Determination of fatigue in the electrically stimulated quadriceps muscle and relative effect of ischaemia

The creation of muscle fatigue using surface electrical stimulation represents a highly reproducible phenomenon in spinal cord injured patients. The torque output was recorded as a function of time. The fatigue curves recorded over 110s exhibited three main parts: first, a plateau of short duration, followed by a more or less steep slope and then a second plateau which was maintained for a long time. This phenomenon was fitted using an exponential equation which had been developed and four parameters introduced that outlined to the muscle behaviour. A set of fatigue indices was defined to characterize the asymptotic value, the slope, the coordinates of the inflexion point, the time constant and the vertical amplitude of the curve recorded. Two populations were studied; a group of 11 thoracic level of injury paraplegic patients and a group of 10 able-bodied control subjects. The computed coefficients of determination, r², were of very high values (0.99). Therefore, fatigue indices gave reliable information. Torque output did not differ between the two populations until 25 s had elapsed, but from 30s onwards it was markedly lower in paraplegics. The residual torque output was 21.1 ± 10.6% in the paraplegic group while it was 58.5 ± 8.9% in the control group. The effect of blood supply in the production of fatigue was also studied by repeating the same test using a tourniquet at the groin level. The residual torque became 14.7 ± 2.3% in the paraplegic group and 42.9 ± 6.3% in the control group. This test permitted the determination at a gross level of the different metabolic phases and therefore the recruitment of the different populations of muscle fibres within the quadriceps. Fast fatiguable fibres (type FF) produced the maximum output during the first phase, then fast resistant (type FR) explained the slope, and slow fibres (type S) were responsible for the second plateau. The effects of the tourniquet were obvious on the fatigue indices in both populations. In the spinal cord injured patients, there appeared to be a lack of slow fibres, probably due to disuse. Fatigue appeared to be independent from blood flow in paraplegics.