DETERMINATION OF FATIGUE IN THE ELECTRICALLY STIMULATED QUADRICEPS MUSCLE AND RELATIVE EFFECT OF ISCHEMIA

The creation of muscle fatigue using surface electrical stimulation re presents a highly reproducible phenomenon in spinal cord injured patie nts. The torque output was recorded as a function of time. The fatigue curves recorded over 110s exhibited three main parts: first, a platea u 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 se t 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 a nd a group of 10 able-bodied control subjects. The computed coefficien ts of determination, r2, were of very high values (0.99). Therefore, f atigue indices gave reliable information. Torque output did not differ between the two populations until 25s had elapsed, but from 30s onwar ds it was markedly lower in paraplegics. The residual torque output wa s 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 fa tigue was also studied by repeating the same tests using a tourniquet at the groin level. The residual torque became 14.7 +/- 2.3% in the pa raplegic group and 42.9 +/- 6.3% in the control group. This test permi tted the determination at a gross level of the different metabolic pha ses and therefore the recruitment of the different populations of musc le fibres within the quadriceps. Fast fatiguable fibres (type FF) prod uced the maximum output during the first phase, then fast resistant (t ype 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 p atients, there appeared to be a lack of slow fibres, probably due to d isuse. Fatigue appeared to be independent from blood flow in paraplegi cs.