Trunk postural reactions to the force perturbation intensity and frequency during sitting astride in children with cerebral palsy.

The purpose of this study was to examine kinematic and neuromuscular responses of the head and body to pelvis perturbations with different intensities and frequencies during sitting astride in children with CP. Sixteen children with spastic CP (mean age 7.4 ± 2.4 years old) were recruited in this study. A custom designed cable-driven robotic horse was used to apply controlled force perturbations to the pelvis during sitting astride. Each participant was tested in four force intensity conditions (i.e., 10%, 15%, 20%, and 25% of body weight (BW), frequency = 1 Hz), and six force frequency conditions (i.e., 0.5 Hz, 1 Hz, 1.5 Hz, 2 Hz, 2.5 Hz, and 3 Hz, intensity = 20% of BW). Each testing session lasted for one minute with a one-minute rest break inserted between two sessions. Kinematic data of the head, trunk, and legs were recorded using wearable sensors, and EMG signals of neck, trunk, and leg muscles were recorded. Children with CP showed direction-specific trunk and neck muscle activity in response to the pelvis perturbations during sitting astride. Greater EMG activities of trunk and neck muscles were observed for the greater intensities of force perturbations (P < .05). Participants also showed enhanced activation of antagonistic muscles rather than direction-specific trunk and neck muscle activities for the conditions of higher frequency perturbations (P < .05). Children with CP may modulate trunk and neck muscle activities in response to greater changes in intensity of pelvis perturbation during sitting astride. Perturbations with too high frequency may be less effective in inducing direction-specific trunk and neck muscle activities.

Overground walking with a constraint force on the nonparetic leg during swing improves weight shift toward the paretic side in people after stroke.

Targeting enhancing the use of the paretic leg during locomotor practice might improve motor function of the paretic leg. The purpose of this study was to determine whether application of constraint force to the nonparetic leg in the posterior direction during overground walking would enhance the use of the paretic leg in people with chronic stroke. Fifteen individuals after stroke participated in two experimental conditions, i.e., overground walking with a constraint force applied to the nonparetic leg and overground walking only. Each participant was tested in the following procedures that consisted of overground walking with either constraint force or no constraint force, instrumented split-belt treadmill walking, and pressure-sensitive gait mat walking before and after the overground walking. Overground walking practice with constraint force resulted in greater enhancement in lateral weight shift toward the paretic side (P < 0.01), muscle activity of the paretic hip abductors (P = 0.04), and propulsion force of the paretic leg (P = 0.05) compared with the results of the no-constraint condition. Overground walking practice with constraint force tended to induce greater increase in self-selected overground walking speed (P = 0.06) compared with the effect of the no-constraint condition. The increase in propulsion force from the paretic leg was positively correlated with the increase in self-selected walking speed (r = 0.6, P = 0.03). Overground walking with constraint force applied to the nonparetic leg during swing phase of gait may enhance use of the paretic leg, improve weight shifting toward the paretic side and propulsion of the paretic leg, and consequently increase walking speed.NEW & NOTEWORTHY Application of constraint force to the nonparetic leg during overground walking induced improved lateral weight shifts toward the paretic leg and enhanced muscle activity of the paretic leg during walking. In addition, one session of overground walking with constraint force might induce an increase in propulsive force of the paretic leg and an increase in self-selected overground walking speed, which might be partially due to the improvement in motor control of the paretic leg.

Improving Trunk Postural Control Facilitates Walking in Children With Cerebral Palsy: A Pilot Study.

OBJECTIVE: The aim of this study is to determine the effects of bilateral trunk support during walking on trunk and leg kinematics and neuromuscular responses in children with cerebral palsy. DESIGN: Fourteen children with spastic cerebral palsy (Gross Motor Function Classification System level I to III) participated in this study. Children walked on a treadmill under four different conditions, that is, without support (Baseline), with bilateral support applied to the upper trunk (upper trunk support), the lower trunk (lower trunk support), and combined upper and lower trunk (combined trunk support). The trunk and leg kinematics and muscle activity were recorded. RESULTS: Providing bilateral support to the trunk had a significant impact on the displacement of the pelvis and trunk ( P < 0.003) during walking. Children's weaker leg showed greater step length ( P = 0.032) and step height ( P = 0.012) in combined trunk support compared with baseline and greater step length in upper trunk support ( P = 0.02) and combined trunk support ( P = 0.022) compared with lower trunk support. Changes in soleus electromyographic activity during stance phase of gait mirrored the changes in step length across all conditions. CONCLUSIONS: Providing bilateral upper or combined upper and lower trunk support during walking may induce improvements in gait performance, which may be due to improved pelvis kinematics. Improving trunk postural control may facilitate walking in children with cerebral palsy.

Enhanced phasic sensory afferents paired with controlled constraint force improve weight shift toward the paretic side in individuals post-stroke.

PURPOSE: The goal of this study was to determine whether enhanced phasic sensory afferent input paired with the application of controlled constraint force during walking would improve weight shift toward the paretic side and enhance use of the paretic leg. METHODS: Fourteen stroke survivors participated in two experimental conditions, sessions that consisted of 1 min treadmill walking without force and stimulation (baseline), 7 min walking with either "constraint force and sensory stimulation (constraint+stim)" or "constraint force only (constraint)" (adaptation), and then 2 min walking without force and stimulation (post-adaptation). Kinematics of the pelvis and legs, and muscle activity of the paretic leg were recorded. RESULTS: Participants showed greater increases in hip abductor (p < 0.001) and adductor (p = 0.04) muscle activities, weight shift toward the paretic side (p = 0.002), and step length symmetry (p < 0.01) during the late post-adaptation period in the "constraint+stim" condition, compared with the effect of the "constraint" condition. In addition, changes in overground walking speed from baseline to 10 min post treadmill walking was significantly greater for the "constraint force and stimulation" condition than for the "constraint force only" condition (p = 0.04). CONCLUSION: Enhanced targeted sensory afferent input during locomotor training may facilitate recruitment of targeted muscles of the paretic leg and facilitate use-dependent motor learning of locomotor tasks, which might retain longer and partially transfer from treadmill to overground walking, in stroke survivors.

Swing-phase pelvis perturbation improves dynamic lateral balance during walking in individuals with spinal cord injury.

The purpose of this study was to determine whether the control of lateral balance can be improved by applying repeated lateral perturbation force to the pelvis during swing versus stance phase walking in individuals with spinal cord injury (SCI). Fourteen individuals with incomplete SCI were recruited in this study. Each participant visited the lab once and was tested in two experimental sessions that consisted of (1) treadmill walking with bilateral perturbation force applied to the pelvis in the lateral direction during either swing or stance phase of each leg and (2) overground walking pre- and post-treadmill walking. Applying the swing-phase perturbation during walking induced a greater increase in the muscle activation of hip abductors and ankle plantar flexors and a greater improvement in lateral balance control after the removal of perturbation force, in comparison to the results of the stance-phase perturbation condition (P ≤ 0.03). Participants also exhibited a greater reduction in overground step width and a greater improvement in overground walking speed after a session of treadmill walking practice with the swing-phase perturbation, compared with the result of the stance-phase perturbation (P = 0.01). These findings suggest that applying perturbation force to the pelvis during the swing phase of gait while walking may enhance muscle activities of hip abductors and improve lateral balance control in individuals with SCI. A walking practice with the swing-phase pelvis perturbation can be used as a rehabilitation approach to improve the control of lateral balance during walking in people with SCI.

Repeated adaptation and de-adaptation to the pelvis resistance force facilitate retention of motor learning in stroke survivors.

Locomotor adaptation to novel walking patterns induced by external perturbation has been tested to enhance motor learning for improving gait parameters in individuals poststroke. However, little is known regarding whether repeated adaptation and de-adaptation to the externally perturbed walking pattern may facilitate or degrade the retention of locomotor learning. In this study, we examined whether the intermittent adaptation to novel walking patterns elicited by external perturbation induces greater retention of the adapted locomotion in stroke survivors, compared with effects of the continuous adaptation. Fifteen individuals poststroke participated in two experimental conditions consisting of 1) treadmill walking with intermittent (i.e., interspersed 2 intervals of no perturbation) or continuous (no interval) adaptation to externally perturbed walking patterns and 2) overground walking before, immediately, and 10 min after treadmill walking. During the treadmill walking, we applied a laterally pulling force to the pelvis toward the nonparetic side during the stance phase of the paretic leg to disturb weight shifts toward the paretic side. Participants showed improved weight shift toward the paretic side and enhanced muscle activation of hip abductor/adductors immediately after the removal of the pelvis perturbation for both intermittent and continuous conditions (P < 0.05) and showed longer retention of the improved weight shift and enhanced muscle activation for the intermittent condition, which transferred from treadmill to overground walking (P < 0.05). In conclusion, repeated motor adaptation and de-adaptation to the pelvis resistance force during walking may promote the retention of error-based motor learning for improving weight shift toward the paretic side in individuals poststroke.NEW & NOTEWORTHY We examined whether the intermittent versus the continuous adaptation to external perturbation induces greater retention of the adapted locomotion in stroke survivors. We found that participants showed longer retention of the improved weight shift and enhanced muscle activation for the intermittent versus the continuous conditions, suggesting that repeated motor adaptation and de-adaptation to the pelvis perturbation may promote the retention of error-based motor learning for improving weight shift toward the paretic side in individuals poststroke.

Enhanced error facilitates motor learning in weight shift and increases use of the paretic leg during walking at chronic stage after stroke.

The purpose of this study was to determine whether the application of lateral pelvis pulling force toward the non-paretic side during the stance phase of the paretic leg would enhance forced use of the paretic leg and increase weight shift toward the paretic side in stroke survivors. Eleven chronic stroke survivors participated in two experimental sessions, which consisted of (1) treadmill walking with the application of "pelvis resistance" or "pelvis assistance" and (2) overground walking. During the treadmill walking, the laterally pulling force was applied during the stance phase of the paretic leg toward the non-paretic side for the "pelvis resistance" condition or toward the paretic side for the "pelvis assistance" condition during the stance phase of the paretic leg. After force release, the "pelvis resistance" condition exhibited greater enhancement in muscle activation of hip ABD, ADD, and SOL and greater improvement in lateral weight shift toward the paretic side, compared with the effect of the "pelvis assistance" condition (P < 0.03). This improved lateral weight shift was associated with the enhanced muscle activation of hip ABD and ADD (R2 = 0.67, P = 0.01). The pelvis resistance condition also improved overground walking speed and stance phase symmetry when measured 10 min after the treadmill walking (P = 0.004). In conclusion, applying pelvis resistance forces to increase error signals may facilitate motor learning of weight shift toward the paretic side and enhance use of the paretic leg in chronic stroke survivors. Results from this study may be utilized to develop an intervention approach to improve walking in stroke survivors.

Gradual adaptation to pelvis perturbation during walking reinforces motor learning of weight shift toward the paretic side in individuals post-stroke.

The purpose of this study was to determine whether the gradual versus abrupt adaptation to lateral pelvis assistance force improves weight shift toward the paretic side and enhance forced use of the paretic leg during walking. Sixteen individuals who had sustained a hemispheric stroke participated in two experimental sessions, which consisted of (1) treadmill walking with the application of lateral pelvis assistance force (gradual vs. abrupt condition) and (2) overground walking. In the "gradual" condition, during treadmill walking, the assistance force was gradually increased from 0 to 100% of the predetermined force step by step. In the abrupt condition, the force was applied at 100% of the predetermined force throughout treadmill walking. Participants exhibited significant improvements in hip abductor and adductor, ankle dorsiflexor, and knee extensor muscle activities, weight shift toward the paretic side, and overground walking speed in the gradual condition (P < 0.05), but showed no significant changes in the abrupt condition (P > 0.20). Changes in weight shift toward the paretic side were statistically different between conditions (P < 0.001), although changes in muscle activities were not (P > 0.11). In the gradual condition, the error amplitude was proportional to the improvement in weight shift during the late post-adaptation (R2 = 0.32, P = 0.03), but not in the abrupt condition (R2 = 0.001, P = 0.93). In conclusion, the "gradual adaptation" inducing "small errors" during constraint-induced walking may improve weight shift and enhance forced use of the paretic leg in individuals post-stroke. Applying gradual pelvis assistance force during walking may be used as an intervention strategy to improve walking in individuals post-stroke.

Increased motor variability facilitates motor learning in weight shift toward the paretic side during walking in individuals post-stroke.

The purpose of this study was to determine whether applying "varied" versus constant pelvis assistance force mediolaterally toward the paretic side of stroke survivors during walking would result in short-term improvement in weight shift toward the paretic side. Twelve individuals post-stroke (60.4 ± 6.2 years; gait speed: 0.53 ± 0.19 m/s) were tested under two conditions (varied vs. constant). Each condition was conducted in a single separate session, which consisted of (a) treadmill walking with no assistance force for 1 min (baseline), pelvis assistance toward the paretic side for 9 min (adaptation), and then no force for additional 1 min (post-adaptation), and (b) overground walking. In the "varied" condition, the magnitude of force was randomly changed across steps between 30% and 100% of the predetermined amount. In the abrupt condition, the magnitude of force was kept constant at 100% of the predetermined amount. Participants exhibited greater improvements in weight shift toward the paretic side (p < 0.01) and in muscle activity of plantar flexors and hip adductors of the paretic leg (p = 0.02) from baseline to late post-adaptation period for the varied condition than for the constant condition. Motor variability of the peak pelvis displacement at baseline was correlated with improvement in weight shift toward the paretic side after training for the varied (R2  = 0.64, p = 0.01) and the constant condition (R2  = 0.39, p = 0.03). These findings suggest that increased motor variability, induced by applying the varied pelvis assistance, may facilitate motor learning in weight shift and gait symmetry during walking in individuals post-stroke.

Targeted Pelvic Constraint Force Induces Enhanced Use of the Paretic Leg During Walking in Persons Post-Stroke.

The purpose of this study was to determine whether activation of muscles in the paretic leg, particularly contributing to propulsion, and gait symmetry can be improved by applying a targeted resistance force to the pelvis in the backward direction during stance phase while walking in individuals post-stroke. Thirteen individuals post-stroke participated in two experimental sessions, which consisted of treadmill walking, with either targeted or constant resistances, together with overground walking. For the targeted condition, a resistance force was applied to the pelvis during the stance phase of the paretic leg. For the constant condition, the resistance force was applied throughout the whole gait cycle. Participants showed greater increase in medial hamstring muscle activity in the paretic leg and improved step length symmetry after the removal of targeted resistance force, compared to effects of a constant resistance force (P < 0.03). In addition, treadmill walking with the targeted resistance induced more symmetrical step length during overground walking 10 min after the treadmill walking, compared to the result of the constant resistance force (P = 0.01). Applying a targeted resistance force to the pelvis during the stance phase of the paretic leg may induce an enhanced use of the paretic leg and an improvement in gait symmetry in individuals post-stroke. These results provide evidence showing that applying a targeted resistance to the pelvis may induce a forced use of the paretic leg during walking.

Reaction to a Visual Stimulus: Anticipation with Steady and Dynamic Contractions.

Reacting fast to visual stimuli is important for many activities of daily living and sports. It remains unknown whether the strategy used during the anticipatory period influences the speed of the reaction. The purpose of this study was to determine if reaction time (RT) differs following a steady and a dynamic anticipatory strategy. Twenty-two young adults (21.0 ± 2.2 yrs, 13 women) participated in this study. Participants performed 15 trials of a reaction time task with ankle dorsiflexion using a steady (steady force at 15% MVC) and a dynamic (oscillating force from 10-20% MVC) anticipatory strategy. We recorded primary agonist muscle (tibialis anterior; TA) electromyographic (EMG) activity. We quantified RT as the time interval from the onset of the stimulus to the onset of force. We found that a dynamic anticipatory strategy, compared to the steady anticipatory strategy, resulted in a longer RT (p = 0.04). We classified trials of the dynamic condition based on the level and direction of anticipatory force at the moment of the response. We found that RT was longer during the middle descending relative to the middle ascending and the steady conditions (p < 0.01). All together, these results suggest that RT is longer when preceded by a dynamic anticipatory strategy. Specifically, the longer RT is a consequence of the variable direction of force at which the response can occur, which challenges the motor planning process.

Functional motor control deficits in older FMR1 premutation carriers.

Individuals with fragile X mental retardation 1 (FMR1) gene premutations are at increased risk for fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. However, it is unknown whether older FMR1 premutation carriers, with or without FXTAS, exhibit functional motor control deficits compared with healthy individuals. The purpose of this study, therefore, was to determine whether older FMR1 premutation carriers exhibit impaired ability to perform functional motor tasks. Eight FMR1 premutation carriers (age: 58.88 ± 8.36 years) and eight age- and sex-matched healthy individuals (60.13 ± 9.25 years) performed (1) a steady isometric force control task with the index finger at 20% of their maximum voluntary contraction (MVC) and; (2) a single-step task. During the finger abduction task, firing rate of multiple motor units of the first dorsal interosseous (FDI) muscle was recorded. Compared with healthy controls, FMR1 premutation carriers exhibited (1) greater force variability (coefficient of variation of force) during isometric force (1.48 ± 1.02 vs. 0.63 ± 0.37%; P = 0.04); (2) reduced firing rate of multiple motor units during steady force, and; (3) reduced velocity of their weight transfer during stepping (156.62 ± 26.24 vs. 191.86 ± 18.83 cm/s; P = 0.01). These findings suggest that older FMR1 premutation carriers exhibit functional motor control deficits that reflect either subclinical issues associated with premutations independent of FXTAS, or prodromal markers of the development of FXTAS.

Control of oscillatory force tasks: Low-frequency oscillations in force and muscle activity.

Force variability during steady force tasks is strongly related to low-frequency oscillations (<0.25 Hz) in force. However, it is unknown whether low-frequency oscillations also contribute to the variability of oscillatory force tasks. To address this, twelve healthy young participants (21.08 ±â€¯2.99 years, 6 females) performed a sinusoidal force task at 15% MVC at two different frequencies (0.5 and 1 Hz) with isometric abduction of the index finger. We recorded the force from the index finger and surface EMG from the first dorsal interosseous muscle and quantified the following outcomes: 1) trajectory variability and accuracy; 2) power spectrum of force and EMG bursting below 2 Hz; 3) power spectrum of the interference EMG from 4 to 60 Hz. The trajectory variability and error significantly increased from 0.5 to 1 Hz task (P < 0.01). Increased force oscillations <0.25 Hz contributed to greater trajectory variability and error for both the 0.5 and 1 Hz oscillatory task (R2 > 0.33; P < 0.05). The <0.25 Hz oscillations in force were positively associated with greater power in the <0.25 Hz for EMG bursting (R2 > 0.52; P < 0.01). The modulation of the interference EMG from 35 to 60 Hz was a good predictor of the <0.25 Hz force oscillations for both the 0.5 Hz task and 1 Hz task (R2 > 0.66; P < 0.01). These results provide novel evidence that, similar to steady contractions, low-frequency oscillations of the motor neuron pool appear to be a significant mechanism that controls force during oscillatory force tasks.

Motor planning perturbation: muscle activation and reaction time.

Reaction time (RT) is the time interval between the appearance of a stimulus and initiation of a motor response. Within RT, two processes occur, selection of motor goals and motor planning. An unresolved question is whether perturbation to the motor planning component of RT slows the response and alters the voluntary activation of muscle. The purpose of this study was to determine how the modulation of muscle activity during an RT response changes with motor plan perturbation. Twenty-four young adults (20.5 ±1.1 yr, 13 women) performed 15 trials of an isometric RT task with ankle dorsiflexion using a sinusoidal anticipatory strategy (10-20% maximum voluntary contraction). We compared the processing part of the RT and modulation of muscle activity from 10 to 60 Hz of the tibialis anterior (primary agonist) when the stimulus appeared at the trough or at the peak of the sinusoidal task. We found that RT ( P = 0.003) was longer when the stimulus occurred at the peak compared with the trough. During the time of the reaction, the electromyography (EMG) power from 10 to 35 Hz was less at the peak than the trough ( P = 0.019), whereas the EMG power from 35 to 60 Hz was similar between the peak and trough ( P = 0.92). These results suggest that perturbation to motor planning lengthens the processing part of RT and alters the voluntary activation of the muscle by decreasing the relative amount of power from 10 to 35 Hz. NEW & NOTEWORTHY We aimed to determine whether perturbation to motor planning would alter the speed and muscle activity of the response. We compared trials when a stimulus appeared at the peak or trough of an oscillatory reaction time task. When the stimulus occurred at the trough, participants responded faster, with greater force, and less EMG power from 10-35 Hz. We provide evidence that motor planning perturbation slows the response and alters the voluntary activity of the muscle.

Motor transfer from the corticospinal to the corticobulbar pathway.

There are multiple descending neural pathways, including the corticospinal pathway (CS) and the corticobulbar pathway (CB). The corticospinal pathway has been shown to exhibit within-pathway (CS-to-CS) motor transfer. However, motor transfer across each pathway (CS-to-CB or CB-to-CS) has yet to be studied in depth. The aim of the present study was to examine the effects of cross-pathway motor transfer between the ankle (CS) and tongue (CB) after training with a ballistic goal-directed motor task. Twelve healthy participants were recruited for this two-day experimental study. Six participants performed a ballistic goal-directed task with their ankle on Day 1 (ankle dorsiflexion), then tongue on Day 2 (elevate tongue against IOPI). The other 6 participants performed the same task with their tongue on Day 1, then ankle on Day 2. Both the ankle and tongue tasks (50 trials each) required matching force and time to a visual target. Our findings indicate that participants who underwent ankle training on Day 1 exhibited decreased tongue force error on Day 2 compared with participants who completed the tongue training on Day 1, with no prior ankle training (p = 0.02) (i.e. greater accuracy). This finding suggests that cross-pathway transfer from the corticospinal pathway to the corticobulbar pathway occurred with respect to force error. In other words, training of the ankle (CS) translated to improved training performance of the tongue (CB) through a reduction in force error. However, the reverse was not true - training the tongue did not elicit improved performance of the ankle. Nonetheless, if training with the corticospinal pathway can lead to improved corticobulbar pathway functioning, incorporating multi-pathway rehabilitation techniques might be valuable for clinicians across medical disciplines.

Integration of visual feedback and motor learning: Corticospinal vs. corticobulbar pathway.

Although movement is controlled by different descending pathways, it remains unknown whether the integration of visual feedback and motor learning differs for movements controlled by different descending pathways. Here, we compare motor control and learning of the ankle joint and tongue because they are primarily controlled by the corticospinal and corticobulbar pathways, respectively. Twelve young adults (19.63 ±â€¯2.11 years, 6 females) practiced a tracking task (combination of 0.02, 0.37, 0.5, and 1 Hz) with ankle dorsiflexion and with tongue elevation for 100 trials. The participants practiced each effector (ankle and tongue) in different days and the order of the effector was counterbalanced. Following practice, participants performed the same tracking task with concurrent contractions of the tongue and ankle (dual tracking task; transfer) with three different visual feedback conditions (no visual feedback, visual feedback only for ankle, visual feedback only for tongue). We quantified the force accuracy (RMSE) from each effector during the practice and transfer periods. During practice, the force accuracy and performance improvement to the visuomotor task was greater for the ankle dorsiflexion than tongue elevation. During the transfer task, the ankle dorsiflexion was more accurate than tongue elevation, independent of whether visual feedback was given for the ankle or tongue. The greater performance improvement for the ankle dorsiflexion during practice was related to superior transfer performance. These findings suggest that the corticospinal pathway integrates visual feedback more efficiently than the corticobulbar pathway, which enhances performance and learning of visuomotor tasks.

Voluntary reduction of force variability via modulation of low-frequency oscillations.

Visual feedback can influence the force output by changing the power in frequencies below 1 Hz. However, it remains unknown whether visual guidance can help an individual reduce force variability voluntarily. The purpose of this study, therefore, was to determine whether an individual can voluntarily reduce force variability during constant contractions with visual guidance, and whether this reduction is associated with a decrease in the power of low-frequency oscillations (0-1 Hz) in force and muscle activity. Twenty young adults (27.6 ± 3.4 years) matched a force target of 15% MVC (maximal voluntary contraction) with ankle dorsiflexion. Participants performed six visually unrestricted contractions, from which we selected the trial with the least variability. Following, participants performed six visually guided contractions and were encouraged to reduce their force variability within two guidelines (±1 SD of the least variable unrestricted trial). Participants decreased the SD of force by 45% (P < 0.001) during the guided condition, without changing mean force (P > 0.2). The decrease in force variability was associated with decreased low-frequency oscillations (0-1 Hz) in force (R 2 = 0.59), which was associated with decreased low-frequency oscillations in EMG bursts (R 2 = 0.35). The reduction in low-frequency oscillations in EMG burst was positively associated with power in the interference EMG from 35 to 60 Hz (R 2 = 0.47). In conclusion, voluntary reduction of force variability is associated with decreased low-frequency oscillations in EMG bursts and consequently force output. We provide novel evidence that visual guidance allows healthy young adults to reduce force variability voluntarily likely by adjusting the low-frequency oscillations in the neural drive.

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.