Visual Information Processing in Older Adults: Force Control and Motor Unit Pool Modulation.

Increased visual information about a task impairs force control in older adults. To date, however, it remains unclear how increased visual information changes the activation of the motor unit pool differently for young and older adults. Therefore, this study aimed to determine how increased visual information alters the activation of the motor neuron pool and influences force control in older adults. Fifteen older adults (66-86 years, seven women) and fifteen young adults (18-30 years, eight women) conducted a submaximal constant force task (15% of maximum) with ankle dorsiflexion for 20 s. The visual information processing was manipulated by changing the amount of force visual feedback into a low-gain (0.05°) or high-gain (1.2°) condition. Older adults exhibited greater force variability, especially at high-gain visual feedback. This exacerbated force variability from low- to high-gain visual feedback was associated with modulations of multiple motor units, not single motor units. Specifically, increased modulation of multiple motor units from 10 to 35 Hz may contribute to the amplification in force variability. Therefore, our findings suggest evidence that high-gain visual feedback amplifies force variability of older adults which is related to increases in the activation of motor neuron pool from 10 to 35 Hz.

Fatigability of the knee extensor muscles during high-load fast and low-load slow resistance exercise in young and older adults.

Resistance exercise training is a cornerstone in preventing age-related declines in muscle mass and strength, and fatigability of limb muscle is important to this adaptive response. It is unknown, however, whether fatigability and the underlying mechanisms differ between different resistance exercise protocols in young and older adults. The purpose of this study was to quantify the fatigability of the knee extensors and identify the mechanisms in 20 young (22.2 ± 1.3 yr, 10 women) and 20 older adults (73.8 ± 5.4 yr, 10 women) elicited by a single session of high- and low-load resistance exercise. One leg completed a high-load protocol with contractions performed as fast as possible (HL-fast, ~80% 1 Repetition Max, 1RM), and the contralateral leg a low-load protocol performed with slow contractions (LL-slow, ~30% 1RM, 6 s concentric, 6 s eccentric). Each exercise involved four sets of eight repetitions. Before and immediately following each set, maximal voluntary isometric contractions (MVC) were performed, and voluntary activation and contractile properties quantified using electrical stimulation. The reduction in MVC was greater following the LL-slow (20%) than the HL-fast (12%, P = 0.004), with no age or sex differences. Similarly, the reduction in the amplitude of the involuntary electrically-evoked twitch was greater in the LL-slow (14%) than the HL-fast (7%, P = 0.014) and correlated with the reduction in MVC (r = 0.546, P < 0.001), whereas voluntary activation decreased only for the LL-slow protocol (5%, P < 0.001). Thus, low-load resistance exercise with slow contractions induced greater fatigability within the muscle than a more traditional high-load resistance protocol for both young and older men and women.

Attenuated activation of knee extensor muscles during fast contractions in older men and women.

AIM: Reduced physical function and increased risk of falls in older adults are accompanied by age-related reductions in torque development of leg muscles, although the mechanisms and potential sex differences are not understood. PURPOSE: To determine the mechanistic origins (neural vs. muscular) for the age-related reduction in torque development, we compared the peak rates of torque development (RTD) during electrically-evoked and fast voluntary contractions of the knee extensors between young and older men and women. METHODS: Sets of single- and double-pulse electrical stimulations evoked contractions of the knee extensor muscles in 20 young (23.0 ± 0.8 years; 10 women) and 20 older adults (78.2 ± 1.5 years; 10 women), followed by voluntary isometric knee extension contractions with torque development as fast as possible that matched the torque during electrically-evoked contraction (10-40% maximal torque). RESULTS: Peak RTD during fast-voluntary contractions was 41% less than electrically-evoked contractions (p < 0.001), but more so for older adults (44%) than young (38%, p = 0.04), with no sex differences. Peak RTD during fast-voluntary contractions was more variable between contractions for the older than young adults (77%MVC s-1 vs. 47%MVC s-1, p < 0.001). Additionally, older women exhibited greater variability than older men (81%MVC s-1 vs. 72%MVC s-1, p = 0.04) with no sex-related differences within the young adults. CONCLUSION: Older adults had slower and more variable RTD during voluntary contractions than young adults, particularly older women. The limited age-related differences in electrically-evoked RTD suggest the primary mechanism for the slower torque development of the knee extensor muscles in older men and women involve reduced neural activation.

Visual information processing in older adults: reaction time and motor unit pool modulation.

Presently, there is no evidence that magnification of visual feedback has motor implications beyond impairments in force control during a visuomotor task. We hypothesized that magnification of visual feedback would increase visual information processing, alter the muscle activation, and exacerbate the response time in older adults. To test this hypothesis, we examined whether magnification of visual feedback during a reaction time task alters the premotor time and the motor unit pool activation of older adults. Participants responded as fast as possible to a visual stimulus while they maintained a steady ankle dorsiflexion force (15% maximum) either with low-gain or high-gain visual feedback of force. We quantified the following: 1) response time and its components (premotor and motor time), 2) force variability, and 3) motor unit pool activity of the tibialis anterior muscle. Older adults exhibited longer premotor time and greater force variability than young adults. Only in older adults, magnification of visual feedback lengthened the premotor time and exacerbated force variability. The slower premotor time in older adults with high-gain visual feedback was associated with increased force variability and an altered modulation of the motor unit pool. In conclusion, our findings provide novel evidence that magnification of visual feedback also exacerbates premotor time during a reaction time task in older adults, which is correlated with force variability and an altered modulation of motor unit pool. Thus these findings suggest that visual information processing deficiencies in older adults could result in force control and reaction time impairments. NEW & NOTEWORTHY It is unknown whether magnification of visual feedback has motor implications beyond impairments in force control for older adults. We examined whether it impairs reaction time and motor unit pool activation. The findings provide novel evidence that magnification of visual feedback exacerbates reaction time by lengthening premotor time, which implicates time for information processing in older adults, which is correlated with force variability and an altered modulation of motor unit pool.

Neuromuscular variability and spatial accuracy in children and older adults.

Our ability to control movements is influenced by the developmental status of the neuromuscular system. Consequently, movement control improves from childhood to early adulthood but gradually declines thereafter. However, no study has compared movement accuracy between children and older adults. The purpose of this study was to compare endpoint accuracy during a fast goal-directed movement task in children and older adults. Ten pre-adolescent children (9.7 ±â€¯0.67 yrs) and 19 older adults (71.95 ±â€¯6.99 yrs) attempted to accurately match a peak displacement of the foot to a target (9° in 180 ms) with a dorsiflexion movement. We recorded electromyographic activity from the tibialis anterior (agonist) and soleus (antagonist) muscles. We quantified position error (i.e. spatial accuracy) as well as the coordination, magnitude, and variability of the antagonistic muscles. Children exhibited greater position error than older adults (36.4 ±â€¯13.4% vs. 27.0 ±â€¯9.8%). This age-related difference in spatial accuracy, was related to a more variable activation of the agonist muscle (R2: 0.358; P < 0.01). These results suggest that an immature neuromuscular system, compared to an aged one, affects the generation and refinement of the motor plan which increases the variability in the neural drive to the muscle and reduces spatial accuracy in children.

Increased Force Variability Is Associated with Altered Modulation of the Motorneuron Pool Activity in Autism Spectrum Disorder (ASD).

Force control deficits have been repeatedly documented in autism spectrum disorder (ASD). They are associated with worse social and daily living skill impairments in patients suggesting that developing a more mechanistic understanding of the central and peripheral processes that cause them may help guide the development of treatments that improve multiple outcomes in ASD. The neuromuscular mechanisms underlying force control deficits are not yet understood. Seventeen individuals with ASD and 14 matched healthy controls completed an isometric index finger abduction test at 60% of their maximum voluntary contraction (MVC) during recording of the first dorsal interosseous (FDI) muscle to determine the neuromuscular processes associated with sustained force variability. Central modulation of the motorneuron pool activation of the FDI muscle was evaluated at delta (0-4 Hz), alpha (4-10 Hz), beta (10-35 Hz) and gamma (35-60 Hz) frequency bands. ASD patients showed greater force variability than controls when attempting to maintain a constant force. Relative to controls, patients also showed increased central modulation of the motorneuron pool at beta and gamma bands. For controls, reduced force variability was associated with reduced delta frequency modulation of the motorneuron pool activity of the FDI muscle and increased modulation at beta and gamma bands. In contrast, delta, beta, and gamma frequency oscillations were not associated with force variability in ASD. These findings suggest that alterations of central mechanisms that control motorneuron pool firing may underlie the common and often impairing symptoms of ASD.

Motor output oscillations with magnification of visual feedback in older adults.

Magnification of task visual feedback increases force variability in older adults. Although the increased force variability with magnified visual feedback in older adults relates to the amplification of oscillations in force below 0.5Hz, the related frequency modulation in muscle activity remains unknown. The purpose of this study, therefore, was to characterize the oscillations in muscle activity that contribute to the amplification of force variability with magnified visual feedback in older adults. Fifteen older adults (76.7±6.4years, 7 females) performed isometric contractions at 15% of maximal voluntary contraction (MVC) with ankle dorsiflexion with low-gain (0.05°) or high-gain visual feedback (1.2°). The standard deviation (SD) of force increased significantly (55%) from low- to high-gain visual feedback condition (P<0.0001), without changing the mean force (P>0.5). The increase in force variability was related to greater power in force oscillations from 0 to 0.5Hz (R2=0.37). The increase in force oscillations was associated with greater power in EMG burst oscillations from 0.5 to 1.0Hz (R2=0.50). In conclusion, these findings suggest that magnification of visual feedback alters the modulation of the motor neuron pool in older adults and exacerbates force variability by increasing the oscillations in force below 0.5Hz.

Motor control differs for increasing and releasing force.

Control of the motor output depends on our ability to precisely increase and release force. However, the influence of aging on force increase and release remains unknown. The purpose of this study, therefore, was to determine whether force control differs while increasing and releasing force in young and older adults. Sixteen young adults (22.5 ± 4 yr, 8 females) and 16 older adults (75.7 ± 6.4 yr, 8 females) increased and released force at a constant rate (10% maximum voluntary contraction force/s) during an ankle dorsiflexion isometric task. We recorded the force output and multiple motor unit activity from the tibialis anterior (TA) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) mean discharge rate and variability of discharge rate of multiple motor units; and 3) power spectrum of the multiple motor units from 0-4, 4-10, 10-35, and 35-60 Hz. Participants exhibited greater force variability while releasing force, independent of age (P < 0.001). Increased force variability during force release was associated with decreased modulation of multiple motor units from 35 to 60 Hz (R(2) = 0.38). Modulation of multiple motor units from 35 to 60 Hz was further correlated to the change in mean discharge rate of multiple motor units (r = 0.66) and modulation from 0 to 4 Hz (r = -0.64). In conclusion, these findings suggest that force control is altered while releasing due to an altered modulation of the motor units.

Differential contribution of visual and auditory information to accurately predict the direction and rotational motion of a visual stimulus.

Vision and auditory information are critical for perception and to enhance the ability of an individual to respond accurately to a stimulus. However, it is unknown whether visual and auditory information contribute differentially to identify the direction and rotational motion of the stimulus. The purpose of this study was to determine the ability of an individual to accurately predict the direction and rotational motion of the stimulus based on visual and auditory information. In this study, we recruited 9 expert table-tennis players and used table-tennis service as our experimental model. Participants watched recorded services with different levels of visual and auditory information. The goal was to anticipate the direction of the service (left or right) and the rotational motion of service (topspin, sidespin, or cut). We recorded their responses and quantified the following outcomes: (i) directional accuracy and (ii) rotational motion accuracy. The response accuracy was the accurate predictions relative to the total number of trials. The ability of the participants to predict the direction of the service accurately increased with additional visual information but not with auditory information. In contrast, the ability of the participants to predict the rotational motion of the service accurately increased with the addition of auditory information to visual information but not with additional visual information alone. In conclusion, this finding demonstrates that visual information enhances the ability of an individual to accurately predict the direction of the stimulus, whereas additional auditory information enhances the ability of an individual to accurately predict the rotational motion of stimulus.

Processing of visual information compromises the ability of older adults to control novel fine motor tasks.

We performed two experiments to determine whether amplified motor output variability and compromised processing of visual information in older adults impair short-term adaptations when learning novel fine motor tasks. In Experiment 1, 12 young and 12 older adults underwent training to learn how to accurately trace a sinusoidal position target with abduction-adduction of their index finger. They performed 48 trials, which included 8 blocks of 6 trials (the last trial of each block was performed without visual feedback). Afterward, subjects received an interference task (watched a movie) for 60 min. We tested retention by asking subjects to perform the sinusoidal task (5 trials) with and without visual feedback. In Experiment 2, 12 young and 10 older adults traced the same sinusoidal position target with their index finger and ankle at three distinct visual angles (0.25°, 1° and 5.4°). In Experiment 1, the movement error and variability were greater for older adults during the visual feedback trials when compared with young adults. In contrast, during the no-vision trials, age-associated differences in movement error and variability were ameliorated. Short-term adaptations in learning the sinusoidal task were similar for young and older adults. In Experiment 2, lower amount of visual feedback minimized the age-associated differences in movement variability for both the index finger and ankle movements. We demonstrate that although short-term adaptations are similar for young and older adults, older adults do not process visual information as well as young adults and that compromises their ability to control novel fine motor tasks during acquisition, which could influence long-term retention and transfer.

Force dysmetria in spinocerebellar ataxia 6 correlates with functional capacity.

Spinocerebellar ataxia type 6 (SCA6) is a genetic disease that causes pure cerebellar degeneration affecting walking, balance, and coordination. One of the main symptoms of SCA6 is dysmetria. The magnitude of dysmetria and its relation to functional capacity in SCA6 has not been studied. Our purpose was to quantify dysmetria and determine the relation between dysmetria and functional capacity in SCA6. Ten individuals diagnosed and genetically confirmed with SCA6 (63.7 ± 7.02 years) and nine age-matched healthy controls (65.9 ± 8.5 years) performed goal-directed isometric contractions with the ankle joint. Dysmetria was quantified as the force and time error during goal-directed contractions. SCA6 functional capacity was determined by ICARS and SARA clinical assessments. We found that SCA6 participants exhibited greater force dysmetria than healthy controls (P < 0.05), and reduced time dysmetria than healthy controls (P < 0.05). Only force dysmetria was significantly related to SCA6 functional capacity, as measured with ICARS kinetic score (R(2) = 0.63), ICARS total score (R(2) = 0.43), and SARA total score (R(2) = 0.46). Our findings demonstrate that SCA6 exhibit force dysmetria and that force dysmetria is associated to SCA6 functional capacity. Quantifying force and time dysmetria in individuals with SCA6 could provide a more objective evaluation of the functional capacity and disease state in SCA6.

High-gain visual feedback exacerbates ankle movement variability in children.

The purpose was to compare the effect of low- and high-gain visual feedback on ankle movement variability and muscle activation in children and young adults. Six young adults (19.8 ± 0.6 years) and nine children (9.4 ± 1.6 years) traced a sinusoidal target by performing ankle plantar/dorsiflexion movements. The targeted range of motion was 10°, and the frequency of the sinusoidal target was 0.4 Hz for 35 s. Low-gain visual feedback was 0.66°, and high-gain visual feedback was 4.68°. Surface EMG was recorded from the tibialis anterior (TA) muscle. Movement variability amplitude was quantified as the standard deviation of the position fluctuations after the task frequency was removed with a notch filter (second-order; 0.3-0.5 Hz). We quantified the oscillations in movement variability and TA EMG burst using the following frequency bands: 0-0.3, 0.3-0.6, 0.6-0.9, 0.9-1.2, and 1.2-1.5 Hz. Children exhibited greater movement variability than young adults, which was exacerbated during the high-gain visual feedback condition (P < 0.05). The greater ankle movement variability in children at the high-gain visual feedback condition was predicted by greater power within the 0-0.3 Hz of their movement variability (R (2) = 0.51, P < 0.001). The greater power in movement variability from 0 to 0.3 Hz in children was predicted by greater power within the 0-0.3 Hz in their TA EMG burst activity (R (2) = 0.6, P < 0.001). The observed deficiency in movement control with amplified visual feedback in children may be related to an ineffective use of visual feedback and the immaturity of the cortico-motor systems.

Force control is related to low-frequency oscillations in force and surface EMG.

Force variability during constant force tasks is directly related to oscillations below 0.5 Hz in force. However, it is unknown whether such oscillations exist in muscle activity. The purpose of this paper, therefore, was to determine whether oscillations below 0.5 Hz in force are evident in the activation of muscle. Fourteen young adults (21.07 ± 2.76 years, 7 women) performed constant isometric force tasks at 5% and 30% MVC by abducting the left index finger. We recorded the force output from the index finger and surface EMG from the first dorsal interosseous (FDI) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) power spectrum of force below 2 Hz; 3) EMG bursts; 4) power spectrum of EMG bursts below 2 Hz; and 5) power spectrum of the interference EMG from 10-300 Hz. The SD of force increased significantly from 5 to 30% MVC and this increase was significantly related to the increase in force oscillations below 0.5 Hz (R(2) = 0.82). For both force levels, the power spectrum for force and EMG burst was similar and contained most of the power from 0-0.5 Hz. Force and EMG burst oscillations below 0.5 Hz were highly coherent (coherence = 0.68). The increase in force oscillations below 0.5 Hz from 5 to 30% MVC was related to an increase in EMG burst oscillations below 0.5 Hz (R(2) = 0.51). Finally, there was a strong association between the increase in EMG burst oscillations below 0.5 Hz and the interference EMG from 35-60 Hz (R(2) = 0.95). In conclusion, this finding demonstrates that bursting of the EMG signal contains low-frequency oscillations below 0.5 Hz, which are associated with oscillations in force below 0.5 Hz.

Neuromuscular control of goal-directed ankle movements differs for healthy children and adults.

PURPOSE: The purpose was to compare neuromuscular control of rapid ankle goal-directed movements in healthy preadolescent children and young adults. METHODS: Ten young adults (20.0 ± 0.9 years) and ten children (9.5 ± 0.7 years) attempted to accurately match the peak displacement of the foot to a spatiotemporal target with an ankle dorsiflexion movement. The targeted displacement was 9° of ankle dorsiflexion, and the targeted time was 180 ms. Surface electromyograms (EMGs) were recorded from the tibialis anterior (TA; agonist) and soleus (SOL; antagonist) muscles. Ankle movement control was quantified with endpoint accuracy and variability. The activation of the involved muscles was quantified with an EMG burst analysis. RESULTS: Children exhibited decreased endpoint accuracy and control compared with young adults, as indicated by greater endpoint errors (47.6 ± 15.2 vs. 25.8 ± 9.0%) and position variability (29.5 ± 5.7 vs. 15.2 ± 6.1 %). In addition, children exhibited differences in muscle activation, as evidenced by greater TA (53.2 ± 19.1 vs. 33.0 ± 19.0%) and SOL (19.9 ± 12.0 vs. 9.6 ± 5.4%) amplitudes of EMG burst, shorter TA duration (251.3 ± 43.6 vs. 296.1 ± 27.6%), and greater variability in the activation of these muscles. The endpoint error (R (2) = 0.7) and position variability (R (2) = 0.67) were predicted from the TA burst amplitude variability and TA burst duration. CONCLUSION: The differences in muscle activation and deficient control of rapid goal-directed ankle movements exhibited by children are likely due to their incomplete development of higher centers.

Altered activation of the antagonist muscle during practice compromises motor learning in older adults.

Aging impairs the activation of muscle; however, it remains unclear whether it contributes to deficits in motor learning in older adults. The purpose of this study was to determine whether altered activation of antagonistic muscles in older adults during practice inhibits their ability to transfer a motor task ipsilaterally. Twenty young (25.1 ± 3.9 yr; 10 men, 10 women) and twenty older adults (71.5 ± 4.8 yr; 10 men, 10 women) participated. Half of the subjects practiced 100 trials of a rapid goal-directed task with ankle dorsiflexion and were tested 1 day later with elbow flexion (transfer). The rest did not perform any ankle practice and only performed the task with elbow flexion. The goal-directed task consisted of rapid movement (180 ms) to match a spatiotemporal target. For each limb, we recorded the EMG burst activity of the primary agonist and antagonist muscles. The rate of improvement during task acquisition (practice) was similar for young and older adults (P > 0.3). In contrast, only young adults were able to transfer the task to the upper limb. Specifically, young adults who practiced ankle dorsiflexion exhibited ∼30% (P < 0.05) lower movement error and ∼60% (P < 0.05) lower antagonist EMG burst activity compared with older adults who received equal practice and young adults who did not receive any ankle dorsiflexion practice. These results provide novel evidence that the deficient motor learning in older adults may be related to a differential activation of the antagonist muscle, which compromises their ability to acquire the task during practice.

Aging and limb alter the neuromuscular control of goal-directed movements.

The purpose of this study was to determine whether the neuromuscular control of goal-directed movements is different for young and older adults with the upper and lower limbs. Twenty young (25.1 ± 3.9 years) and twenty older adults (71.5 ± 4.8 years) attempted to accurately match the displacement of their limb to a spatiotemporal target during ankle dorsiflexion or elbow flexion movements. We quantified neuromuscular control by examining the movement endpoint accuracy and variability, and the antagonistic muscle activity using surface electromyography (EMG). Our results indicate that older adults exhibit impaired endpoint accuracy with both limbs due to greater time variability. In addition, older adults exhibit greater EMG burst and lower EMG burst variability as well as lower coactivation of the antagonistic muscles. The impaired accuracy of older adults during upper limb movements was related to lower coactivation of the antagonistic muscles, whereas their impaired accuracy during lower limb movements was related to the amplified EMG bursts. The upper limb exhibited greater movement control than the lower limb, and different neuromuscular parameters were related to the accuracy and consistency for each limb. Greater endpoint error during upper limb movements was related to lower coactivation of the antagonistic muscles, whereas greater endpoint error during lower limb movements was related to the amplified EMG bursts. These findings indicate that the age-associated impairments in movement control are associated with altered activation of the involved antagonistic muscles. In addition, independent of age, the neuromuscular control of goal-directed movements is different for the upper and lower limbs.

Ankle variability is amplified in older adults due to lower EMG power from 30-60 Hz.

The purpose of this study was to determine the neuromuscular mechanisms of the involved muscles that contribute to the greater positional variability at the ankle joint in older adults compared with young adults. Eleven young adults (25.6±4.9 years) and nine older adults (76.9±5.9 years) were asked to accurately match and maintain a horizontal target line with 5° dorsiflexion of their ankle for 20 s. The loads were 5 and 15% of the one repetition maximum load (1 RM). The visual gain was kept constant at 1° for all trials. Positional variability was quantified as the standard deviation (SD) of the detrended position signal. The neural activation of the tibialis anterior and soleus muscles was quantified as the normalized EMG amplitude, power spectrum density (PSD; EMG oscillations) and coactivation of the two muscles. As expected, positional variability was greater in older adults (older: 0.11±0.06° vs. young: 0.04±0.02°; p=.003). The only significant neural difference occurred for the PSD of the tibialis anterior muscle, where young adults exhibited significantly greater power than older adults from 30-60 Hz. The amplified positional variability of ankle joint in older adults was associated with lower power from 30-60 Hz oscillations in the tibialis anterior muscle (r(2)=.3, p=.01). These results provide novel evidence that older adults exhibit greater positional variability with the ankle joint relative to young adults likely due to their inability to activate the tibialis anterior muscle from 30-60 Hz.

Magnified visual feedback exacerbates positional variability in older adults due to altered modulation of the primary agonist muscle.

The purpose of this study was to determine whether magnified visual feedback during position-holding contractions exacerbates the age-associated differences in motor output variability due to changes in the neural activation of the agonist muscle in the upper and lower limb. Twelve young (18-35 years) and ten older adults (65-85 years) were instructed to accurately match a target position at 5° of index finger abduction and ankle dorsiflexion while lifting 10 % of their 1 repetition maximum (1RM) load. Position was maintained at three different visual angles (0.1°, 1°, and 4°) that varied across trials. Each trial lasted 25 s and visual feedback of position was removed from 15 to 25 s. Positional error was quantified as the root mean square error (RMSE) of the subject's performance from the target. Positional variability was quantified as the standard deviation of the position data. The neural activation of the first dorsal interosseus and tibialis anterior was measured with surface electromyography (EMG). Older adults were less accurate compared with young adults and the RMSE decreased significantly with an increase in visual gain. As expected, and independent of limb, older adults exhibited significantly greater positional variability compared with young adults that was exacerbated with magnification of visual feedback (1° and 4°). This increase in variability at the highest magnification of visual feedback was predicted by a decrease in power from 12 to 30 Hz of the agonist EMG signal. These findings demonstrate that motor control in older adults is impaired by magnified visual feedback during positional tasks.

Age-associated differences in positional variability are greater with the lower limb.

The authors' purpose was to determine the interaction of age and limb used on positional variability at different loads. Eleven young adults and 10 older adults were asked to accurately match and maintain a horizontal target line with 5° abduction of their index finger and 5° dorsiflexion of their ankle for 20 s at loads ranging from 2 to 50% of the maximal load that could be lifted with each limb. The visual gain was kept constant at 1° (visual angle). Positional variability was greater in older adults for both limbs, nonetheless age-associated differences were greater for the ankle dorsiflexion task compared with the abduction of the index finger task. In addition, we found that, independent of age, motor output variability was greater with the lower limb. These results provide novel evidence that older adults may exhibit greater impairments in motor control with the foot compared with the finger. Furthermore, these findings support the idea, using a different task than previous literature, that the lower limb has greater motor output variability than the upper limb.