Force and electromyography responses during isometric force release of different rates and step-down magnitudes
Introduction
Humans not only increase but also release force to perform activities of daily living. Recent studies have shown that motor unit control for regulating force release is independent from the one for force increase (Andrzejewska et al., 2014, Onushko et al., 2013, Orizio et al., 2010). This difference of motor unit control might emerge as altered behavioral responses, such as increased variability during force release (Park, Kwon, Solis, Lodha, & Christou, 2016).
Force release and muscle relaxation are prerequisites for skillful and precise movements (Sugawara et al., 2016, Suzuki et al., 2015). Nonetheless, several studies regarding sports, playing instruments, and caregiving have indicated that muscle relaxation might be difficult especially when the movements are unfamiliar (Daikoku and Saito, 2008, Kato et al., 2014, Sakurai and Ohtsuki, 2000, Wada and Yoshida, 2016). Similarly, recent studies on human-robot cooperation showed that although assistive force is provided to aid joint movements, robot users might not easily release their muscular force (Muraki et al., 2018, Rosen and Perry, 2007). The common feature of those movements is that muscular force is released to a particular step-down magnitude, rather than complete relaxation to achieve precise and smooth performance. However, few studies have focused on how the force output and corresponding muscle activity are altered under varying conditions of force release to step-down magnitudes.
Several studies showed that varying ramp rates to guide isometric force release were associated with altered force variability and error during joint movements (Naik et al., 2011, Orizio et al., 2010). Additionally, gradual force release increased corticospinal excitability in the antagonist muscle (Yoshida, Yamaguchi, Saitou, Tanabe, & Sugawara, 2015). On the other hand, maintaining varying degrees of step-down magnitude, which follows force release might also affect behavioral responses. There is evidence that force variability and error increased if isometric force was greatly released to lower magnitude (Choi et al., 2018, Masumoto and Inui, 2010). Further, maintaining lower force magnitudes might raise force variability (Galganski et al., 1993, Griffin et al., 2009, Kumar et al., 2017).
Regulation of force release to step-down magnitude can be a type of target-directed aiming movements. Specifically, fast and forceful movements might cause noises such as overshoot, which represents the degree of passing through the target (Elliott, Hansen, Mendoza, & Tremblay, 2004). The overshoot arisen during bottom-up target-directed force control is known to be costlier than the undershoot (Kesar, Chou, & Binder-Macleod, 2008). It is because corrective and time-consuming sub-movement is required to overcome the inertia and recover the target after the overshoot (Elliott et al., 2004, Sparrow and Newell, 1998). Conversely, if overshoot occurs during top-down force control, this might be even costlier as the sub-movement to recover the target level requires force increase with additional muscle contraction. Although previous studies have shown that relatively small and rapid changes in force cause larger overshoot during bottom-up force control (Dideriksen et al., 2017, Hu et al., 2011), it is unclear how top-down force control with varying ramp rates and step-down magnitudes affect the degree of overshoot.
The objective of this study is to examine the force output and muscle activity during the isometric force release and maintenance. Force variability and overshoot ratio are calculated from the force output to evaluate the motor performance. Simple voluntary exertion of isometric elbow flexion is investigated as a basic study, where this exercise excludes various biomechanical factors, such as angle movement and velocity that may affect the sole performance of motor control by agonist and antagonist muscles. Visuomotor tracking tasks are conducted to guide force control with different levels of ramp duration and step-down magnitude, in which the isometric force of elbow flexion and the corresponding muscle activity are simultaneously measured. We hypothesize that different ramp durations and step-down magnitudes would alter the motor performance and muscle activity during isometric elbow flexion.
Section snippets
Participants
Twelve healthy male adults (age: 23.1 ± 1.1 years, height: 173.3 ± 6.3 cm, weight: 67.0 ± 8.6 kg) were recruited. None of the participants reported any history of musculoskeletal disorders or functional upper limb impairment. Hand dominance was determined using the Edinburgh Handedness Inventory (Oldfield, 1971). Each participant provided informed consent prior to participating in the experiment. This study was approved by the Research Ethics Committee of the Faculty of Design, Kyushu
Force output and muscle activity
The normalized isometric force (%MVC force) was released in accordance with the four conditions of guided trajectory during the simulation (Fig. 3a). The activities of the BB and TB muscles (%MVC) showed trends similar to those for force output (Fig. 3b & c). The BB was more involved as the agonist muscle during elbow flexion than the TB. The last 3 s of the simulation (12 s–15 s for the conditions involving ramp duration of 1 s; 16 s–19 s for the conditions involving ramp duration of 5 s) were
Discussion
This study aimed to determine whether motor performance and EMG responses were affected by varying ramp durations and by followed step-down magnitudes. The results of the last 3 s of force output indicated that force was released and maintained as it was guided by the experimental task; however, relatively short ramp duration (1 s) increased BB activity in the last seconds of force maintenance. We also found that both force variability and overshoot ratio decreased when ramp duration was
Conclusion
This study focused on the force and EMG responses in the process of force release to a certain magnitude and its maintenance during isometric elbow flexion. Although force release guided by longer ramp duration lowered the force variability and overshoot ratio than that guided by shorter ramp duration, it also accompanied increased RPE and co-contraction ratio. When force was greatly released, force variability increased while overshoot ratio decreased. Specifically, the results of overshoot
Acknowledgements
The authors would like to thank Yuanyuan Wang and Angelie Revilla for their assistance in conducting our experiment. This work was supported by JSPS KAKENHI Grant Number JP17H01454.
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