Stimulus Prediction and Postural Reaction: Phase-Specific Modulation of Soleus H-Reflexes Is Related to Changes in Joint Kinematics and Segmental Strategy in Perturbed Upright Stance

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.

Biarticular muscles are most responsive to upper-body pitch perturbations in human standing

“…When mechanical perturbations are applied, humans use different strategies to prepare or counteract the effect of the perturbation 17,61,62 . For instance, when a perturbation is expected or predicted, activity of antagonistic muscles can be increased simultaneously (co-contraction) in order to build up joint impedance before the perturbation onset 63 .…”

Functional deficits in chronic mechanical ankle instability

“…Measurements started with the unaffected leg first and two trials were performed over a period of 30 s each, separated by 1 min breaks as previously described in the literature [41]. The center of pressure (COP) displacement was recorded and averaged for the medio-lateral and the anterior-posterior direction; the 90% ellipse of the COP's area was calculated [42]. This has previously been used a valid and reliable measure of postural control [8,41].…”

Trunk Angle Modulates Feedforward and Feedback Control during Single-Limb Squatting

“…Feedback responses to perturbations include the short-latency reflex (0-50 ms), the long-latency reflex (51-200 ms), and volitional reaction time (>200 ms) [10]. The longlatency reflex is the first feedback response with cortical involvement [11] and it occurs at a latency and amplitude that may facilitate a corrective response before soft tissue damage occurs (200 ms) [12].…”

“…Feedforward factors such as biomechanical position [19][20][21][22], activation of the muscles prior to the perturbation [10,13], and the magnitude of the perturbation [13,18] all determine the initial conditions for a long-latency reflex response. Body position, such as a flexed trunk, is a key way to manipulate the feedforward background muscle activity level [23][24][25].…”