Abstract: BACKGROUND: Early postural adjustments serve as preparatory measures for forthcoming actions or potential disruptions in posture, thereby facilitating improved movement execution and mitigating destabilizing effects caused by posture interference.
OBJECTIVE: To investigate the characteristics of temporal and intensity parameters of key lower limb muscles during early postural adjustment phase when stroke patients with varying levels of balance initiate walking at a self-selected comfortable pace. 
METHODS: The characteristics of early postural adjustments in 16 stroke patients were observed. Sixteen patients were divided into a non-fall group (n=8) and a fall group (n=8) based on the history of falls and Berg Balance Scale scores. Noraxon inertial sensors and Noraxon Ultium EMG wireless surface electromyography were utilized to collect body kinematic data and surface electromyography data during gait initiation. Muscle activation time and activation sequence of six key muscles in the lower limbs (tibialis anterior, medial and lateral gastrocnemius, rectus femoris, lateral femoris and biceps femoris muscles) during the early postural adjustment phase, as well as normalized electromyography integral values for the four time windows (each 150 ms) before gait initiation, were analyzed.
RESULTS AND CONCLUSION: Stroke patients with a history of falls exhibited earlier activation times for the six key muscles in the lower limbs during gait initiation compared with those in the non-fall group. The fall group demonstrated significantly earlier activation times for tibialis anterior, lateral head of gastrocnemius, and vastus lateralis (P < 0.01, P < 0.05). In contrast, the non-fall group displayed a consistent pattern of activating extensor muscles before flexor muscles, with thigh muscle activation preceding calf muscle activation. However, in the fall group, calf extensor muscle activation occurred prior to thigh extensor muscle activation, and the vastus lateralis was activated even earlier. The tibialis anterior was the last activated muscle in both groups. Specifically during T3 (> -300 to -150 ms), the tibialis anterior exhibited significantly higher activity in the fall group compared with the non-fall group (P < 0.05), while the lateral head of gastrocnemius demonstrated significant inhibition during T3 (P < 0.05) and the medial head of gastrocnemius showed significant inhibition during both T3 and T4 (> -150 to 0 ms) stages compared with the non-fall group (P < 0.01, P < 0.05). To conclude, stroke patients with varying balance abilities employ distinct early postural adjustment strategies prior to stepping, as evidenced by differences in muscle activation timing, recruitment order, and muscle activity amplitude. Patients at a high risk of falling exhibit prolonged duration of early postural adjustment and delayed initiation of gait, indicating earlier activation of the tibialis anterior muscle and inhibition of gastrocnemius muscle activity. These delays in gait initiation and variations in muscle recruitment strategies may contribute to unstable posture and an increased susceptibility to falls.

Key words: feedforward postural adjustments, early postural adjustments, stroke, gait initiation, fall