Selective recruitment of high-threshold human motor units during voluntary isotonic lengthening of active muscles.
TL;DR: The possibility that voluntary muscle lengthening contractions can be performed by selective recruitment of fast‐twitch motor units, accompanied by derecruitment of slow‐ twitch motor units is investigated.
Abstract: 1. We have investigated the possibility that voluntary muscle lengthening contractions can be performed by selective recruitment of fast-twitch motor units, accompanied by derecruitment of slow-twitch motor units. 2. The behaviour of motor units in soleus, gastrocnemius lateralis and gastrocnemius medialis muscles was studied during (a) controlled isotonic plantar flexion against a constant load (shortening contraction, S), maintained plantar flexion, or dorsal flexion resisting the load and gradually yielding to it (lengthening contraction, L), (b) isometric increasing or decreasing plantar torque accomplished by graded contraction or relaxation of the triceps surae muscles, (c) isometric or isotonic ballistic contractions, and (d) periodic, quasi-sinusoidal isotonic contractions at different velocities. The above tasks were performed under visual control of foot position, without activation of antagonist muscles. The motor units discharging during foot rotation were grouped on the basis of the phase(s) during which they were active as S, S + L and L. The units were also characterized according to both the level of isometric ramp plantar torque at which they were first recruited and the amplitude of their action potential. 3. S units were never active during dorsal flexion; some of them were active during the sustained contraction between plantar and dorsal flexion. Most S + L units were active also during the maintenance phase and were slowly derecruited during lengthening; their behaviour during foot rotations was similar to that during isometric contractions or relaxations. L units were never active during either plantar or maintained flexion, but discharged during lengthening contraction in a given range of rotation velocities; the velocity of lengthening consistently influenced the firing frequency of these units. Such dependence on velocity was not observed in S + L units. 4. A correlation was found between the amplitude of the action potential and the threshold torque of recruitment among all the units. In addition, the amplitudes of both the action potential and the threshold torque were higher in the case of L units than in the case of S and S + L units. Most L units could be voluntarily recruited only in the case of ballistic isometric or isotonic contraction. 5. Occasionally, L units were directly activated by electrical stimulation of motor fibres and their conduction velocity was in the higher range for alpha-axons. In contrast, nerve stimulation could induce a reflex activation of S and S + L units.(ABSTRACT TRUNCATED AT 400 WORDS)
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TL;DR: This work proposes four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms and suggests a number of experiments to advance the understanding of the neurobiology of muscle fatigue.
Abstract: Muscle fatigue encompasses a class of acute effects that impair motor performance. The mechanisms that can produce fatigue involve all elements of the motor system, from a failure of the formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of the contractile proteins. We propose four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms: 1) task dependency to identify the conditions that activate the various mechanisms; 2) force-fatigability relationship to explore the interaction between the mechanisms that results in a hyperbolic relationship between force and endurance time; 3) muscle wisdom to examine the association among a concurrent decline in force, relaxation rate, and motor neuron discharge that results in an optimization of force; and 4) sense of effort to determine the role of effort in the impairment of performance. On the basis of this perspective with an emphasis on neural mechanisms, we suggest a number of experiments to advance our understanding of the neurobiology of muscle fatigue.
1,391 citations
TL;DR: The purpose of this paper is to extensively review the literature as to the mechanisms of muscle hypertrophy and their application to exercise training and to draw conclusions from the research as toThe optimal protocol for maximizing muscle growth.
Abstract: The quest to increase lean body mass is widely pursued by those who lift weights. Research is lacking, however, as to the best approach for maximizing exercise-induced muscle growth. Bodybuilders generally train with moderate loads and fairly short rest intervals that induce high amounts of metabolic stress. Powerlifters, on the other hand, routinely train with high-intensity loads and lengthy rest periods between sets. Although both groups are known to display impressive muscularity, it is not clear which method is superior for hypertrophic gains. It has been shown that many factors mediate the hypertrophic process and that mechanical tension, muscle damage, and metabolic stress all can play a role in exercise-induced muscle growth. Therefore, the purpose of this paper is twofold: (a) to extensively review the literature as to the mechanisms of muscle hypertrophy and their application to exercise training and (b) to draw conclusions from the research as to the optimal protocol for maximizing muscle growth.
798 citations
TL;DR: It is suggested that resistance exercise at an intensity even lower than 50% 1 RM is effective in inducing muscular hypertrophy and concomitant increase in strength when combined with vascular occlusion.
Abstract: Acute and long-term effects of resistance exercise combined with vascular occlusion on muscular function were investigated. Changes in integrated electromyogram with respect to time (iEMG), vascula...
698 citations
Book•
01 Jan 1987
TL;DR: In this paper, the processing of information in face-to-face communication when a speaker makes both audible and visible information available to a perceiver is discussed. But the evaluation of the information source provides information about the strength of alternative interpretations, rather than just all-or-none categorical information, as claimed by "categorical perception" theory.
Abstract: This book is about the processing of information in face-to-face communication when a speaker makes both audible and visible information available to a perceiver. Both auditory and visual sources of information are evaluated and integrated to achieve speech perception. The evaluation of the information source provides information about the strength of alternative interpretations, rather than just all-or-none categorical information, as claimed by “categorical perception” theory. Information sources are evaluated independently; the integration process insures that the least ambiguous sources have the most influences on the judgment. Similar processes occur in a variety of other behaviors, ranging from personality judgments and categorization to sentence interpretation and decision making. The experimental results are consistent with a fuzzy logical model of perception, positing three operations in perceptual (primary) recognition: feature evaluation, feature integration, and pattern classification. Continuously valued features are first evaluated, then integrated and matched against prototype descriptions in memory; finally, an identification decision is made on the basis of the relative goodness-of-match of the stimulus information with the relevant prototype descriptions.
678 citations
TL;DR: This review examines the experimental evidence that provides the foundation for the current understanding of the benefits, consequences, and control of eccentric contractions and suggests a new hypothesis: that the neural commands controlling eccentric contraction are unique.
Abstract: Enoka, Roger M. Eccentric contractions require unique activation strategies by the nervous system. J. Appl. Physiol. 81(6): 2339–2346, 1996.—Eccentric contractions occur when activated muscles are ...
645 citations
References
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TL;DR: Intracellular stimulation of individual motoneurones ensured functional isolation of the muscle units innervated by them in pentobarbitone‐anaesthetized cats.
Abstract: 1. A variety of physiological properties of single motor units have been studied in the gastrocnemius muscle (primarily in the medial head) of pentobarbitone-anaesthetized cats. Intracellular stimulation of individual motoneurones ensured functional isolation of the muscle units innervated by them.
2. A system for muscle unit classification was developed using a combination of two physiological properties. Almost all of the units studied could be classified into one of three major types, including two groups with relatively short twitch contraction times (types FF and FR, which were differentiable from one another on the basis of sensitivity to fatigue) and one group with relatively long contraction times (type S, which were extremely resistant to fatigue and were differentiable from FF and FR units on the basis of the shape of unfused tetani). Post-tetanic potentiation of twitch responses was observed in all three muscle unit types. The distributions of axonal conduction velocities for motoneurones innervating FF and FR muscle units were essentially the same, while conduction velocities for motoneurones innervating type S units were, in general, slower.
3. Histochemical profiles of muscle units representative of each of the physiological classes present in the gastrocnemius pool were determined using a method of glycogen depletion for muscle unit identification. Each of the physiological categories of muscle units exhibited a corresponding unique set of muscle fibre staining reactions, or histochemical profile. Within each physiological type, all of the units examined had the same histochemical profile. The results generally support the hypothesis that the histochemical characteristics of muscle fibres are meaningfully related to the physiological properties of the same fibres. However, certain limitations in the detailed application of the hypothesis were also apparent.
4. Systematic assessment of the histochemical profiles of relatively large numbers of fibres belonging to single muscle units provided strong support for the hypothesis that all of the muscle fibres innervated by a single α-motoneurone are histochemically identical.
1,514 citations
TL;DR: The contractile properties of human motor units from the first dorsal interosseus muscle of the hand were studied during voluntary isometric contractions using recently developed techniques.
Abstract: 1. The contractile properties of human motor units from the first dorsal interosseus muscle of the hand were studied during voluntary isometric contractions using recently developed techniques.
2. The twitch tensions produced by motor units varied widely from about 0·1–10 g. The twitch tension of a motor unit varied nearly linearly as a function of the level of voluntary force at which it was recruited over the entire range of forces studied (0–2 kg).
3. The number of additional motor units recruited during a given increment in force declined sharply at high levels of voluntary force. This suggests that even though the high threshold units generate more tension, the contribution of recruitment to increases in voluntary force declines at higher force levels.
4. Contraction times for these motor units varied from 30 to 100 msec. Over 80% had contraction times less than 70 msec, and might be classed as fast twitch motor units. The larger motor units, which were recruited at higher threshold forces, tended to have shorter contraction times than the smaller units.
903 citations
TL;DR: It is shown that the rate at which energy is liberated by a tetanized muscle increases linearly as the external force is reduced below the isometric tension.
Abstract: IN a recent paper Hill [1938b] has shown that the rate at which energy is liberated by a tetanized muscle increases linearly as the external force is reduced below the isometric tension. If a muscle shortens against a force P, which is smaller than the isometric tension PO, work is done by the muscle, and in addition heat is liberated, in excess of the isometric, at a rate proportional to the speed of shortening v. The rate of total energy production exceeds the isometric heat rate by an amount proportional to the reduction of external tension PO-P. These findings were expressed by the equation v(P+a)=b (PO-P), ...... (1) where a [g.] and b [cm./sec.] are constants (a g.cm. being the \" extra heat\" for 1 cm. shortening, b g.cm./sec. the increase of the rate of energy production for 1 g. tension loss). This formula relates two variables, the speed of shortening v and the external force P, which can be determined without any heat measurements. According to this formula, which can also be written as
793 citations
TL;DR: The division of labor between MG and SOL and the absolute force levels required from the MG during the full range of hindlimb movements in posture, locomotion, and jumping appear to be precisely matched to the very different characteristics of the motor-units composing these synergistic muscles.
Abstract: 5. During the step cycle the peaks of both MG and SOL force profiles occur before the end of the yield (E2) phase, as both muscles undergo active lengthening. In contrast, peak MG and SOL forces during vertical jumping coincide with the end of active lengthening and the onset of ankle extensor shortening. 6. The results are consistent with the hypothesis that the inherent stiffness of active muscle during the step cycle is an important factor in the control of force output from hindlimb extensor muscles in locomotion. The division of labor between MG and SOL and the absolute force levels required from the MG during the full range of hindlimb movements in posture, locomotion, and jumping appear to be precisely matched to the very different characteristics of the motor-unit populations composing these synergistic muscles.
699 citations
601 citations