MinHyuk Kwon

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.

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.

Age-Associated Differences in Positional Variability Are Greater With the Lower Limb

ABSTRACT 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.

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.

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.

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 (R2 = 0.63), ICARS total score (R2 = 0.43), and SARA total score (R2 = 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.

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.

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.

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 ± 0.67 yrs) and 19 older adults (71.95 ± 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 ± 13.4% vs. 27.0 ± 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.