Motor performance patterns between unilateral mechanical assistance and bilateral muscle contraction

Transient Electromyographic Responses by Isokinetic Torque Release during Mechanically Assisted Elbow Flexion

“…Similar to previous studies on isometric force control, greater variability was observed when torque was released from a higher baseline (GIT) ( Choi et al, 2020 a, 2019). Hence, if the magnitude of the initial manual control is greater, the torque mismatch between the joint inertia and power assistance could prevent the smooth transition from manual control to collaboration.…”

“…In contrast, our results of TB muscle activation and CCI support the hypothesis that the activation patterns of the TB muscle are distinct from those of agonist muscles as they did not decrease from pre-release to 200 ms after release. Such patterns could reflect that the joint stiffness strategy to maintain postural stability can be applied for sudden joint torque release by power assistance ( Hasan, 2005 ; Latash, 1992 ) as well as for sudden joint loading ( Holmes and Keir, 2012 ).This is interesting because our previous studies on force release by visually guided force tracking ( Choi et al, 2019 ) and mechanical assistance ( Choi et al, 2020 a) during isometric elbow flexion did not find distinct patterns of muscle co-contraction although EMG amplitude of the elbow flexor was reduced. Therefore, the need for postural stabilization mechanisms by the co-contraction of the antagonist muscle could increase when transient changes in the joint angle and torque release take place due to power assistance.…”

“…For sample size calculation, a priori power analysis was conducted using G*Power (v 3.1.9.7, University of Dusseldorf, Dusseldorf, Germany) based on recently published data from a previous study ( Choi et al, 2020 b) involving a similar population (i.e., male undergraduate and graduate students [n = 11]) and similar tasks of muscle force release. To satisfy the usual recommendation of 80% power (1–β error probability) at the 5% level of α error needed to conclude that a result is significant for the major dependent variable of torque variability, a minimum sample size of n = 10 was required.…”

“…Greater variability was also observed during linear torque release in an isometric contraction ( Choi et al, 2019 ; Orizio et al, 2010 ). Previous studies that simulated power assistance on isometric elbow flexion have demonstrated that a slow force releasing rate reduces force variability, and greater change of force magnitude results in high variability while target overshoots decrease by adopting a conservative control strategy ( Choi et al, 2020 a, 2019). Specifically, the role of the agonist muscle has been highlighted during isometric exercise, as it tries to stabilize motor performance when power assistance is provided and causes external perturbation of the elbow joint.…”

“…Herein, we investigated how these joint stabilizing mechanisms functioned when power assistance was provided in the same direction as elbow flexion, causing different magnitudes of joint torque to be released with varying angular speed. Recently, distinct patterns of antagonist muscle activation and joint stiffness were barely observed during collaborations ( Choi et al, 2020 a; Loh et al, 2020 ), probably because the simulation settings for power assistance were mainly based on static force control in isometric contraction.…”