Neuromechanics of Dynamic Balance Tasks in the Presence of Perturbations

Abstract

Understanding the neuromechanical responses to perturbations in humans may help to explain the reported improvements in stability performance and muscle strength after perturbation-based training. In this study, we investigated the effects of perturbations, induced by unstable surfaces, on the mechanical loading and the modular organization of motor control in the lower limb muscles during lunging forward and backward. Fifteen healthy adults performed 50 forward and 50 backward lunges on stable and unstable ground. Ground reaction forces, joint kinematics, and the electromyogram (EMG) of 13 lower limb muscles were recorded. We calculated the resultant joint moments and extracted muscle synergies from the stepping limb. We found sparse alterations in the resultant joint moments and EMG activity, indicating a little if any effect of perturbations on muscle mechanical loading. The time-dependent structure of the muscle synergy responsible for the stabilization of the body was modified in the perturbed lunges by a shift in the center of activity (later in the forward and earlier in the backward lunge) and a widening (in the backward lunge). Moreover, in the perturbed backward lunge, the synergy related to the body weight acceptance was not present. The found modulation of the modular organization of motor control in the unstable condition and related minor alteration in joint kinetics indicates increased control robustness that allowed the participants to maintain functionality in postural challenging settings. Triggering specific modulations in motor control to regulate robustness in the presence of perturbations may be associated with the reported benefits of perturbation-based training.

Proactive Modulation in the Spatiotemporal Structure of Muscle Synergies Minimizes Reactive Responses in Perturbed Landings

“…Hence, it seems that predictive adjustment made by the participants during the single-leg landings were sufficient to cope with the UG and the unstable ground did not trigger reactive modulations of the neuromotor control which might be elicited if the difficulty of the task is increased. The result of our present setup is somewhat in disagreement with our previous findings during forward and backward lunging onto a foam beam—with similar mechanical characteristics to the current UG surface—where we found a modulation of the touchdown as well as the weight acceptance and stabilization synergies leading to a higher frequency of overlaps in the unstable condition ( Munoz-Martel et al, 2021 ). From a biomechanical point of view, a basic difference between single-leg landings and lunges is the dynamic state of the body mass at touchdown.…”

“…The damping behavior of the foam pads due to its viscoelastic properties might explain the significantly lower development of the GRF after touchdown and the reduced rate of joint moment development; the shorter lever arm of the GRF at the ankle joint, however, indicates an additional mechanism that explains the lower maximum ankle joint moment in UG. We found similar results (i.e., scarce differences in the EMG activity and a tendency toward lower resultant joint moments in the lower extremities) during forward and backward lunges on stable and unstable surfaces ( Munoz-Martel et al, 2021 ). Therefore, we can conclude that using unstable surfaces does not necessarily increase muscle loading per se.…”

“…In response, the neuromotor system modifies its strategies to increase control’s robustness (i.e., the ability to cope with perturbations) ( Santuz et al, 2018 ; Munoz-Martel et al, 2019 ). In earlier studies adopting the muscle synergies approach, we found specific modulations (i.e., wider, less unstable and less complex basic activation patterns of muscle groups) of the temporal structure of muscle synergies in the presence of perturbations ( Santuz et al, 2018 , 2020 ; Munoz-Martel et al, 2021 ). Such regulations of motor function in the presence of perturbations might be related to the efficacy of perturbation-based exercise interventions and its potential to enhance the ability of the motor system to respond and adapt to challenging conditions related to environmental changes during the daily life.…”

“…We performed an a priori power analysis using the findings from our earlier study investigating forward and backward lunges in stable and unstable surfaces ( Munoz-Martel et al, 2021 ). We found an effect size of 1.17 for the differences in the temporal structure of muscle synergies (i.e., width of the motor primitives) between stable and unstable condition, and assuming type I and type II errors of 0.05, we calculated that seven participants were sufficient for the designed study.…”

“…Recently, investigating forward and backward lunges on stable and unstable surfaces and using the muscle synergies approach, we found alterations in the spatiotemporal structure of muscle synergies during the stance phase (i.e., weight acceptance and stabilization synergy), resulting in an increased overlap between chronologically adjacent synergies in the unstable condition ( Munoz-Martel et al, 2021 ). However, studies investigating the EMG activity in the lower leg muscles during landings on stable and unstable grounds reported marginal effects of landing surface on the EMG activity ( Prieske et al, 2013 ; Hollville et al, 2020 ).…”