Electrical stimulation of peripheral nerves for the recovery of hand function in tetraplegic patients of fundamental importance in improving quality of life.

The muscle selectivity index will be obtained by intraneural stimulation with a TIME-4H astimulator. Single stimulation pulses (1 Hz) will be applied from each active site of each implanted TIME-4H electrode. Simultaneously, electromyographic (EMG) signals of the flexor and extensor muscles of the forearm and hand, innervated by the median and radial nerves, respectively, will be recorded. For the EMG acquisitions we will use the CE-marked Sessantaquattro+ system. The peak-to-peak value of the compound muscle action potential (CMAP), evoked by each stimulation pulse, will be calculated and normalised with respect to the maximum value obtained for each muscle during the overall experimental session. These normalised levels of muscle activation will be used to define the relationship between current intensity and muscle recruitment for each active site and so to calculate an index of muscle selectivity.

To assess functional selectivity, bursts of stimulation, with a duration of 0.5-1 sec, will be applied from each active site of each implanted TIME-4H. Evoked movements will be assessed using the Medical Research Council (MRC) scale. In addition, we will characterise evoked movements in terms of muscle activity patterns by recording the EMG activity of the muscles with the Sessantaquattro+ system, and in terms of kinematic patterns (amplitude and velocity) and generated force. The same experiments will then be performed by modulating the frequency of the stimulation bursts (in the range 10-100 Hz) while keeping the amplitude fixed at a value capable of generating functional movements. Thus, we will evaluate the possibility of gradually controlling the amplitude and force of the movement by adjusting the different stimulation parameters.

To measure muscle fatigue, experiments will be conducted in which stimulation trains will be applied for approximately 20-30 seconds. At the same time, the EMG activity of the forearm and hand muscles and the grip force generated will be recorded. Some indices of muscle fatigue found in the literature will then be calculated, which measure, for example, the rate at which muscle strength decreases over time. The values obtained will be compared with those measured during voluntary contractions and with those induced by other stimulation modalities, such as FES with surface electrodes.

Somatosensory cortical mapping of the paralysed hand will be performed using somatosensory evoked potentials (PESs). The median and ulnar nerves will be stimulated using bipolar surface electrodes at a frequency of 1 Hz. The amplitude of the response will be measured automatically by the BE Plus PRO system, as the difference between the peak potential and the signal baseline, for each electrode.