improve the checklist and the training of raters in the assessment of external validity It has been well recognized that the steady-state isometric force after active muscle/fiber stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions ( MVC ) . However , recent evidence obtained for sub-MVC suggests that force enhancement properties are different from those observed for electrically induced and MVC . Specifically , it appears that force enhancement is activation-dependent and that there is a subject-specific threshold for force enhancement in sub-MVC . To address these suggestions , the relationship between force enhancement and voluntary activation during stretch was investigated in 11 healthy subjects . Human adductor pollicis muscles were studied and force enhancement was measured while muscle activation during the steady-state isometric phase was controlled at a level of 30 % of MVC . In order to study the effects of activation on force enhancement , subjects performed stretch contractions at 0 , 10 , 30 , 60 , and 100 % of maximal voluntary effort while the steady-state isometric force after stretch , obtained at 30 % of activation in all cases , was compared to the corresponding values measured in the isometric reference contractions . There was no force enhancement if muscle stretching occurred passively but all subjects showed force enhancement when muscle stretching occurred at maximal voluntary effort . When increasing the level of activation during the stretch phase , force enhancement increased , and the number of subjects who showed force enhancement increased as well . We conclude from these results that force enhancement during voluntary contractions is activation-dependent with a threshold that is subject-specific 
    Output:
    
Muscle performance is dependent on shortening velocity , shortening distance , stretch distance , the time ( transition phase ) between stretch and shortening and the active prephase duration .
The findings from this systematic review indicate that the mechanisms in the early phase of shortening are associated with pre-activation effects , elastic recoil and stretch reflex .
Furthermore , we speculate that residual force enhancement ( RFE ) is mainly responsible for an increased steady-state force compared to a pure shortening contraction