Triphasic behavioral response of motor units to submaximal fatiguing exercise

Abstract

We have measured the firing rate and amplitude of 4551 motor unit action potentials (MUAPs) recorded with concentric needle electrodes from the brachial biceps muscles of 10 healthy young adults before, during, and after 45 minutes of intermittent isometric exercise at 20% of maximum voluntary contraction (MVC), using an automatic method for decomposition of electromyographic activity (ADEMG). During and after exercise, MUAPs derived from contractions of 30% MVC showed progressive increase in mean firing rate (P less than or equal to .01) and amplitude (P less than or equal to .05). The firing rate increase preceded the rise in mean amplitude, and was evident prior to the development of fatigue, defined as reduction of MVC. Analysis of individual potentials revealed that the increase in firing rate and in amplitude reflected different MUAP subpopulations. A short-term (less than 1 minute) reduction in MUAP firing rates (P less than or equal to .05) was also observed at the onset of each test contraction. These findings suggest that motor units exhibit a triphasic behavioral response to prolonged submaximal exercise: (1) short-term decline and stabilization of onset firing rates, followed by (2) gradual and progressive increase in firing rates and firing variability, and then by (3) recruitment of additional (larger) motor units. The (2) and (3) components presumably compensate for loss of force-generating capacity in the exercising muscle, and give rise jointly to the well-known increase in total surface EMG which accompanies muscle fatigue.

Motor unit activity during long-lasting intermittent muscle contractions in humans

“…Contradictory ®ndings demonstrating an increase of the MU discharge rate have also been reported (Bigland-Ritchie et al 1986a;Dorfman et al 1990;Miller et al 1996). Maton and Gamet (1989) have found a slightly increased or unchanged ®ring rate in the biceps brachii and brachioradialis muscles.…”

Dissociation of the electrical and contractile properties in single human motor units during fatigue

“…4 If the fatiguing contractions continue, the unrelenting accumulation of K+ in the interstitial spaces will eventually overwhelm the capacity of the Na/K pump, resulting in electrical propagation failure and a decline in the size of the MUAP. 7,15 However, given that fast fatigable MUs are usually recruited at higher thresholds, sustained firing probably only occurs infrequently in common daily tasks.…”

“…The progressive enlargement in its rectified integrated area during submaximal fatiguing contraction is thought to primarily reflect an increase in the number of recruited motor units (MUs) and in their firing rates. 7,10,14 A third possibility is that the size of the motor unit action potential (MUAP) might change during fatiguing contraction. Such possibility indeed has been shown in animal studies, 1,8,18 but has not been systematically investigated in humans.…”

Spatial differences in fatigue‐associated electromyographic behaviour of the human first dorsal interosseus muscle.

“…With regard to mechanism (ii), the alterations of motoneuronal discharge rate and recruitment during submaximal constant force contractions, no consistent view emerges from previously published studies. Some authors have seen a decrease in firing rate together with an increase in recruitment (17-35% MVC, rectus femoris muscle; Person & Kudina, 1972), whereas others have found the maintenance of force to be associated with no change (30% MVC for 1 min, quadriceps muscle; BiglandRitchie, Cafarelli & V0llestad, 1986) or an increase of discharge frequency (20-30% MVC, biceps brachii and brachioradialis muscle; Maton & Gamet, 1989) combined with an increase in recruitment (20 % MVC, biceps brachii muscle; Dorfman, Howard & McGill, 1990). Some of these differences might be related to differences between target muscles and/or between contractions performed at different relative force levels.…”