Abstract
Anticipation determines the timing and efficiency of human motor performance. This study aimed to evaluate the effect of stimulus anticipation on proactive (prior to the event) and reactive (after the event) postural adjustments in response to perturbations. Postural set was manipulated by providing either (i) predictable, (ii) unpredictable, or (iii) cheated perturbations which require balance corrections to maintain postural stability. In 29 subjects, a protocol of anterior and posterior perturbations was applied for the conditions (i–iii). Center of pressure (COP) displacement, ankle, knee, and hip joint kinematics and electromyographic activity (EMG) of the soleus (SOL) and tibialis anterior (TA) muscles were recorded prior (PRE) and after posterior perturbations. SOL H-reflexes at the peak of the short-, medium- ,and long-latency responses (SLR, MLR, LLR) were assessed. For conditions (i to iii) EMG activity and COP differed prior to perturbation onset (p < 0.05). After perturbation, results demonstrated a progressively increased H-reflex amplitude in the MLR and LLR (p < 0.05), delayed muscle activities (p < 0.05), and shifted activation patterns, with muscles of the proximal segment being more involved in the compensatory postural response (p < 0.05). COP displacements and ankle, knee, and hip joint deflections progressively increased (p < 0.05). Neuromechanical coupling showed positive correlations for the anticipation-induced changes in EMG activity and H-reflex amplitude with that of COP displacement (p < 0.05). In conclusion, proactive and reactive postural responses indicated setting dependent modulations of segmental and phasic muscle activation. A shift to proximal muscle groups and facilitated late reflex responses compensating for cheated or unpredicted perturbations was found to recover a safe body equilibrium. In consideration of the phase-specific adaptation and its interrelationship to the kinematics, it suggested that changes in stimulus prediction challenged the central nervous system to appropriately counteract the higher postural challenges. The outcomes of this experiment are of functional relevance for experimental and training settings involving perturbation stimuli. These findings provide fundamental information of the mechanisms underlying postural adjustments in response to external perturbations.
Highlights
Humans experience perturbations applied to their body (Macpherson et al, 1989) due to the displacement of the body’s center of mass (COM) beyond the boundaries of the base of support (Maki and McIlroy, 1996)
The central nervous system (CNS) uses 2 main strategies to restore balance if it is disrupted by a perturbation: (1) the proactive postural adjustments based on feed-forward mechanisms made throughout the anticipation phase at a conscious and subconscious level prior to perturbations (Belen’kii et al, 1967; Mohapatra et al, 2012), and (2) the compensatory postural adjustments based on feedback initiated by sensory signals after perturbations (Horak and Nashner, 1986; Horak et al, 1989; Mohapatra et al, 2012)
Three different protocols were used applying anterior/posterior surface translations (Figure 1A): (i) the perturbation direction was known and predictable, this protocol served as a control; (ii) aimed to assess perturbation-induced effects when unpredictable perturbations occurred randomly in an anterior or posterior direction and the direction of deterioration stimulus was unknown (Okai and Fujiwara, 2013; Kanekar and Aruin, 2014a,b); and (iii) we investigated the influence of the perturbation when subjects were cheated and the direction was indicated incorrectly
Summary
Humans experience perturbations applied to their body (Macpherson et al, 1989) due to the displacement of the body’s center of mass (COM) beyond the boundaries of the base of support (Maki and McIlroy, 1996). The predictability has been clustered in three modalities based on an increased level of difficulty: (i) predictable, (ii) unpredictable, and (iii) wrongly predicted perturbation of postural stability (Horak et al, 1989). The neuromechanics of the recovery response in regard to the predictability of the perturbation, whether (i, ii, or iii), is the topic of the current paper Despite articles comparing these three modalities being rare, the state of the art in terms of neuromuscular and kinematic distinctions is described below
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