No abstract

This study aimed to assess modulation of lower leg muscle reflex excitability and co-contraction during unipedal balancing on compliant surfaces in young and older adults. Twenty healthy adults (ten aged 18-30 years and ten aged 65-80 years) were recruited. Soleus muscle H-reflexes were elicited by electrical stimulation of the tibial nerve, while participants stood unipedally on a robot-controlled balance platform, simulating different levels of surface compliance. In addition, electromyographic data (EMG) of soleus (SOL), tibialis anterior (TA), and peroneus longus (PL) and full-body 3D kinematic data were collected. The mean absolute center of mass velocity was determined as a measure of balance performance. Soleus H-reflex data were analyzed in terms of the amplitude related to the M wave and the background EMG activity 100 ms prior to the stimulation. The relative duration of co-contraction was calculated for soleus and tibialis anterior, as well as for peroneus longus and tibialis anterior. Center of mass velocity was significantly higher in older adults compared to young adults (p < 0.001) and increased with increasing surface compliance in both groups (p < 0.001). The soleus H-reflex gain decreased with surface compliance in young adults (p = 0.003) , while co-contraction increased (p SOL,TA = 0.003 and p PL,TA < 0.001). Older adults did not show such modulations, but showed overall lower H-reflex gains (p < 0.001) and higher co-contraction than young adults (p SOL,TA < 0.001 and p PL,TA = 0.002). These results suggest an overall shift in balance control from the spinal level to supraspinal levels in older adults, which also occurred in young adults when balancing at more compliant surfaces.

With training older adults can improve balance control, but the time course and neural mechanisms underlying these improvements are unclear. We studied changes in balance (robustness and performance), as well as in H-reflex gains, paired reflex depression (PRD) and co-contraction duration (CCI) in ankle muscles after short-term (1 session; STT) and long-term (3 weeks; LTT) balance training in 22 older adults. Mediolateral balance robustness during unipedal stance (time to balance loss in unipedal standing on a robotic platform with decreasing rotational stiffness) improved (33%) after STT, with no further improvement after LTT. Balance performance (mean absolute mediolateral center of mass velocity) improved (18.75%) after STT in perturbed unipedal standing and after LTT (18.18%) in unperturbed unipedal standing. CCI of soleus/tibialis anterior did not change after STT but increased (16%) after LTT. H-reflex gain and PRD excitability did not change with training. Cross-correlations showed that H-reflex gains in unipedal stance were lower and CCI was higher in participants with a more robust balance at the last time-point measurement and, CCI was higher in participants with better balance performance at several time-points. However, changes in robustness and performance were uncorrelated with changes in CCI, H-reflex gain, or PRD. Our results indicate that balance robustness improves over a single session, while balance performance improves more gradually over multiple sessions. Changes in co-contraction and motor neuron excitability of ankle muscles are not exclusive causes of improved balance performance and robustness.

Recovering balance after perturbations becomes challenging with aging, but an effective balance training could reduce such challenges. In this study, we examined the effect of balance training on feedback control after unpredictable perturbations by investigating balance performance, recovery strategy, and muscle synergies. We assessed the effect of balance training on unipedal perturbed balance in twenty older adults (>65 years) after short-term (one session) and long-term (3-weeks) training. Participants were exposed to random medial and lateral perturbations consisting of 8-degree rotations of a robot-controlled balance platform. We measured full-body 3D kinematics and activation of 9 muscles (8 stance leg muscles, one trunk muscle) during 2.5 s after the onset of perturbation. The perturbation was divided into 3 phases: phase1 from the onset to maximum rotation of the platform, phase 2 from the maximum rotation angle to the 0-degree angle and phase 3 after platform movement. Balance performance improved after long-term training as evidenced by decreased amplitudes of center of mass acceleration and rate of change of body angular momentum. The rate of change of angular momentum did not directly contribute to return of the center of mass within the base of support, but it reoriented the body to an aligned and vertical position. The improved performance coincided with altered activation of synergies depending on the direction and phase of the perturbation. We concluded that balance training improves control of perturbed balance, and reorganizes feedback responses, by changing temporal patterns of muscle activation. These effects were more pronounced after long-term than short-term training.