Movement features and H-reflex modulation. II. Passive rotation, movement velocity and single leg movement

. Corticospinal excitability is lower during rhythmic arm movement than during tonic contraction. J Neurophysiol 95: 914 -921, 2006. First published October 26, 2005 doi:10.1152/jn.00684.2005. Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motorevoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion (F ϭ 13.51, P ϭ 0.006; F ϭ 11.83, P ϭ 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.

Section: Discussionsupporting

confidence: 90%

Corticospinal Excitability Is Lower During Rhythmic Arm Movement Than During Tonic Contraction

Carroll

Baldwin²,

Collins³

et al. 2006

“…During active pedaling with one leg, the soleus H-reflex in the contralateral resting limb undergoes a profound modulation ) which is absent during passive pedaling movements, a condition in which the Sol H-reflex is tonically depressed (Cheng et al 1998;Collins et al 1993;McIlroy et al 1992). Voluntary oscillations of the whole arm (Hiraoka 2001) as well as passive oscillations of the forearm (Hiraoka and Nagata 1999) have both been reported to modulate the Sol H-reflex excitability in a phase-dependent way.…”

Section: Discussionmentioning

confidence: 99%

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

Cerri¹,

Borroni²,

Baldissera³

2003

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.

“…When actively or passively pedaling, increasing the velocity of the movement inhibits the soleus H-reflex (McIlroy et al 1992). To our knowledge, similar data are not available for the upper limb, although Carroll et al (2006) showed an increase of corticospinal excitability (H-reflex and motor evoked potentials) during cycling movements of the arm compared with tonic contractions.…”

Section: Methodological Issuesmentioning

confidence: 99%

Physiological Basis of Limb-Impedance Modulation During Free and Constrained Movements

Damm

McIntyre²

2008

Damm L, McIntyre J. Physiological basis of limb-impedance modulation during free and constrained movements. J Neurophysiol 100: 2577-2588, 2008. First published August 20, 2008 doi:10.1152/jn.90471.2008. Arm stiffness is a critical factor underlying stable interactions with the environment. When the hand moves freely through space, a stiff limb would most effectively maintain the hand on the desired path in the face of external perturbations. Conversely, when constrained by a rigid surface, a compliant limb would allow the surface to guide the hand while minimizing variations in contact forces. We aimed to identify the physiological basis of stiffness adaptation for these two classes of movement. Stiffness can be regulated by two mechanisms: coactivation of antagonistic muscles and modulation of reflex gains. We hypothesized that subjects would select high stiffness (high coactivation and/or reflex gains) in free space and high compliance (low coactivation and reflex gains) for constrained movements. We measured EMG and the H-reflex during constrained and unconstrained movement of the wrist. As predicted, subjects coactivated antagonist muscles more when performing the unconstrained movement. Contrary to our hypothesis, however, Hreflex amplitude was higher for the constrained movement despite the a priori preference for lower reflex gains in this situation. In addition, the H-reflex depended on the task and the net force exerted by the limb on the environment, rather than showing a simple dependence on the level of muscle activation. Thus stiffness seems to increase in free space compared with constrained motion through the use of coactivation, whereas spinal loop gains are adjusted to better regulate the influence of afferences on the ongoing movement. These observations support the hypothesis of movement programming in terms of impedance.