Calibration of the Leg Muscle Responses Elicited by Predictable Perturbations of Stance and the Effect of Vision

Abstract

Motor adaptation due to task practice implies a gradual shift from deliberate control of behavior to automatic processing, which is less resource- and effort-demanding. This is true both for deliberate aiming movements and for more stereotyped movements such as locomotion and equilibrium maintenance. Balance control under persisting critical conditions would require large conscious and motor effort in the absence of gradual modification of the behavior. We defined time-course of kinematic and muscle features of the process of adaptation to repeated, predictable perturbations of balance eliciting both reflex and anticipatory responses. Fifty-nine sinusoidal (10 cm, 0.6 Hz) platform displacement cycles were administered to 10 subjects eyes-closed (EC) and eyes-open (EO). Head and Center of Mass (CoM) position, ankle angle and Tibialis Anterior (TA) and Soleus (Sol) EMG were assessed. EMG bursts were classified as reflex or anticipatory based on the relationship between burst amplitude and ankle angular velocity. Muscle activity decreased over time, to a much larger extent for TA than Sol. The attenuation was larger for the reflex than the anticipatory responses. Regardless of muscle activity attenuation, latency of muscle bursts and peak-to-peak CoM displacement did not change across perturbation cycles. Vision more than doubled speed and the amount of EMG adaptation particularly for TA activity, rapidly enhanced body segment coordination, and crucially reduced head displacement. The findings give new insight on the mode of amplitude- and time-modulation of motor output during adaptation in a balancing task, advocate a protocol for assessing flexibility of balance strategies, and provide a reference for addressing balance problems in patients with movement disorders.

Reductions in body sway responses to a rhythmic support surface tilt perturbation can be caused by other mechanisms than prediction

“…Using a similar setup, the same authors also showed that adaptations to sequences that were repeated within (three identical 15-s sequences per trial) and across trials did not differ from exposing subjects to random sequences (Van Ooteghem et al 2010). Similarly, during continuous sinusoidal support translations, a decrease in body sway relative to the platform was found across time (Schmid et al 2011;Sozzi et al 2016). In summary, some studies using predictable perturbation sequences found changes in sway responses across sequence repetitions.…”

“…Several studies used perturbation sequences that can easily be predicted (Corna et al 1999;Oie et al 2002;Mergner et al 2003;Ravaioli et al 2005;Maurer et al 2006;Polastri et al 2012;Sozzi et al 2016). The most common example are sinusoidal stimuli.…”

“…In contrast, balancing in the presence of predictable perturbations could allow subjects to adapt the balance control mechanism and include predictive components. While a number of studies have used predictable perturbations (Corna et al 1999;Oie et al 2002;Mergner et al 2003;Ravaioli et al 2005;Maurer et al 2006;Polastri et al 2012;Sozzi et al 2016), the adaptations of the balance control mechanism taking place while performing a series of predictable perturbations are not well understood.…”

Human Balance in Response to Continuous, Predictable Translations of the Support Base: Integration of Sensory Information, Adaptation to Perturbations, and the Effect of Age, Neuropathy and Parkinson’s Disease

“…The anticipatory activities appear when the leg muscles are not stretched and are appropriate for counteracting the inertia of the body at the time of the turning points of the platform. Anticipatory activities may not be optimally tuned to the complex combination of active and passive body movements from the beginning of the perturbation sequence and may be adjusted as the perturbation proceeds [32,33]. configure anticipatory postural adjustments.…”

“…Under these circumstances, the sensorimotor processes underlying the adaptation to the repeated perturbations would require the collection of information about the amplitude velocity and frequency of the displacement of the support surface and of the instantaneous and continuously varying position of the body's CoM relative to the platform. This information would improve the performance by reducing redundant muscle activity (the initial co-contraction of the antagonist leg muscles) and enhancing the intersegmental coordination [33,41]. The reduction in the muscle activity concerns both the reflex responses and the anticipatory activities, a sign of coordinated modulation of responses occurring at different times of the perturbation cycle.…”