Abstract
BackgroundModulation of nociceptive withdrawal reflex (NWR) excitability was evaluated during gait initiation in 10 healthy subjects to investigate how load- and movement-related joint inputs activate lower spinal centres in the transition from quiet stance to walking. A motion analysis system integrated with a surface EMG device was used to acquire kinematic, kinetic and EMG variables. Starting from a quiet stance, subjects were asked to walk forward, at their natural speed. The sural nerve was stimulated and EMG responses were recorded from major hip, knee and ankle muscles. Gait initiation was divided into four subphases based on centre of pressure and centre of mass behaviours, while joint displacements were used to categorise joint motion as flexion or extension. The reflex parameters were measured and compared between subphases and in relation to the joint kinematics.ResultsThe NWR was found to be subphase-dependent. NWR excitability was increased in the hip and knee flexor muscles of the starting leg, just prior to the occurrence of any movement, and in the knee flexor muscles of the same leg as soon as it was unloaded. The NWR was hip joint kinematics-dependent in a crossed manner. The excitability of the reflex was enhanced in the extensor muscles of the standing leg during the hip flexion of the starting leg, and in the hip flexors of the standing leg during the hip extension of the starting leg. No notable reflex modulation was observed in the ankle muscles.ConclusionsOur findings show that the NWR is modulated during the gait initiation phase. Leg unloading and hip joint motion are the main sources of the observed modulation and work in concert to prepare and assist the starting leg in the first step while supporting the contralateral leg, thereby possibly predisposing the lower limbs to the cyclical pattern of walking.
Highlights
Modulation of nociceptive withdrawal reflex (NWR) excitability was evaluated during gait initiation in 10 healthy subjects to investigate how load- and movement-related joint inputs activate lower spinal centres in the transition from quiet stance to walking
Modulation related to joint kinematics In the starting leg, significant differences in reflex probability rate (Table 1) and reflex size, related to the hip joint kinematics of both legs, were observed in the rectus femoris (RF) and vastus medialis (VM) muscles
In the RF muscle, the reflex size values were higher during hip extension, compared to flexion, of the starting leg [r. size: 6.8 (1.5) vs 4.0 (1.0), paired t-test, t = −2.585, df = 9, p = 0.029] and both reflex probability and size values were higher during hip flexion, compared to extension, of the standing leg [r. size: 6.1 (1.1) vs 3.2 (0.4), paired t-test, t = 3.372, df = 9, p = 0.008]
Summary
Modulation of nociceptive withdrawal reflex (NWR) excitability was evaluated during gait initiation in 10 healthy subjects to investigate how load- and movement-related joint inputs activate lower spinal centres in the transition from quiet stance to walking. Gait initiation comprises a preparatory and a stepping phase [7,8] in which the legs In this task, descending commands and load- and movement-related joint inputs converge on the spinal cord, probably interacting with the central pattern generator (CPG) to generate and control joint and muscle synchronisation during walking [11]. Knowledge of these interactions would improve understanding of how the muscle synergies essential to walking [12] are automatically selected at spinal cord level. The NWR is not involved in voluntary movements, afferents belonging to this reflex do participate in movement through alternative excitatory and inhibitory spinal neural pathways [27,28]
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