Modulation of excitability in the cerebral cortex projecting to upper extremity muscles by rotational positioning of the forearm.

The forearm rotation changes sensory inputs to the central nervous system, thereby providing orientation of the hand for grasping an object. Electrical activities of the muscles, induced by transcranial magnetic stimulation to the brain, i.e., motor evoked potentials (MEPs), are used for estimation of the excitability of motor neurons in the brain and spinal cord. It is well known that rotational positioning of the forearm influences MEPs of forearm muscles through modulation of excitability in the central nervous system. In the present study, we investigated whether such a posture-dependent change of MEPs could be found in upper arm and intrinsic hand muscles at three different rotational forearm positions: the most internal (pronation), neutral, and most external (supination) positions of rotation. MEPs were simultaneously recorded from the four muscles, biceps brachii (BB), triceps brachii (TB), abductor digiti minimi (ADM), and abductor pollicis brevis (AbPB). MEP amplitudes and latencies in BB, TB and ADM were significantly larger and shorter, respectively, in supination compared to the values in other positions. By contrast, MEP of AbPB in supination was lower in amplitude and longer in latency. Importantly, muscle lengths of TB, ADM and AbPB are constant in any rotational forearm positions, excluding the possibility of the muscle-length dependent change of spinal reflex. Therefore, these results might be attributable to the posture-dependent modulation of the motor cortex activity for the upper limb. The motor cortex probably changes the control strategy for the upper limb muscles in accordance with the sensory input from the forearm.