Pointing movement in short and long-term exposure to hypoxia.

Kinematics variables of pointing movements where assessed in five adult subjects exposed acutely (30 min) and chronically (10 days) to a low O2 mixture (13.5% O2 in N2). Amplitude of displacement did not vary in both experimental conditions but movement duration markedly increased compared to pre and post exposure conditions. While in acute hypoxia the times of acceleration and deceleration are almost equal, in chronic hypoxia deceleration time exceeded of 100 ms the time of acceleration. The time from the peak acceleration to the peak of deceleration ("switch" time) increased in both experimental conditions and was about 50% of the movement duration. This time lengthening at hypoxia may be explained either by alteration of propioceptive loops or by a different strategy elaborated by the CNS to generally slow accurate movements.

Termination of upper limb movement by cutaneous afferents.

We tested whether cutaneous afferents from the skin field close to an upper limb muscular region would carry information to spinal neurons at the onset or at the offset of a voluntary elbow extension movement lasting 1 s. We detected a depression of EMG activity both at onset and at the offset of the reaching movement but in the latter case depression was significantly larger and immediate. The marked depression of EMG activity suggests an inhibition, via spinal neurons, of the descending excitation to the motoneurones supplying the triceps brachii. This spinal control might be a very efficient mechanism for the termination of voluntary movement.