Cyclic H-Reflex Modulation in Resting Forearm Related to Contractions of Foot Movers, Not to Foot Movement

Abstract

During rhythmic voluntary oscillations of the foot, the excitability of the H-reflex in the Flexor Carpi Radialis (FCR) muscle of the resting prone forearm increases during the foot plantar-flexion and decreases during dorsiflexion. It is known that, when the two extremities are moved together, isodirectional (in-phase) coupling is the preferred form of movement association. Thus the above pattern of the H-reflex excitability modulation may favor the preferred coupling between the two limbs. To gain some clues about its origin, FCR H-reflex excitability was tested before and after modifying the phase relations between the activation [electromyogram (EMG)] of foot movers and foot movement, either by loading of the foot or by changing the movement frequency. After foot loading, the movement cycle was consistently delayed with respect to the onset of the EMG in Soleus (Sol) or Tibialis Anterior (TA) muscles. Simultaneously, the FCR H-reflex modulation advanced by that same amount with respect to the foot movement, thus remaining phase-locked to the EMG onsets. Similarly, when movement frequency was varied step-wise between 1.0 and 2.0 Hz, the foot movement was progressively delayed with respect to both the EMG onset (Sol and TA) and the FCR H-reflex modulation, so that the phase relation between the motor command to the foot and the H-modulation in the forearm remained constant. These results suggest that modulation of H-reflex in the forearm is tied to leg muscle contraction, rather than to foot kinematics, and point to a central, rather than kinesthetic, origin for the modulation.

Postural constraints to coupling of ipsilateral hand–foot movements

“…With regard to the present results, it is of special interest that during both impulsive and cyclic foot movements and irrespective of the movement direction (plantarflexion or dorsiflexion) and forearm position (prone or supine), APAs always arise in those forearm muscles that are 'isodirectional' with the foot prime movers. This distribution replicates the pattern of the subliminal excitability modulation observed in the resting forearm muscle when foot oscillations are performed in the sitting position [7][8][9], suggesting that the excitability modulation and APAs 1617 are the subliminal and supraliminal versions of one and the same phenomenon. It should be underlined, however, that when the foot movements are cyclic, the sine wave modulation of the forearm EMG may include not only APAs but also feedback components.…”

“…It has been recently observed [7][8][9] that, in sitting study participants performing voluntary rhythmic flexion-extensions of the foot, the excitability of the corticospinal pathway to the resting wrist muscles is sinusoidally modulated at the foot rhythm. With the forearm in a prone position, excitability increases in the flexors during foot plantarflexion and decreases during dorsiflexion, while in the extensors the opposite occurs.…”

The effect of immobilization of the head and trunk on arm-cycling-induced depression of soleus motoneuron pool excitability

“…However, other previous studies indicate that interlimb neural communication is phase-dependently modulated during rhythmic movement. Soleus H-reflexes were phase-dependently modulated during rhythmic arm swing (Hiraoka 2001;Hiraoka and Iwata 2006), interlimb cutaneous reflexes were phase-dependently modulated during rhythmic cycling movement (Balter and Zehr 2007) or walking Zehr and Haridas 2003), and oscillatory movement of the foot sinusoidally modulated the H-reflex in the flexor carpi radialis muscle (Baldissera et al 2002;Cerri et al 2003;Borroni et al 2004). In our study and previous studies on armcycling-induced depression of the soleus H-reflex, both arms moved cyclically at 180 degrees out of phase.…”

“…Several studies using the H-reflex have investigated this issue in humans. Oscillatory movement of the foot modulated H-reflex excitability in the forearm muscle (Baldissera et al 2002;Cerri et al 2003;Borroni et al 2004). The H-reflex in the soleus muscle, an ankle extensor, was depressed during rhythmic arm cycling (Frigon et al 2004;Loadman and Zehr 2007;Dragert and Zehr 2009;Hundza and Zehr 2009) and during rhythmic arm swing (Hiraoka 2001;Hiraoka and Iwata 2006).…”

APAs Constraints to Voluntary Movements: The Case for Limb Movements Coupling

“…After foot inertial loading (B) the foot flexion peak lags the onset of the TA burst by about 90°, while the H-reflex modulation remains phase-linked to the TA activation, showing to be insensitive to the afferent signals monitoring the phase shift of the foot position. Figure assembled with data from Cerri et al (2003) and Borroni et al (2004). …”