***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: Background Mental fatigue is a psychobiological state induced by sustained periods of dem and ing cognitive activity and is characterized by feelings of tiredness which are common in everyday life . Recently , it has been hypothesized that mental fatigue might have an impact on gait performance in old adults . Therefore , the effect of mental fatigue on gait performance under single- and dual-task conditions was investigated in young and old participants . Methods Spatio-temporal gait parameters of 16 young and 16 old healthy participants were measured using a photoelectric system during single- and dual-task walking before and after a r and omly assigned mental fatigue ( performing a stop-signal task for 90 minutes ) and control intervention ( watching a video for 90 minutes ) , respectively . Changes in subjective fatigue , wakefulness , mood , arousal , and psychophysiological workload ( heart rate variability indices ) were assessed . Results Psychometric measures indicated increased subjective fatigue and arousal as well as decreased mood and wakefulness after the mental fatigue task . Heart rate variability indices revealed a higher psychophysiological workload during the mental fatigue intervention in old compared to young participants . Gait measures ( coefficient of variation of speed , stride length , and stance time ) revealed impaired dual-task walking performance following the mental fatigue intervention only in old participants . Conclusion Data indicate that mental fatigue , induced by sustained cognitive activity , can impair gait performance during dual-task walking in old adults . The susceptibility to mental fatigue could be a new intrinsic risk factor for falls in older people and should be taken into account when dual-task gait analyses are performed The purpose of this study was to quantify changes in postural stability with fatigue of the frontal and sagittal movers of the lower extremities . There were four test sessions , with a r and omized order assigned according to the muscles tested and the plane of motion . Subjects were 20 healthy men ( age : 22.6+/-2.4 years , height : 173.7+/-3.6 cm , weight : 63.3+/-7.9 kg ) . During each session , one set of muscle groups was fatigued using isokinetic contractions : ankle plantar/dorsi flexors , ankle evertor/invertors , hip flexor/extensors or hip abductor/adductors . The Biodex Stability System was used to assess anterior/posterior and medial/lateral stability before and after muscle fatigue . Repeated measures ANOVAs revealed that fatigue was associated with a significant increase in all stability indices . Fatigue of the hip movers , whether in the frontal or sagittal planes , led to greater increments in stability indices than fatigue of the ankle musculature . Fatigue of the frontal movers result ed in greater increases in the medial/lateral stability index compared to fatigue of the sagittal movers . In conclusion , fatigue of proximal lower extremity muscles affects postural stability and fatigue of the frontal movers is associated with postural instability in the frontal plane Repeated isometric or shortening contractions of skeletal muscle cause muscle fatigue but several prior studies have reported an apparent absence of muscle fatigue when humans performed up to 70 lengthening contractions . We pursued the hypothesis that perhaps muscle excitability is a factor that aids force preservation with repeated eccentric actions . Soleus compound muscle action potential ( M-wave ) latency , peak-to-peak amplitude ( PPA ) , duration , and area under the curve were examined in 12 subjects ( mean age 24.3 y ) over 4 testing days that included : no exercise , isometric exercise ( neutral ankle angle ) , isokinetic ( 0.5 rad.s-1 ) concentric and eccentric exercise of the plantar flexors in the seated position on a Biodex dynamometer . Supramaximal shocks were delivered to the tibial nerve in the popliteal fossa at baseline ( 3 shocks , 1 min apart ) , during exercise ( 1 shock-after each of 5 bouts/10 contractions ) , and during 10-min recovery . From initial to final contractions , concentric , isometric , and eccentric fatigue was -32 , -41 and + 2 % ( Condition by Trial Interaction , F2,22 = 25.1 , p = 0.000 ) . No changes occurred in latency or duration ( p > 0.05 ) , but PPA ( Condition by Time interaction , F51,561 = 3.7 , p = 0.000 ) increased during isometric and eccentric exercise and remained elevated during recovery . Area increased ( F51,561 = 3.1 , p = 0.000 ) significantly during all three exercise conditions and approximated baseline by minute 8 of recovery . It was concluded that although the potentiation of the action potential of individual muscle fibers seems to be the common mechanism underlying the increase in muscle excitability during plantar flexion exercise , it is possible that different factors could cause such a non-specific response Tripping over obstacles is a common cause of falls in older adults , and muscle fatigue , which can alter walking patterns , may add to this risk . To date , no study has examined the effect of lower limb muscle fatigue on obstacle negotiation in older adults . 30 older adults ( 13 women , aged 78.3 [ 6.2 ] years )  negotiated a 12 m obstacle course , while completing a visual secondary task , under two r and omized conditions : rested or fatigued . For the fatigue condition , participants performed a repeated sit-to-st and movement , as fast as possible , until they could no longer continue . Participants then immediately began walking trials . Kinematic and kinetic data were collected on approach to , during , and after crossing a height-adjustable target obstacle ( 10 % and 20 % of leg length ) . Repeated measures ANOVA showed a statistically significant increase in lead limb vertical loading rate after stepping over the 10 % obstacle when fatigued , relative to rested ( P=0.046 ) . No other significant between-condition differences ( > 0.05 ) were observed for the other kinematic variables when negotiating the 10 % obstacle . Furthermore , no significant between-condition differences ( P>0.05 ) were observed for any kinetic or kinematic variables when negotiating the 20 % obstacle . This study describes a feasible method for investigating the consequences of lower limb muscle fatigue on obstacle crossing . The current finding of increased vertical loading rate when fatigued supports the need for further investigation into the effect of muscle fatigue on gait under different environmental conditions , fatiguing a range of muscles , analyzing a more comprehensive array of kinetic and kinematic measures , and in healthy and clinical population BACKGROUND AND PURPOSE Research ers study ing central activation of muscles in elderly subjects ( > or = 65 years of age ) have investigated activation in only the nonfatigued state . This study examined the ability of young and elderly people to activate their quadriceps femoris muscles voluntarily under both fatigued and nonfatigued conditions to determine the effect of central activation failure on age-related loss of force . SUBJECTS AND METHODS Twenty young subjects ( 11 men , 9 women ; mean age = 22.67 years , SD = 4.14 , range = 18 - 32 years ) and 17 elderly subjects ( 8 men , 9 women ; mean age = 71.5 years , SD = 5.85 , range = 65 - 84 years ) participated in this study . Subjects were seated on a dynamometer and stabilized .  Central activation was quantified , based on the change in force produced by a 100-Hz , 12-pulse electrical train that was delivered during a 3- to 5-second isometric maximum voluntary contraction ( MVC ) of the quadriceps femoris muscle . Next , subjects performed 25 MVCs ( a 5-second contraction with 2 seconds of rest ) to fatigue the muscle . During the last MVC , central activation was measured again . RESULTS In the nonfatigued state , elderly subjects had lower central activation than younger subjects . In the fatigued state , this difference became larger . DISCUSSION AND CONCLUSION Central activation of the quadriceps femoris muscle in elderly subjects was reduced in both the fatigued and nonfatigued states when compared with young subjects . Some part of age-related weakness , therefore , may be attributed to failure of central activation in both the fatigued and nonfatigued states This study assessed effects of unilateral leg muscle fatigue ( ULMF ) on balance control in gait during the stance and swing phases of the fatigued leg in healthy elderly , to test the assumption that leg muscle strength limits balance control during the stance-phase . Ten subjects ( aged 63.4 , SD 5.5 years ) walked on a treadmill in 4 conditions : unperturbed unfatigued , unperturbed fatigued , perturbed unfatigued , and perturbed fatigued . The perturbations were lateral trunk pulls just before contralateral heel contact . ULMF was evoked by unilateral squat exercise until task failure . Isometric knee extension strength was measured to verify the presence of muscle fatigue . Between-stride st and ard deviations and Lyapunov exponents of trunk kinematics were used as indicators of balance control . Required perturbation force and the deviation of trunk kinematics from unperturbed gait were used to assess perturbation responses . Knee extension strength decreased considerably ( 17.3 % SD 8.6 % ) as a result ULMF . ULMF did not affect steady-state gait balance . Less force was required to perturb subjects when the fatigued leg was in the stance-phase compared to the swing-phase . Subjects showed a faster return to the unperturbed gait pattern in the fatigued than in the unfatigued condition , after perturbations in swing and stance of the fatigued leg . The results of this study are not in line with the hypothesized effects of leg muscle fatigue on balance in gait . The healthy elderly subjects were able to cope with substantial ULMF during steady-state gait and demonstrated faster balance recovery after laterally directed mechanical perturbations in the fatigued than in the unfatigued condition ABSTRACT Purpose To develop and vali date a novel model for assessment of peripheral fatigue progression during dynamic exercise . Methods Seven males and four females ( 24 ± 3 yr ) completed one-legged knee-extensor exercise  at 40 % of peak power output ( PPO ) for 12 min ( 40 % PPO ) . Additionally , an exhaustive bout lasting approximately 6 min ( 84 % ± 2 % PPO ; 59 ± 10 W ) was contrasted to two bouts completed at ±5 % of the target workload . Trials were completed in r and omized order . Percutaneous electrical stimulation of m. vastus lateralis during the passive knee-flexion phase allowed quantification of maximal twitch force every 30 s in parallel with RPE . Results Elicited twitch force remained unchanged during the 40 % PPO trial . During the three exhaustive bouts , exercise differed in duration ( 561 ± 154 s , 366 ± 64 s , 245 ± 61 s ; P < 0.001 ) and amplitude of elicited twitch force showed a curvilinear decline across time . Elicited twitch force at exhaustion was approximately 60 % reduced and similar between the exhaustive trials ( intraclass correlation coefficient , 0.76 ; 95 % confidence interval , 0.48–0.92 ) . The increase in RPE during exercise was strongly correlated to the gradually reduced evoked twitch force ( repeated- measures correlation , 0.89 ; 95 % confidence interval , 0.62–0.97 ) . Conclusions The developed model permits quantification of muscle fatigue progression during continuous dynamic one-legged knee-extension and a biphasic fatigue pattern is demonstrated during intense exercise . The model is sensitive to small changes in intensity , and it provides a novel approach for study ing muscular mechanisms and their temporal relation to fatigue progression in vivo 
    Output:
    
Conclusion Muscle performance fatigability affects spatial and temporal features of gait and , mainly , inter-stride trunk acceleration variability .
In contrast , sustained mental activity tends only to affect step variability during dual tasking .