Fatigue‐related changes in motor unit action potentials of adult cats

Abstract

The purpose of this study was to quantify the changes in motor-unit action potentials (MUAP) and force during a standard motor-unit fatigue test. MUAP waveforms were characterized by the measurement of amplitude, duration, area, and shape (as reflected in a coefficient of proportionality). Fatigue-resistant motor units exhibited small, but statistically significant, changes in MUAP amplitude and area during the fatigue test, whereas fatigable motor units displayed variable changes in MUAP amplitude, duration, and area. For all motor-unit types, the coefficient of proportionality did not change, and hence the change in MUAP area was proportional to the combined changes in amplitude and duration. The between- and within-train changes in MUAP were also distinct for the fatigue-resistant and fatigable motor units. Although several mechanisms could be responsible for the changes in the MUAP as the fatigue test proceeded, the dissociation of the time courses for MUAP and force indicated that these MUAP changes were not the principal reason for the decline in force under these conditions.

EMG Changes in Human Thenar Motor Units With Force Potentiation and Fatigue

“…These results also show that excitation of the sarcolemma and transmission across the neuromuscular junctions was effective and changes in these processes cannot explain the force loss. Similar conclusions have been reached in studies of single cat and rat motor units (Burke et al 1973;Celichowski et al 1991;Enoka et al 1992;Hamm et al 1989;Sandercock et al 1985), where it has been suggested that force loss relates to changes in phosphate metabolism, intercellular Ca 2ϩ handling and altered cross-bridge kinetics (Edman 1995;Westerblad and Allen 1991;Westerblad et al 1998).…”

“…However, for thenar units, there was usually an increase in the amplitude of the first EMG potential in each train of stimuli (Fig. 6A), whereas more variable amplitude effects have been observed in cat units exposed to the same fatigue protocol (Enoka et al 1992;Sandercock et al 1985). These results may reflect differences in relative contraction intensity.…”

“…Likewise, during muscle fatigue, the EMG may not change, may increase, or may decline for units that show large or intermediate force declines. In units that are more fatigue resistant, either no change or potentiation of EMG may accompany the decline in force (Burke et al 1973;Celichowski et al 1991;Enoka et al 1992;Gardiner and Olha 1987;Hamm et al 1989;Sandercock et al 1985). Furthermore, changes in EMG potentials, when present, usually recover before the force is restored (Sandercock et al 1985).…”

Changes in time and frequency related aspects of motor unit action potentials during fatigue

“…Application of sag and fatigue index according to [2,4] did not enable reliable classification of fast fatiguing, fatigue intermediate and fatigue resistant motor units in our rat EDLs. We noted that the motor unit characteristics in our research varied considerably within a category.…”

“…Fatigue itself is normally quantified with the mechanical behavior, but comes to expression in the electrical signals as well. For action potentials of different types of motor units the time characteristics have been followed during fatigue in animal experiments [ 1,2,3,4]. In human recordings a lot of attention has been given to the changes in the frequency spectra of the electromyogram [e.g.…”