Brenda Bigland-Ritchie

Muscle fatigue induced by stimulation with and without doublets.

Muscles are usually stimulated by shocks delivered at some constant rate. However, human thenar motor units generate optimum force per pulse when excited by impulse trains that begin with one or two short interpulse intervals (“doublets”), followed by longer intervals. Our aim was to determine whether the rate of force and force–time integral reduction during fatigue of thenar muscles is influenced by an initial doublet, and/or the number of pulses per train. We first matched thenar force–time integral using two different pulse patterns, one of which began with a doublet. Fatigue induced by trains that contained a doublet resulted in slower rates of force and force–time integral reduction and smaller increases in half‐relaxation time than that evoked by bursts of 40‐HZ stimulation. When the force was measured in each protocol after equal numbers of pulses had been delivered, the force loss was still significantly less for pulse trains containing a doublet. These results have useful implications when designing stimulation to strengthen weak muscles or to drive paralyzed muscles.

Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor

1 While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean ±s.e.m., 183 ± 27 spikes per train; range, 100–782 spikes) from 26 single motor units in extensor hallucis longus during sustained (60–180 s) maximal dorsiflexions of the big toe in seven human subjects. 2 Long spike trains were recorded from 13 units during the first 30 s of a maximal voluntary contraction (mean train duration, 9.6 ± 1.2 s; range, 3.6–21.9 s) and from 13 units after 30 s (mean train duration, 16.6 ± 3.7 s; range, 7.1–58.1 s). Maximal isometric force generated by the big toe declined to 78.3 ± 6.3 % of its control level by 60–90 s and to 39.5 ± 1.4 % of control by 120–150 s. Despite this substantial fatigue, mean firing rates did not change significantly over time, declining only slightly from 15.8 ± 0.7 Hz in the first 30 s to 14.0 ± 0.5 Hz by 60–90 s and 13.6 ± 0.3 Hz by 120–150 s. 3 To assess fatigue‐related adaptation in discharge frequency and variability of individual motor units, each spike train was divided into 2–15 equal segments containing at least 50 interspike intervals. Discharge variability was measured from the coefficient of variation (s.d./mean) in the interspike intervals, with the s.d. being calculated using a floating mean of 19 consecutive intervals. Adaptation was computed as the average change in firing rate or variability that would occur for each 1 s of activity. There were no systematic changes in either firing rate or variability with time. 4 We conclude that single motoneurones supplying the extensor hallucis longus, a muscle comprised primarily of slow twitch muscle units, show little adaptation in firing with fatigue, suggesting that a progressive reduction in firing rate is not an invariable consequence of the fatigue associated with sustained maximal voluntary contractions.

Contractile Properties of Human Motor Units: Is Man a Gat?

A major goal in neuroscience is to understand how the CNS controls posture and movement in humans. This requires an understanding of individual human motor unit properties and how they interact within the muscle to perform different tasks. This article describes differences and similarities between the contractile properties of human motor units and those of the cat prototype medial gastrocnemius (MG) muscle, on which so many studies have been conducted. The article describes the methods available for measuring human motor unit properties and their limitations, and it discusses how far the behavior of whole muscles can be predicted from their histochemistry. It questions the extent to which human motor units conform to the conventional criteria by which S (slow, fatigue resistant), FR (fast but fatigue resistant) and FF (fast, fatigable) unit types are usually classified. An important difference between human and cat MG data is that weak human motor units are not necessarily slow, nor strong ones fast; that is, generally, human unit force is not correlated with contractile speed. Also, unlike cat MG, the few human muscles studied so far contain few if any FF units but a high proportion of units with intermediate fatigue resistance (Flnt). These apparently aberrant human properties, however, are also found in other cat and rat muscles. Thus, cat MG may not be the best model for motor unit behavior generally. Finally, the influence of human motor unit properties on force output by recruitment and/or rate coding is discussed.

Incidence of F waves in single human thenar motor units.

F‐wave generation, axon conduction velocities, and contractile properties were compared in 44 healthy individual human thenar motor units. Force and muscle action potentials were recorded when single motor axons were stimulated intraneurally about 10 cm proximal to the elbow. Each stimulus usually evoked only one electromyographic (EMG) potential. However, in seven units (16%), a single stimulus elicited an F wave in response to 1.7 ± 1.6% (mean ± SD) of the stimuli applied. Axon conduction velocity proximal to the site of stimulation was faster than distal conduction velocity (72.7 ± 8.0 m/s versus 64.2 ± 10.5 m/s). Distal conduction velocities, twitch forces, and contraction times were similar for units that did and did not generate F waves. Thus, no obvious subset of thenar motor units generated F waves. These results provide valuable baseline information on F waves that can be used to assess changes in axon conduction, motor unit contractile properties, and motoneuron excitability in disease.