Unaccustomed eccentric exercise has been shown to lead to muscle damage. Previous research suggests that fatigue reduces the complexity of muscle force output. However, previous research has not investigated whether the force complexity response to fatigue is affected by eccentric exercise. Therefore, the purpose of the present study was to examine the effect of eccentric exercise and a subsequent fatiguing task on force variability and complexity. Methods: Twelve individuals (six females) participated in the study. The participants performed eccentric exercise consisting of six sets of 10 repetitions of the knee extensors to elicit muscle damage. Before and 48 hr after eccentric exercise, the participants were assessed for maximal voluntary contraction and performed a sustained isometric time to task failure protocol at 25% of maximal voluntary contraction. Force variability was assessed in the first 10 s (Fresh) and last 10 s (End-Task) of the time to task failure via standard deviation, coefficient of variation, detrended fluctuation analysis, sample entropy, and multiscale sample entropy. Results: Maximal voluntary contraction (532.67 +/- 177.49 N vs. 448.54 +/- 186.92 N, p < .001) and time to task failure (197.10 +/- 44.81 s vs. 157.70 +/- 62.52 s, p = .031) were reduced after eccentric exercise. There were no differences in force variability following eccentric exercise at either the Fresh or End-Task time points. Force complexity was reduced at both Fresh (detrended fluctuation analysis: 1.37 +/- 0.18 vs. 1.47 +/- 0.21; sample entropy: 0.88 +/- 0.27 vs. 0.71 +/- 0.027) and End-Task (sample entropy: 0.63 +/- 0.17 vs. 0.53 +/- 0.15), following eccentric exercise compared with the pre-eccentric exercise condition. Conclusions: Reductions in force complexity, but not force variability, were observed following eccentric exercise. These changes are thought to be due to changes in motor control strategies to compensate for damaged tissue.
Recent studies have demonstrated that isometric wrist actions performed with an open hand have a direction-dependent influence on upper arm muscle activity. It is presently unclear if these findings are translatable to motor tasks requiring a closed hand, as it common in most hand-object interactions. Thus, the purpose of the present study was to determine the influence of hand posture on upper arm muscle activity during isometric wrist exertions in different directions. Participants were seated with their forearm unsupported within a custom-built apparatus that could position an adjustable force transducer either above, below, or to the right/left of the participant's hand. Muscle activity was recorded from the biceps and triceps brachii while participants exerted 50% of their maximal force against the transducer in one of four randomized wrist actions: flexion, extension, or radial or ulnar deviation. Results demonstrated that a closed-hand posture produced greater biceps (open: 4.92 +/- 4.17% maximal voluntary excitation [MVE], closed: 8.18 +/- 6.44% MVE, p <.001) and muscle activity when exerting wrist forces downwards but less activity when forces were exerted to the right. The influence of wrist exertion direction on upper arm muscle activity also depended on hand posture. These findings add valuable new evidence that a neural coupling likely exists between the distal and proximal regions of the upper extremity. They also indicate that research into open-hand postures may not fully translate to motor tasks requiring a closed hand.
Lifting objects challenges postural stability and requires the generation of anticipatory (APA) and compensatory postural adjustments. This study investigated the combined effects of perturbation predictability and asymmetric load placement on postural control during a bimanual lifting task. Sixteen healthy adults completed a randomized crossover study involving repeated lifts of a box containing a load equal to 5% of their body weight that was positioned asymmetrically under three conditions: known, unpredictable, and predictable. Surface electromyography from trunk and lower limb muscles and center-of-pressure (COP) displacements were recorded. Electromyography integrals and COP displacements were analyzed separately for APA and compensatory postural adjustment phases using linear mixed-effects models. Explicit knowledge of load position resulted in larger and more direction-specific APAs, characterized by larger anticipatory trunk muscle activation and a proactive anterior shift of the COP (all p < .05). Predictability acquired through repeated exposure produced smaller APAs ( p < .05). Under unpredictable conditions, participants exhibited reduced anticipatory COP shifts and larger compensatory responses for right-sided loads (all p < .05), whereas left-sided loads were characterized by a posterior COP shift before lift initiation. Postural adjustments during lifting are modulated by both explicit task information and experience-based predictability, and load position asymmetry alters anticipatory and compensatory control strategies. Enhancing information about load position may therefore improve postural stability and reduce reliance on compensatory mechanisms during manual material handling.
This study aimed to use nonlinear dynamical measures to examine the function of plantar tactile feedback during quiet standing without vision in order to elucidate the stimulation effects on postural sway. Detrended fluctuation analysis, sample entropy, and rambling and trembling analysis were applied to analyze the dynamics of postural sway in the anterior-posterior and mediolateral directions. Twenty young adults stood on a force plate with eyes closed for 35 s under three conditions: no plantar stimulation, plantar stimulation under the forefoot, and plantar stimulation under the midfoot (MS). The results showed that postural dynamics were affected by the placement of plantar stimulation in the absence of visual information. In the anterior-posterior direction, increased detrended fluctuation analysis values for the center of pressure and rambling displacements were revealed in the MS condition than in the plantar stimulation under the forefoot condition, indicating greater adaptability in postural sway. There were also increased detrended fluctuation analysis values in the rambling displacement in the MS condition compared with the no plantar stimulation condition. In the mediolateral direction, the sample entropy in the trembling displacement was significantly greater in the MS condition than in the no plantar stimulation condition, indicating more automaticity in postural sway. These results suggest that plantar stimulation may contribute to balance regulation in an axis- and component-specific manner during quiet standing without vision.
This study investigated the neural timing control characteristics of countermovement jumps between elite athletes and nonathletes using muscle synergy theory. Twenty male participants (high-level group [HLG, N = 10] and untrained group [UG, N = 10]) performed the countermovement jump while surface electromyography from eight lower limb muscles and three-dimensional force plate data were recorded. Nonnegative matrix factorization extracted three muscle synergy modules, and bootstrap resampling with bias-corrected 95% confidence intervals (CIs) analyzed temporal parameters. Results demonstrated: (a) HLG achieved significantly greater jump height (95% CI: [-0.139, -0.026]); (b) HLG exhibited delayed activation onset (95% CI: [-0.227, -0.015]), peak timing (95% CI: [-0.300, -0.028]), and offset (95% CI: [-0.216, -0.014]) in Synergy module 1, delayed onset (95% CI: [-0.225, -0.054]) and offset (95% CI: [-0.224, -0.072]) in Synergy module 2, and delayed offset in Synergy module 3 (95% CI: [-0.197, -0.017]); and (c) HLG showed narrower full width at half maximum in Synergy module 1 (95% CI: [0.009, 0.094]), with no group differences in amplitude parameters. These findings suggest that, compared with UG, HLG exhibit delayed synergy activation timing during the countermovement jump. This temporal pattern may be associated with more effective use of the eccentric phase and more coordinated interjoint timing during propulsion.
