Voluntary activation of skeletal muscle in young and old women and men

University dissertation from Michael Miller, Department of Physical Therapy, Lasarettsgatan 7, 221 85 Lund, Sweden

Abstract: The evaluation of skeletal muscle strength is an important part of the physiotherapist’s clinical examination in order to plan and execute appropriate interventions. This requires the objective assessment of muscle strength and the determination of the underlying factors of muscle weakness. One factor that can contribute to muscle weakness is the inability of the central nervous system to voluntarily recruit and optimally activate the alpha motor neurons. A sub-optimal voluntary activation (VA) is termed central activation failure (CAF). A common way to assess VA and to detect and quantify CAF is to superimpose an electrical stimulus over the motor nerve to the muscle during a maximal voluntary contraction (MVC). If the stimulus augments torque, then a CAF may be inferred. The overall objectives of this thesis were: i) to develop the methodology of the superimposed electrical stimulation technique; ii) to use the technique to assess the degree of voluntary activation during isometric knee extension in healthy, moderately active, young and older women and men; and iii) to evaluate the degree of discomfort caused by the superimposed electrical stimulation technique. Two methodological studies were performed using a model of CAF whereby the subjects were stimulated at 80% MVC of the knee extensors. The effects of the stimuli on torque increment size, the variability of torque increment size and the perceived discomfort from the stimuli were assessed. Discomfort was assessed using a 100 mm visual analogue scale for pain (VAS-pain). The main findings were that a pulse train stimulus at 100 Hz was superior to single impulses to illicit torque increments during high levels of voluntary action and should be the preferred stimulus mode to study VA. There was no difference in the size of the torque increments when pulse trains of lengths 100 ms, 200 ms or 300 ms were used or when pulse trains at 150 V and 200 V were used. Pulse trains of 100 ms length gave rise to less variability in the torque increment size than trains of 50 ms, but the discomfort from 100 ms trains was somewhat greater than from 50 ms trains. Pulse durations of 0.1 ms and 0.2 ms gave similar torque increments, but pulse durations of 0.1 ms induced significantly less discomfort at sub-maximal effort. The results from the VA assessments of healthy, young women and showed that they have the ability to fully voluntarily activate the knee extensors isometrically, but not on every attempt. Older men achieved very high VA levels, similar to those of the younger men. Older women displayed a slightly lower level of VA and some of the older women never achieved full voluntary activation on any attempt. The mean score of perceived discomfort during 100% MVC was 35 mm for the young women and men and 40 mm for the older women and men. These findings indicate that, with the electrical stimulation parameters used (pulse trains 100 Hz, 100 ms length, pulse duration 0.1 ms or 0.2 ms, 150 V), the level of discomfort was generally moderate and tolerable but not negligible. In conclusion, these developments of the superimposed electrical stimulation technique will allow a more detailed and adequate assessment of VA and CAF. The findings of very high VA levels (>97%) in healthy, active, young and old women and men will aid the interpretation of VA assessments in patients. The results from the discomfort evaluations will assist the researcher or clinician to better inform future patients about the general level of discomfort to be expected from assessments using the superimposed electrical stimulation technique.

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