Electrical stimulation to promote muscle and motor unit force and endurance after spinal cord injury.

We asked whether 20 Hz electrical stimulation for 5% of each day (2.5 sec on and 2.5 sec off for 3 hours; 5%-ES) preserves medial gastrocnemius and tibialis anterior muscle and MU isometric contractile forces and increases their endurance after paralysis. Daily 5%-ES promoted increased muscle endurance irrespective of the muscle length or loading but to a lesser extent than daily 50%-ES (20 Hz ES 2.5 sec on and 2.5 sec off for 24 hours). 5%-ES was effective in counteracting decline and slowing of muscle force that resulted from 50%-ES. Motor units (MUs) were converted from fast fatigable to fatigue intermediate and resistant MUs, comprising ∼80% as compared to ∼10% in the control normal groups. We conclude that the 5%-ES regimen counteracts the fatigue of paralyzed muscle without compromising contractile force, and thereby is effective in conditioning the muscle for effective movement. Abstract figure legend: Chronic electrical stimulation (ES) at 20 Hz of the motor nerve in the paralyzed hindlimb of the awake cat was delivered at 2.5 seconds on and 2.5 seconds off for 3 hours (5% ES) or for 24 hours (50% ES). Regular isometric force recordings made under deep anaesthesia with the hindlimb stabilized and the foot coupled to a force transducer. Comparisons of the endurance of the muscle to repetitive stimulation between a control cat with no ES and with experimental cats whose nerves were stimulated with 5% and 50% ES, showed that the 5% ES regimen prevented the fall in endurance following paralysis and sustained muscle force in contrast to the 50% ES regimen that increased endurance but promoted a profound decline in muscle force. These findings demonstrate the efficacy of a 5% ES regimen for preventing muscle fatigue without a detrimental decline in force capacity of the paralyzed muscle. This article is protected by copyright. All rights reserved.