Contraction Times and Fibre Types in Intact Human Muscle
TL;DR: The histograms of contraction times agreed with histochemical findings; contraction times longer than 60 msec corresponded to the proportion of fibres rich in mitochondria, and long contraction times predominated in the soleus and gastrocnemius muscles.
Abstract: Twitches were recorded in situ in muscles of normal humans by small transducers connected to a needle in the tendon. The twitches were elicited by stimuli in the endplate zone, the force of the smallest twitches corresponding to the average of one or two motor units. The time to peak contraction was measured of 20–30 bundles of fibres in each muscle examined. The histograms of contraction times agreed with histochemical findings; contraction times longer than 60 msec corresponded to the proportion of fibres rich in mitochondria. Thus, long contraction times predominated in the soleus and gastrocnemius muscles, comprised half the times in the anterior tibia], a third in the brachial biceps (long head) and a few per cent in the brachial triceps muscle (lateral head). Bundles of fibres with short contraction times, those poor in mitochondria had a higher temperature coefficient of the time to peak than bundles with long contraction times. During hypoxia the contraction times shortened. The contraction time of motor units first activated voluntarily lay near the mean of bundles activated electrically. The contraction times of fasciculating fibres were in the range of electrically evoked twitches; the force was about three times that of voluntarily activated units.
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TL;DR: A mathematical model is presented which predicts both the major qualitative features and, within experimental error, the quantitative details of a class of perturbed and unperturbed large-amplitude, voluntary movements performed at intermediate speed by primates.
Abstract: This paper presents a mathematical model which predicts both the major qualitative features and, within experimental error, the quantitative details of a class of perturbed and unperturbed large-amplitude, voluntary movements performed at intermediate speed by primates. A feature of the mathematical model is that a concise description of the behavioral organization of the movement has been formulated which is separate and distinct from the description of the dynamics of movement execution. Based on observations of voluntary movements in primates, the organization has been described as though the goal were to make the smoothest movement possible under the circumstances, i.e., to minimize the accelerative transients. This has been formalized by using dynamic optimization theory to determine the movement which minimizes the rate of change of acceleration (jerk) of the limb. Based on observations of muscle mechanics, the concept of a “virtual position” determined by the active states of the muscles is rigorously defined as one of the mechanical consequences of the neural commands to the muscles. This provides insight into the mechanics of perturbed and unperturbed movements and is a useful aid in the separation of the descriptions of movement organization and movement execution.
1,228 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
01 Jan 1985
TL;DR: The sections in this article are: Fiber Composition in Human Skeletal Muscle, Motor Unit Recruitment, Adaptive Response, and Significance of Adaptation.
891 citations
TL;DR: The sections in this article are:============\/\/\/\/\/\/£££1.11\/\/£2.10\/\/£3\/\/£4\/\/£5\/\/£6.1\/\/£7.2\/\/£8\/\/£9\/\/£10\/\/ £1.7\/\/£11.5\/\/ £2.9\/\/ £3£3£4£4 £6.2£5.3£7\/\/ £7.4£8£9.5£10.2 £7======
Abstract: The sections in this article are:
1
Motor Unit
1.1
Fibers per Motor Unit
1.2
Contractile Properties
1.3
Biochemical Basis for Differences in Twitch Properties
1.4
Histochemical Differentiation of Muscle Fibers
1.5
Ultrastructural Basis for Skeletal Muscle Fiber Typing
1.6
Maximal Contractile Force
1.7
Speed of Contraction
1.8
Fatigue Characteristics
1.9
Metabolic Characteristics
1.10
Ionic Composition of Skeletal Muscle
1.11
Summary
2
Muscle Fiber Composition in Human Skeletal Muscle
3
Motor-Unit Recruitment
4
Adaptive Response in Skeletal Muscle
4.1
Muscle Size
4.2
Metabolic Capacity
5
Connective Tissue
6
Capillaries
6.1
Methodology
6.2
Anatomy
6.3
Capillary Density
6.4
Capillary Length and Diameter
6.5
Use and Disuse
6.6
Regulation
7
Significance of Adaptation
7.1
Muscular Size
7.2
Substrate Stores
7.3
Enzyme Activities
7.4
Summary
863 citations
TL;DR: Evidence-based practical recommendations are provided for rational quantification of rate of force development in both laboratory and clinical settings and various methodological considerations inherent to its evaluation are discussed.
Abstract: The evaluation of rate of force development during rapid contractions has recently become quite popular for characterising explosive strength of athletes, elderly individuals and patients. The main aims of this narrative review are to describe the neuromuscular determinants of rate of force development and to discuss various methodological considerations inherent to its evaluation for research and clinical purposes. Rate of force development (1) seems to be mainly determined by the capacity to produce maximal voluntary activation in the early phase of an explosive contraction (first 50–75 ms), particularly as a result of increased motor unit discharge rate; (2) can be improved by both explosive-type and heavy-resistance strength training in different subject populations, mainly through an improvement in rapid muscle activation; (3) is quite difficult to evaluate in a valid and reliable way. Therefore, we provide evidence-based practical recommendations for rational quantification of rate of force development in both laboratory and clinical settings.
791 citations
Cites background from "Contraction Times and Fibre Types i..."
...Although caution is required when deducing the functional significance of differences in single fibre properties (see Enoka and Duchateau 2015), fibre type is often considered a major factor influencing muscular RFD based on the observation that the rate of tension development is faster in type II than type I fibres (Buchthal and Schmalbruch 1970; Harridge et al. 1996)....
[...]
...…differences in single fibre properties (see Enoka and Duchateau 2015), fibre type is often considered a major factor influencing muscular RFD based on the observation that the rate of tension development is faster in type II than type I fibres (Buchthal and Schmalbruch 1970; Harridge et al. 1996)....
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TL;DR: The paper shows that a maximal voluntary effort develops the same tension as a maximal tetanus artificially excited; in the second part the same equality is found to persist during fatigue, implying that in fatigue, too, the limitation of strength is peripheral.
Abstract: In voluntary efforts it is not known for certain whether the force that can be exerted is limited by the capacity of the nervous centres and conducting pathways to deliver motor impulses to the muscle fibres or by the intrinsic contractile properties of the fibres themselves; whether, in fact, a voluntary effort can be bettered by maximal tetanic stimulation of the muscle electrically, or not. Again in fatigue it is undecided whether tension falls because the degree of voluntary innervation drops or because the fibres are biochemically incapable of maintaining their contraction. The experiments described here attempt to settle these questions by comparing directly voluntary tension with that resulting from electrically excited motor volleys. To make a valid comparison in an intact human subject is difficult, but it will be argued that it can be achieved by using a particularly convenient muscle, the adductor of the thumb, and special apparatus. The paper falls into three parts: the first shows that a maximal voluntary effort develops the same tension as a maximal tetanus artificially excited; in the second part the same equality is found to persist during fatigue, implying that in fatigue, too, the limitation of strength is peripheral; finally the effect of ischaemia is described. Preliminary accounts have already appeared (Merton & Pampiglione, 1950; Merton, 1950).
1,437 citations
777 citations
TL;DR: It is shown that the excitability of motoneurons is an inverse function of their size, and the participation of a motor unit in graded motor activity is dictated by the size of its neuron.
Abstract: INTWORECENTLYPUBLISHED PAPERS (11,18)a detailedstudy wasmade of the properties of motor units in the soleus and medial gastrocenmius muscles of the cat. In particular, it was shown that there were great variations in the size of individual motor units and, paralleling them, important differences in their contractile characteristics. The full significance of these findings did not become apparent, however, until the observations reported in the preceding paper (7) had been made, indicating that the excitability of motoneurons is an inverse function of their size. From this it follows that the participation of a motor unit in graded motor activity is dictated by the size of its neuron. A corollary of this conclusion is that the total amount of contractile activity of a unit decreases as its size increases. These new findings on motor units have prompted a histochemical study of the soleus and m. gastrocnemius (MG), using recently developed methods (14) of staining muscle fibers for adenosine triphosphatase (ATPase). The results of this study are presented here. They provide a background for a functional analysis of the data on motor units. The hypotheses emerging from this analysis, in turn, offer a rationale for the histological findings. The conjunction of these different findings, all concerned with the same muscles, permits us to point out some previously unrecognized principles which govern the design of skeletal muscles.
708 citations
TL;DR: In this article, the effects of stimulation of single motor nerve fibres and contractions of a single motor unit were studied instead of the synchronized contractions in the whole muscle, which appeared to provide a picture of the histochemical composition and distribution of the fibres within the muscle.
Abstract: Histochemically, the anterior tibial muscle of the albino rat presents a mosaic pattern of fibres with different enzymatic activity. From the standpoint of esterase and oxidative enzymes, such as succinic dehydrogenase, three different types of fibres have been distinguished: type A fibres, which are the classical white muscle fibres, and B and C fibres, whicharetwokinds ofred fibres (Stein and Padykula, 1962). The A fibres are large with low enzymatic activity and the C fibres small with high enzymatic activity. The B fibres form a more heterogeneous group with various intermediate sizes and intensities of activity. It has been shown that muscle contractions induced by shock stimulation to the sciatic nerve produce striking changes in the phosphorylase activity and glycogen content, which vary in the different types of fibres (Kugelberg and Edstr6m, 1968a, b). The changes were sufficiently distinct and constant to serve as a histochemical index of previous muscle activity. In a few experiments the effect of stimulation of single motor nerve fibres and contractions of single motor units were studied instead of the synchronized contractions of the whole muscle. This seemed to provide a picture of the histochemical composition and distribution of the fibres within a single motor unit and permitted correlation between its histochemical and physiological properties. From the clinical point of view this is of interest for the interpretation of the electromyogram and the findings in muscle biopsies. Some of these aspects of the motor unit have been investigated earlier by less direct methods (see Discussion) and therefore require further study in a heterogeneous muscle. In the present study the effects of stimulation of single motor nerve fibres and contractions of single motor units have been studied in greater detail. A brief account of our findings has been reported (Edstrom and Kugelberg, 1968). METHODS
613 citations
577 citations