1. The discharge properties of single motor units during prolonged maximal voluntary effort have been studied using electromyographic recordings, mainly in the short big toe extensor muscle but also in the anterior tibial muscle. 2. The required selectivity of the e.m.g. recordings was achieved in the short big toe extensor muscle after previous mechanical lesions to the terminal nerve twigs and muscle fibres and consequent collateral sprouting, and in the anterior tibial muscle with the use of a high impedance wire electrode. 3. During the first few hundred milliseconds of sustained maximal effort the motor units fired at rates ranging from about 30‐60 Hz, and the tension was the same as that obtained on electrical tetanization of the nerve to the toe extensor muscles above 50 Hz. 4. During prolonged maximal effort the firing rates and the proportion of motor units firing successively decreased. Motor units initially firing at 30 Hz continued to fire tonically but at 15‐20 Hz. Motor units initially firing at 60 Hz ceased to fire tonically but could still be made to discharge phasically. The period of time during which all motor units responded tonically could be increased from some seconds up to 20 sec by long‐term training. 5. Motor units with a limited endurance fired at a lower tension in the early than in the late stages of maintained contraction. 6. It is suggested that motoneurones innervating slow twitch muscle fibres respond continuously to prolonged voluntary drive at rates sufficient for full fusion but that the threshold of motoneurones innervating fast twitch muscle fibres increases so that they finally mainly fire phasically thus protecting the peripheral excitation and contractile mechanisms from excessive exhaustion.

SUMMARY1. The contraction time and the voluntary discharge properties of forty-five short toe extensor motor units were studied in man.2. The contraction time of the individual motor unit was studied by using selective electrical nerve stimulation or by averaging the increase in force related to its electromyographic potential in tonic voluntary contraction.3. Both methods showed a range of contraction times from 40 to 90 ms.

Study design: Pilot study. Objectives: The aim of the study was to develop a neurophysiological method to diagnose the cranial as well as the caudal level of a complete thoracic spinal cord injury (SCI) with higher precision than today's protocols. Setting: SCI unit Karolinska University Hospital, Stockholm, Sweden. Methods: Bipolar needle electromyography was recorded in intercostal spaces of five patients with chronic, complete thoracic SCI. Tests were performed during rest, during voluntary activation and during activation of lower body spasticity. Magnetic resonance imaging (MRI) was performed in each patient according to a protocol optimized for imaging near metal implants. Results: Three distinct patterns were found in each patient. Above the lesion we found voluntary activated, normal motor unit potentials (MUPs). At the neurological level and a varying number of segments below, denervated intercostal segments with fibrillation potentials and positive sharp waves appeared. Below the neurological level, normal MUP activated in concert with lower body spasticity was found. The number of denervated segments showed a significant correlation to the length of spinal cord discontinuity on MRI (r¼0.97, Po0.05). Conclusion: Intercostal neurophysiology in combination with MRI optimized for imaging near metal implants can be used to determine the extent of a chronic complete thoracic SCI, both anatomically and functionally. The described method increases the sensitivity to detect delicate neurological changes related to the dynamic of the pathology that follows SCI and may be useful in analyzing outcome in clinical trials.

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