***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: We present results from a study of the 6-to 12-Hz  force tremor  in relation to motor unit ( MU ) firing synchrony . Our experimental observations from 32 subjects , 321 contractions , and 427 recorded MUs reveal that tremor is accompanied by corresponding , in-phase MU rhythms that are additional to the ones at the MU intrinsic firing rates . This rhythmical synchrony is widespread and has a uniform strength that ranges from near zero to very large ( MU/MU coherence > 0.50 ) in different contractions . Both the synchrony and the tremor are suppressed during ischemia , and this strongly suggests an involvement of spindle feedback in their generation . Furthermore , in the presence of substantial synchrony , the tremor enhancement , relative to the minimal tremor of ischemia , reflects the strength of the synchrony . Theoretical considerations based on these observations indicate that the muscle force signal is expected to show 1 ) frequency components in the b and of the firing rates of the last-recruited , large MUs , and 2 ) because of the synchronized MU rhythms , an additional , distinct component with a size reflecting the strength of synchrony . Furthermore , synchronized MU rhythms , with frequencies in the 6- to 12-Hz range , are expected to arise from self-oscillations in the monosynaptic stretch reflex loop , due primarily to the associated muscle delay ( several tens of milliseconds ) . Our results therefore reveal the parallel action of two tremor mechanisms , one of which involves MU synchrony probably caused by loop action . Clearly , the results on the synchrony and its impact also apply to other possible generators of tremor synchrony , including supraspinal ones Summary Methods that objective ly assess the muscle fibre-type arrangement may improve the detection of fibre-type grouping , a diagnostic sign of a denervation and reinnervation process . To distinguish between a diseased and a normal muscle , there is a need for quantitative data on the fibre-type arrangement in healthy human muscles at different ages . In this study , cross-sections were prepared of whole autopsied vastus lateralis muscle  from 24 previously physically healthy men , aged 15 to 83 years . The arrangements of type 1 and type 2 fibres were assessed in terms of the number of enclosed fibres in individual fascicles throughout each muscle . Recent improvements to the enclosed fibre method were used to define measures of r and omness which facilitated the combination of several sample areas and the quantification of the fibre-type arrangements . Segregation was typical for young muscles , r and omness was most common between 30 and 50 years of age , while some fibre-type grouping was considered “ normal ” in old muscles . The arrangements of type 1 and type 2 fibres were quantitatively similar , irrespective of the age of the individual . The results imply that the fibre population changes considerably during a lifetime , and that it undergoes a continuous denervation and reinnervation process with normal ageing . Because of its importance , age should be accommo date d in the analysis of a muscle sample , irrespective of the statistical model and method used 1 . Interspike interval distributions from human motor units of a variety of muscles were analysed to assess the role of synaptic noise in excitation . The time course of the underlying post-spike after-hyperpolarization ( AHP ) was deduced by applying a specially developed transform to the interval data . Different firing rates were studied both by varying the firing voluntarily , and by selecting sub population s of spikes for a given firing rate from long recordings with slight variations in frequency . 2 . At low firing rates the interval histograms had an exponential tail . Thus at long intervals , the motoneurone was r and omly excited by noise and its post-spike AHP was complete . This contrasts with the firing produced by intracellular current injection in the cat , when the membrane potential increases linearly until threshold is reached . The interval histogram was therefore analysed with the aid of a model of synaptic excitation to deduce the mean ' trajectory ' of membrane voltage in the last part of the interspike interval . 3 . The computer model , described in the Appendix , was used to determine the effect of the mean level of membrane potential on the probability of a spike being excited , per unit time , during an on-going interspike interval . All variables were treated as voltages , with synaptic noise simulated by time-smoothed Gaussian noise . This enabled an interval distribution to be transformed into a segment of the underlying trajectory of the membrane potential ; the potential was expressed in terms of the noise amplitude and the spike threshold . 4 . At low firing rates , the equilibrium value of the membrane voltage trajectory lay well below threshold ; the deviation typically corresponded to the st and ard deviation of the noise or more . The noise st and ard deviation was estimated to be about 2 mV. 5 . With increasing mean firing rate , the near-threshold portion of the trajectory obtainable from the histogram occurred earlier , was steeper and rose to a higher level . Trajectories for different firing rates fell on the same curve after shifting them vertically by varying amounts . The curve was taken to represent the AHP of the motoneurone and was closely exponential . The shift of the trajectory gave its mean synaptic drive . The duration of the AHP varied between units and was longer than average for units from soleus muscle . 6 . Further modelling showed that summation of noise with the AHP can explain the well-known changes in discharge variability that occur as firing rate increases . 7 . It is concluded that synaptic noise plays a major role in the excitation of tonically firing human motoneurones and that the noiseless motoneurone with a linear trajectory provides an inadequate model for the conscious human . This is of interest in relation to various st and ard measures of human motor unit activity such as short-term synchronization There are conflicting reports of the effects of hormone replacement therapy ( HRT ) on strength preservation in postmenopausal women , while any effect on power output has received little attention . Decreased steadiness of force generation has been reported in older muscles and may be related to the hormonal changes associated with the menopause , but the effect of HRT has not been investigated . We have studied the effect of HRT on strength , power output and isometric force steadiness in healthy women . Sixteen young ( aged 27.4±1.4 years , mean ± SEM ) and 29 postmenopausal women were studied . Fifteen of the latter were taking HRT ( 68.1±1.4 years , HRT+  ) and 14 ( 70.5±1.5 years , HRT− ) had never done so . During isometric quadriceps contractions the force steadiness ( coefficient of variation of force ) was measured at 10 , 25 , 50 and 100 % maximum voluntary contraction ( MVC ) . The average power generated by an explosive leg extension was recorded . The HRT− group generated less power ( 110.2±7.2 W ) than both the HRT+ ( 136.5±10.9 W , P=0.027 ) and young ( 136.2±5.8 W , P=0.027 ) subjects . Power output was similar in the HRT+ and younger subjects . The HRT− subjects were weaker than the younger ones ( 241.3±14.0 N vs. 297.6±13 N , P=0.006 ) . The strength of the HRT+ group ( 255.5±14 N ) was not significantly different to the other two groups . There was no difference in steadiness between the three groups at any of the force levels . HRT appears to maintain power output to a greater extent than isometric strength in postmenopausal women . There was no evidence for an effect of either age or HRT  on isometric steadiness in the quadriceps The purpose of this study was to investigate the effect of altered local temperature on soleus H-reflex and compound muscle action potential ( M wave ) in young and older women . H-reflex and M wave responses were elicited in 10 young ( 22.3±3.3 years ) and 10 older ( 72.5±3.2 years ) women at three muscle temperatures : control ( 34.2±0.3 ° C ) , cold ( 31.3±0.5 ° C ) and warm ( 37.1±0.2 ° C ) . H-reflex output , expressed as the ratio between maximal H-reflex and maximal M wave ( Hmax/Mmax ) , was lower in the older , compared with the younger , group , regardless of temperature . In control temperature conditions , for example , the Hmax/Mmax ratio was 36.8±24 % in the young and 25.4±20 % in the older ( P<0.05 ) . Warming had no effect on the H-reflex output in either group , whilst cooling increased H-reflex output only in the younger group ( + 28 % ) . In both groups ,  cooling increased ( + 5.3 % ) , and warming decreased ( −5.5 % ) the H-reflex latency . This study confirms that older individuals experience a reduced ability to modulate the reflex output in response to a perturbation . In a cold environment , for example , the lack of facilitation in the reflex output , along with a delayed reflex response could be critical to an older individual in responding to postural perturbations thus potentially compromising both static and dynamic balance The firing behavior of  motor units ( MUs ) of the first dorsal interrosseus muscle of the h and was examined during both constant-force and varying-force ( sinusoidal or broadb  and r and om variations ) isometric contractions in healthy adults . The emphasis was on the analysis of MU synchrony with an efficient and sensitive method . In static contractions , widespread and strong MU firing correlations , with the MUs in phase , were present at the frequency of muscle tremor , when the tremor was regular ( narrowb and ) and large . MU correlations could also exist in contractions where the tremor of a subject was irregular ( broadb and ) overall , but they were generally weak . These correlations were at the frequency of the subject 's regular tremor , and the corresponding distinct tremor component was sometimes discernible within the broad tremor-b and . In contrast , the MUs did not show any such correlations in the case of purely irregular and small tremor . On the basis of these observations , it is concluded that the rhythms in the force contributions of the last- recruited , large MUs , which fire near their threshold rate , compose the broadb and frequency content of physiological muscle tremor in every contraction . Within this b and , there is an additional distinct tremor component when MU correlations are present . For widespread and strong MU correlations , this component dominates and constitutes the observed regular tremor . In dynamic contractions , the firing of all MUs was modulated in the frequency b and of both the sinusoidal and the complex variations of the force . The MU modulations showed a time-lead over the force variations and were strongly correlated both to these variations and among themselves . Thus widespread and strong correlations of MU firing modulations seem to provide a mechanism for generation of time-varying voluntary force , under general dynamic conditions . Finally , when regular tremor was present in dynamic contractions , widespread and fairly strong MU correlations also existed at the tremor frequency . It is concluded that at least two mechanisms can cause widespread MU synchrony , and they can act in parallel . They involve two types of correlated inputs to the alpha-motoneurons ( presumably from the muscle spindles and the cortex ) , whose effects combine at the level of the membrane potential of the cells Humans are able to adjust the accuracy of their movements to the dem and s posed by the task at h and . The variability in task execution caused by the inherent noisiness of the neuromuscular system can be tuned to task dem and s by both feedforward ( e.g. , impedance modulation ) and feedback mechanisms . In this experiment , we studied both mechanisms , using mechanical perturbations to estimate stiffness and damping as indices of impedance modulation and submovement scaling as an index of feedback driven corrections .  Eight subjects tracked three differently sized targets ( 0.0135 , 0.0270 , and 0.0405 rad )  moving at three different frequencies  ( 0.20 , 0.25 , and 0.33 Hz ) . Movement variability decreased with both decreasing target size and movement frequency , whereas stiffness and damping increased with decreasing target size , independent of movement frequency . These results are consistent with the theory that mechanical impedance acts as a filter of noisy neuromuscular signals but challenge stochastic theories of motor control that do not account for impedance modulation and only partially for feedback control . Submovements during unperturbed cycles were quantified in terms of their gain , i.e. , the slope between their duration and amplitude in the speed profile . Submovement gain decreased with decreasing movement frequency and increasing target size . The results were interpreted to imply that submovement gain is related to observed tracking errors and that those tracking errors are expressed in units of target size . We conclude that impedance and submovement gain modulation contribute additively to tracking accuracy 1 . Isometric muscle force and the  surface electromyogram ( EMG )  were simulated from a model that predicted recruitment and firing times in a pool of 120 motor units under different levels of excitatory drive . The EMG-force relationships that emerged from simulations using various schedules of recruitment and rate coding were compared with those observed experimentally to determine which of the modeled schemes were plausible representations of the actual organization in motor-unit pools . 2 . The model was comprised of three elements : a motoneuron model , a motor-unit force model , and a model of the surface EMG . Input to the neuron model was an excitatory drive function representing the net synaptic input to motoneurons during voluntary muscle contractions . Recruitment thresholds were assigned such that many motoneurons had low thresholds and relatively few neurons had high thresholds . Motoneuron firing rate increased as a linear function of excitatory drive between recruitment threshold and peak firing rate levels . The sequence of discharge times for each motoneuron was simulated as a r and om renewal process . 3 . Motor-unit twitch force was estimated as an impulse response of a critically damped , second-order system . Twitch amplitudes were assigned according to rank in the recruitment order , and twitch contraction times were inversely related to twitch amplitude . Nonlinear force-firing rate behavior was simulated by varying motor-unit force gain as a function of the instantaneous firing rate and 
    Output:
    
No relations were found of associations between age difference , upper vs. lower extremity muscle location , and dominance and effect size .
Significant relations of effect size with muscle group , proximal vs. distal muscle location and target force level were found .
Also , an interaction effect of muscle group and target force level was suggested .
The meta- analysis results are in line with motor unit loss as the main cause of the effect of ageing on force steadiness and this effect can partially explain decreased motor performance associated with ageing