Muscle strength and geometrical changes in a paralysed muscle following FES

This study proposes the application of a strengthening index to quantify the effect of training, by functional electrical stimulation (FES), on the force capacity of the quadriceps in spinal cord injury (SCI) subjects. The index is based on evaluating the average muscle force per unit area. This measure is shown to express the overall increase in the muscle force capacity, while accounting for the changes taking place in muscle geometry. The proposed index is demonstrated on one subject with SCI, on whom a longitudinal follow-up was conducted. The measurements included the knee extension torque, from which the force in the quadriceps muscle was evaluated. Additionally, in vivo magnetic resonance imaging of the thigh was used to obtain the muscle anthropometry. In the training period followed-up in this study, the average force per unit area was found to increase from 27 N/cm² in the pretrained muscle to 40 N/cm² after eight weeks of training by FES. The major increase in the physiological cross-sectional area of the muscle took place during the first four-week period; 12% of the total 13.5%. Conversely, only a minor change in the average force per unit area of the muscle was observed during the first four weeks of training (28 N/cm² at the end of the fourth-week). Thus, the major increase (43%) in the ratio of peak force to muscle physiological cross-sectional area was observed during the second four-week period of training. This latter response is attributed to neural adaptation of the axons and nueromuscular junction rather than to an increase in the muscle fibre specific tension.