Motor unit recruitment for dynamic tasks: current understanding and future directions

Hodson-Tole, Emma F.; Wakeling, James M.

doi:10.1007/s00360-008-0289-1

Cited by 109 publications

(79 citation statements)

References 110 publications

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“…which include slow-twitch, fatigue-resistant fibers. By contrast, briefly sustained tasks requiring high levels of force generation or contraction velocities involve progressive recruitment of motor units containing fast-twitch fibers, increasingly susceptible to fatigue (Hodson-Tole and Wakeling, 2009). By contrast, fiber recruitment in the hummingbird muscles involves a quantitative increase in the number of physiologically similar fibers contributing to the task.…”

Section: Discussionmentioning

confidence: 99%

Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)

Altshuler

Welch

Cho

et al. 2010

Journal of Experimental Biology

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SUMMARYHummingbirds can maintain the highest wingbeat frequencies of any flying vertebrate -a feat accomplished by the large pectoral muscles that power the wing strokes. An unusual feature of these muscles is that they are activated by one or a few spikes per cycle as revealed by electromyogram recordings (EMGs). The relatively simple nature of this activation pattern provides an opportunity to understand how motor units are recruited to modulate limb kinematics. Hummingbirds made to fly in low-density air responded by moderately increasing wingbeat frequency and substantially increasing the wing stroke amplitude as compared with flight in normal air. There was little change in the number of spikes per EMG burst in the pectoralis major muscle between flight in normal and low-density heliox (mean1.4spikescycle -1 ). However the spike amplitude, which we take to be an indication of the number of active motor units, increased in concert with the wing stroke amplitude, 1.7 times the value in air. We also challenged the hummingbirds using transient load lifting to elicit maximum burst performance. During maximum load lifting, both wing stroke amplitude and wingbeat frequency increased substantially above those values during hovering flight. The number of spikes per EMG burst increased to a mean of 3.3 per cycle, and the maximum spike amplitude increased to approximately 1.6 times those values during flight in heliox. These results suggest that hummingbirds recruit additional motor units (spatial recruitment) to regulate wing stroke amplitude but that temporal recruitment is also required to maintain maximum stroke amplitude at the highest wingbeat frequencies.

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Section: Discussionmentioning

confidence: 99%

Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)

Altshuler

Welch

Cho

et al. 2010

Journal of Experimental Biology

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“…It describes an orderly recruitment pattern beginning with MUs of smaller size and slower conduction velocity, followed by larger and faster MUs that generate stronger force (i.e., the size principle) (Clamann 1993;Henneman 1957;Henneman et al 1965aHenneman et al , 1965bHodson-Tole and Wakeling 2009). For example, in the cat gastrocnemius, MU recruitment begins with slow oxidative (type S) followed by fast fatigue-resistant (type FR) and, lastly, fast-fatigable (type FF) MUs (Cope and Clark 1991;Henneman and Mendell 1981).…”

Section: Size Principle and Muscle Fiber Typementioning

confidence: 97%

External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat

D’Amico

Collins

2012

Journal of Neurophysiology

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In the rat, external urethral sphincter (EUS) activation during micturition consists of three sequential phases: 1) an increase in tonic EUS activity during passive filling and active contraction of the bladder (guarding reflex), 2) synchronized phasic activity (EUS bursting) associated with voiding, and 3) sustained tonic EUS activity that persists after bladder contraction. These phases are perturbed following spinal cord injury. The purpose of the present study was to characterize individual EUS motor unit (MU) patterns during micturition in the spinally intact and transected adult rat. EUS MU activity was recorded from either the L5 or L6 ventral root (intact) or EUS muscle (transected) during continuous flow cystometry in urethane-anesthetized adult female Sprague-Dawley rats. With the use of bladder pressure threshold and timing of activation, four distinct patterns of EUS MU activity were identified in the intact rat: low threshold sustained, medium/high threshold sustained, medium/high threshold not sustained, and burst only. In general, these MUs displayed little frequency modulation during active contraction, generated high-frequency bursts of action potentials during EUS bursting, and varied in terms of the duration of sustained tonic activity. In contrast, three general patterns of EUS MU activity were identified in the transected rat: low threshold, medium threshold, and high threshold. These MUs exhibited considerable frequency modulation during active contraction of the bladder, no bursting behavior and little to no sustained firing. The prominent frequency modulation of EUS MUs is likely due to the enhanced guarding reflex seen in EUS whole muscle electromyogram recordings in transected rats (D'Amico SC, Schuster IP, Collins WF 3rd. Exp Neurol 228: 59-68, 2011). In addition, EUS MU recruitment in transected rats more closely followed predictions by the size principle than in intact rats. This may reflect the influence of local synaptic circuits or intrinsic properties of EUS motoneurons that are active in intact rats but attenuated or absent in transected rats.

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“…The base plan of orderly recruitment would predict that only the slower motor units would be recruited, but mechanical arguments would suggest that for rapid contractions the task may be better achieved by preferential recruitment of faster motor units [44]. Our studies on the cycle ergometer show that despite the muscle gearing mechanisms highlighted at the start of this paper, the shortening velocities of the muscle fibres can exceed the optimal velocities of the slower fibres, and may even reach or exceed the maximum intrinsic speed of the slower fibres [32].…”

Section: Mechanically Appropriate Recruitmentmentioning

confidence: 99%

“…The size principle was originally identified using a stretch reflex protocol in the decerebrate cat, a model in which many pathways that would normally influence motor unit recruitment were not left intact and therefore could not influence the results (for review, see [44]). More recent studies have repeated the protocol, but have additionally shown that soleus motor units can be selectively inhibited by stretch in synergistic muscles [45] and so there are some fundamental connections at the spinal level that allow recruitment reversals.…”

Section: Mechanically Appropriate Recruitmentmentioning

confidence: 99%

Movement mechanics as a determinate of muscle structure, recruitment and coordination

Wakeling

Blake

Wong

et al. 2011

Phil. Trans. R. Soc. B

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135

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During muscle contractions, the muscle fascicles may shorten at a rate different from the muscletendon unit, and the ratio of these velocities is its gearing. Appropriate gearing allows fascicles to reduce their shortening velocities and allows them to operate at effective shortening velocities across a range of movements. Gearing of the muscle fascicles within the muscle belly is the result of rotations of the fascicles and bulging of the belly. Variable gearing can also occur as a result of tendon length changes that can be caused by changes in the relative timing of muscle activity for different mechanical tasks. Recruitment patterns of slow and fast fibres are crucial for achieving optimal muscle performance, and coordination between muscles is related to whole limb performance. Poor coordination leads to inefficiencies and loss of power, and optimal coordination is required for high power outputs and high mechanical efficiencies from the limb. This paper summarizes key studies in these areas of neuromuscular mechanics and results from studies where we have tested these phenomena on a cycle ergometer are presented to highlight novel insights. The studies show how muscle structure and neural activation interact to generate smooth and effective motion of the body.Keywords: power; efficiency; gearing; pennation PROLOGUEMuscles have a surprising variety of functions during locomotion and can serve as motors, brakes, springs and struts [1]. These muscle functions depend on the arrangement of the muscle fibres within the muscle belly, the length and velocity of the fibres and also on the timing of their activation relative to those length changes. When the muscle fibres undergo substantial length change the muscle can act as a motor if it is active during shortening, or a brake if it is active during lengthening; when the length change is minimal then the muscle can act like a strut if it actively develops force to resist stretch [1]. Muscle fibre velocity may differ from the velocity of the muscletendon unit (MTU) in a process known as gearing. Thus, the extent to which the muscle fibres change length during a limb motion depends on how they are geared relative to the MTU. The timing and the level of activation are under neural control. This paper is a synthesis of previous literature on both gearing and activation, with a particular focus on some of the more recent findings in man. We introduce additional findings about structural mechanics in man and propose that some of the features of muscle gearing actually result from the interactions between the structural mechanics and the activation patterns of the muscles. MUSCLE STRUCTURE AND FASCICLE GEARINGMTUs are comprised of both tendinous structures containing a large proportion of collagen fibres, and skeletal muscle fibres containing the actin-and myosin-rich contractile machinery. The skeletal muscle fibres typically lie parallel to each other and are bundled to form fascicles that combine to form the muscle belly. The mechanics of fibres and fascicles can be ...

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Motor unit recruitment for dynamic tasks: current understanding and future directions

Cited by 109 publications

References 110 publications

Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)

Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)

External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat

Movement mechanics as a determinate of muscle structure, recruitment and coordination

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