Variability of the fatigue response of paralyzed skeletal muscle in relation to the time after spinal cord injury: mechanical and electrophysiological characteristics

The aim of this study was to determine the effect of the time after spinal cord injury (less than and greater than 10 months) on the mechanical and el ectrophysiological characteristics of muscle fatigue of the paralyzed elect rically stimulated quadriceps muscle. Morphologically and histochemically, a relationship was observed between muscle fatigue and the delay from injur y, revealing a critical period of enzymatic turning and a maximum peak of a trophy around the 10th month after the injury, followed by a long-term stab ilization. Knee-torque output and M-wave variables (amplitude, latency, dur ation, and root mean square, RMS) of two muscular heads of the quadriceps w ere recorded in 19 paraplegic patients during a 120-s isometric contraction . The fatiguing muscle contraction was elicited by supramaximal continuous 20-Hz electrical stimulation. Compared to the chronic group, the acutely pa ralyzed group showed a greater resistance to fatigue (amount and rate of fo rce decline, P less than or equal to 0.01), smaller alterations of the M-wa ve amplitude and RMS, and a limited decrease of the muscle fiber conduction velocity (P < 0.05). Mechanical and electrophysiological changes during fa tigue provided a clear functional support of the transformation of skeletal muscle under the lesion and of the existence of a critical period of muscu lar turn. In conclusion, when considering the artificial restoration of mot or function, the evolution of the endurance and force-generating capabiliti es of the muscle actuator must be taken into account, particularly when tas ks require important safety conditions (e.g., standing and walking).