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Molecular adaptations to endurance exercise and skeletal muscle fibre plasticity
Published in Adam P. Sharples, James P. Morton, Henning Wackerhage, Molecular Exercise Physiology, 2022
A motor unit is defined as a motor neurone and all of the muscle fibres it innervates (Figure 9.4). There are different types of motor units which are characterised by their α-motor neurone and muscle fibre type: S (slow) motor units comprise small α-motor neurones with a low excitation threshold and small axons that innervate a small number of slow type I fibres.FR (fatigue-resistant) motor units comprise moderate-sized α-motor neurones that innervate intermediate type IIa fibres. The α-motor neurone properties are in between those of Fast Fatiguing (FF – see below) and S α-motor neurones.FF (fast fatiguing) motor units comprise large, hard to excite but fast-conducting α-motor neurons that innervate many large fibres that contract rapidly but fatigue easily (type IIb and IIx fibres);
Biochemical Contributors to Exercise Fatigue
Published in Peter M. Tiidus, Rebecca E. K. MacPherson, Paul J. LeBlanc, Andrea R. Josse, The Routledge Handbook on Biochemistry of Exercise, 2020
Arthur J. Cheng, Maja Schlittler, Håkan Westerblad
Muscle contraction is linked by a series of events, and fatigue can potentially be attributed to failure at any of the cellular sites involved in the activation and contraction of the muscle fibres (Figure 5.4). The activation of skeletal muscle contraction starts at the central nervous system, thereby activating α-motor neurons, and action potentials propagate along the axons of these neurons out to the muscle fibres. The smallest functional unit of the central motor system, the motor unit, consists of one α-motor neuron and the muscle fibres it activates. The number of muscle fibres in a motor unit varies from muscle to muscle. For instance, in the hand an α-motor neuron generally activates fewer than 100 muscle fibres, whereas for the lower leg a single motor unit may contain up to 1,000 muscle fibres (12). Generally, the larger the number of muscle fibres in a motor unit, the less precise the associated movements.
Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The motor unit is the basic unit of contraction of skeletal muscle. An AP along the axon of an α-motoneuron generates single APs in all the muscle fibers of the motor unit, producing a single twitch contraction in each of these fibers. The muscle fibers of each motor unit are dispersed among fibers of other motor units generally in the same region of the muscle or throughout the muscle, depending on muscle size and number of fibers. This ensures that the direction of the force exerted does not depend on the number of motor units activated. It is also advantageous to have active and inactive muscle fibers intermingled at low levels of muscle contraction so as not to compromise the blood supply to the muscle which, as noted earlier, can occur at high levels of contraction.
Effects of whole-body vibration plus hip-knee muscle strengthening training on adult patellofemoral pain syndrome: a randomized controlled trial
Published in Disability and Rehabilitation, 2022
Zhangxiang Wu, Zhi Zou, Jiugen Zhong, Xinbo Fu, Ligen Yu, Jinzhu Wang, Xin Wang, Qianwen Wu, Xiaohui Hou
Similar to muscle resistance training, whole-body vibration (WBV) training is an alternative method of muscle strengthening [15–19]. The WBV vibratory platform can generate vertical vibrations, which can arouse muscle reflex contractions by facilitating homonymous α-motor neurons via tonic vibration reflexes of the muscle spindles [20]. As a result, motor unit recruitment can be enhanced to improve the excitability of muscle spindles [21], thereby promoting muscle strengthening. Furthermore, WBV training can improve muscle balance, power, and function [15–19]. Thus, combining WBV with muscle strengthening may be a more effective method of treating PFPS. Therefore, we designed this study and hypothesized that WBV plus hip-knee muscle strengthening is more efficient in relieving pain and improving function than is hip-knee strengthening alone.
Spinal automaticity of movement control and its role in recovering function after spinal injury
Published in Expert Review of Neurotherapeutics, 2022
According to the size principle, the order of recruitment of motor neurons within a given motor pool starts from small to large. The number of muscle fibers innervated by a single motor neuron provides an accurate estimate of the size of a motor unit and the order of recruitment within each motor pool ([7] and (Figure 1(a,b)). Furthermore, the number of motor units activated within a motor pool determines the force, speed, and power generated by the muscle. The orderly recruitment of muscle fibers of a muscle unit confers biochemical and physiological benefits. The slow oxidative motor unit can perform the most work and has the highest resistance to fatigue (Figure 1(a)). Thus, smaller units tend to be recruited more often compared to larger, more easily fatigable motor units. The size principle assumes that recruiting the slow (S) motor units with a high oxidative capacity, is a more effective strategy for generating movements requiring low force and power. In contrast, the fast fatigable (FF) units relying primarily on glycolytic metabolism generate higher forces, speed, and power, but are needed less often (Figure 1(b,c)). The fast fatigue-resistant (FR) units are more intermediate in force and fatigability.
Design considerations for the development of neuromuscular electrical stimulation (NMES) exercise in cancer rehabilitation
Published in Disability and Rehabilitation, 2021
Dominic O’Connor, Olive Lennon, Conor Minogue, Brian Caulfield
Early work established that phase durations of 20–200 µs were sufficient for motor stimulation [61]. However, research into phase duration manipulation during HF-NMES application has demonstrated that higher phase durations may be optimal, leading to greater levels of muscle activation and higher torque output. Indeed, Gorgey et al. [62] measured the activated cross-sectional area of the quadriceps femoris in seven healthy participants and reported a 40% greater level of motor unit recruitment when comparing 150 µs and 450 µs phase durations. Higher motor unit recruitment logically translates to increased muscle force production. In support of this, Hultman et al. [2] demonstrated that increasing phase duration from 150 to 500 µs achieved a 40% greater torque output. In addition, when comparing low (200 µs) and high (500 µs) phase durations, healthy young individuals have been shown to tolerate and generate higher force tetanic muscle contractions (45% v 49% maximum voluntary contraction (MVC)) with a higher phase duration [63]. However, many HF-NMES applications use phase durations of <300 µs [3], despite the evidence supporting the use of higher phase durations for muscle strengthening applications.