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Skeletal muscles are made up of individual muscle fibers. And like muscles themselves, not all muscle fibers are the same. There are two types of skeletal muscle fibers, fast-twitch and slow-twitch, and they each have different functions that are important to understand when it comes to movement and exercise programming.

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Slow-twitch muscle fibers are fatigue resistant, and focused on sustained, smaller movements and postural control. They contain more mitochondria and myoglobin, and are aerobic in nature compared to fast-twitch fibers. Slow-twitch fibers are also sometimes called type I or red fibers because of their blood supply.

Fast-twitch muscle fibers provide bigger and more powerful forces, but for shorter durations and fatigue quickly. They are more anaerobic with less blood supply, hence they are sometimes referred to as white fibers or type II. Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors including muscle function, age and training.

The two types of skeletal muscle fibers are slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscle fibers support long distance endurance activities like marathon running, while fast-twitch muscle fibers support quick, powerful movements such as sprinting or weightlifting.

Slow-twitch muscle fibers have high concentrations of mitochondria and myoglobin. Although they are smaller than the fast-twitch fibers, they are surrounded by more capillaries (1,2). This combination supports aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal (aerobic) exercise activities.

You and your muscles are not comprised of one type of muscle fiber. All of your muscles are a mix of fast-twitch and slow-twitch muscle fiber types (1).

Aging causes a loss in lean muscle mass, with a decline in our fast-twitch fibers, especially the type IIx, but there is also an increase in our slow-twitch fibers (2-4).

Recall that the fast-twitch fibers are larger in size than the slow-twitch and are metabolically efficient fibers. Thus, loss of lean muscle mass can contribute to age-related metabolic dysfunctions, body composition changes, even an increased risk of falls (2-5).

1. The effects of 1 mM lanthanum on miniature endplate current (MEPC) frequency, amplitude, and decay time course were studied in voltage-clamped twitch and tonic muscle fibres in the garter snake, Thamnophis. 2. Lanthanum produced a marked increase in MEPC frequency in both fibre types. The maximum frequency in lanthanum was greater at twitch endplates than at tonic endplates although the increase in frequency relative to control levels was as great in tonic fibres as in twitch fibres. 3. In twitch fibres continually exposure to lanthanum, the frequency of MEPCs reached a peak value and then declined progressively until, after approximately 6 h, no MEPCs were recorded. In contrast, at tonic endplates exposed to 1 mM lanthanum, MEPC frequency remained elevated above control levels for periods greater than 20 h. 4. Lanthanum decreased the mean amplitude of MEPCs, skewed the amplitude distribution and increased MEPC duration at both twitch and tonic fibre endplates. 5. Ultrastructural analysis showed that after a 15 min exposure to 1 mM lanthanum, approximately half of the boutons innervating a twitch fibre contained fewer synaptic vesicles than boutons at control endplates, whereas nerve terminals innervating tonic fibre endplates were similar in appearance to those in control preparations. At endplates on both fibres, the postsynaptic membrane was more electron dense than that of control preparations. 6. Following a 6 h exposure to lanthanum, all nerve terminals innervating twitch endplates contained only a few synaptic vesicles and numerous intracellular deposits of electron dense material. The nerve terminals innervating tonic endplates still contained many synaptic vesicles, but the number appeared to be less than that of tonic terminals in untreated preparations. 7. The results demonstrate that lanthanum stimulates spontaneous quantal transmitter release from nerve terminals innervating either twitch or tonic fibres. However, the terminals innervating twitch fibres become depleted of synaptic vesicles, whereas this does not occur as readily in nerve terminals innervating tonic fibres.

We hypothesized that the occurrence of spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), in diastole, might be a mechanism for the saturation of twitch potentiation common to a variety of inotropic perturbations that increase the total cell Ca. We used a videomicroscopic technique in single cardiac myocytes to quantify the amplitude of electrically stimulated twitches and to monitor the occurrence of the mechanical manifestation of spontaneous SR Ca2+ release, i.e., the spontaneous contractile wave. In rat myocytes exposed to increasing bathing [Ca2+] (Cao) from 0.25 to 10 mM, the Cao at which the peak twitch amplitude occurred in a given cell was not unique but varied with the rate of stimulation or the presence of drugs: in cells stimulated at 0.2 Hz in the absence of drugs, the maximum twitch amplitude occurred in 2 mM Cao; a brief exposure to 50 nM ryanodine before stimulation at 0.2 Hz shifted the Cao of the maximum twitch amplitude to 7 mM. In cells stimulated at 1 Hz in the absence of drugs, the maximum twitch amplitude occurred in 4 mM Cao; 1 microM isoproterenol shifted the Cao of the maximum twitch amplitude to 3 mM. Regardless of the drug or the stimulation frequency, the Cao at which the twitch amplitude saturated varied linearly with the Cao at which spontaneous Ca2+ release first occurred, and this relationship conformed to a line of identity (r = 0.90, p = less than 0.001, n = 25). The average peak twitch amplitude did not differ among these groups of cells. In other experiments, (a) the extent of rest potentiation of the twitch amplitude in rat myocytes was also limited by the occurrence of spontaneous Ca2+ release, and (b) in both rat and rabbit myocytes continuously stimulated in a given Cao, the twitch amplitude after the addition of ouabain saturated when spontaneous contractile waves first appeared between stimulated twitches. A mathematical model that incorporates this interaction between action potential-mediated SR Ca2+ release and the occurrence of spontaneous Ca2+ release in individual cells predicted the shape of the Cao-twitch relationship observed in other studies in intact muscle. Thus, the occurrence of spontaneous SR Ca2+ release is a plausible mechanism for the saturation of the inotropic response to Ca2+ in the intact myocardium.

We have characterized calreticulin protein and mRNA in fast-twitch and slow-twitch skeletal muscle. SR vesicles isolated from fast-twitch and slow-twitch skeletal muscles contain calreticulin protein immunoreactive with anti-calreticulin antibody. In addition, the fast-twitch skeletal muscle form of calreticulin is shown to be identical to slow-twitch form of the protein based on the identity of cDNA clones encoding fast-twitch and slow-twitch forms of the protein and hybridization of a fast-twitch or slow-twitch calreticulin cDNA probe to the same mRNA species. Based on these observations we conclude that fast-twitch and slow-twitch skeletal muscles express the same form of calreticulin.

Isometric single-twitch force and intracellular Ca2+ transients were recorded simultaneously, using fura-2, from slow- and fast-twitch muscle fibres of the rat, mouse and Etruscan shrew Suncus etruscus. In the slow-twitch rat soleus, force half-relaxation time was three times longer than the 50% decay time of the fura-2 signal. In contrast, in the fast-twitch extensor digitorum longus muscles of all three animals, muscle relaxation preceded Ca2+ decay. It is proposed that this surprising property of fast-twitch muscles is due to their pCa-tension curve, which is shifted to the right compared with that of slow-twitch muscle.

Concentrations of the high-energy phosphates, ATP and creatine phosphate, were investigated in slow-twitch (ST) and fast-twitch (FT) muscle fibres of patients with myotonia congenita (n = 6), dystrophia myotonica (n = 5), myopathia ocularis (n = 2) and hyperornithinemia with gyrate atrophy (HOGA) (n = 3) and compared with those of normal subjects (n = 4). At rest, the patients with HOGA had lower values of ATP in ST muscle fibres than the controls (P less than 0.05). They also had lower values of creatine phosphate in these fibres than the patients with dystrophia myotonica (P less than 0.03) and myotonia congenita (P less than 0.05). After 30 s bicycle ergometer exercises there was an increase in ATP in the ST muscle fibres of the patients with myotonia congenita, but in all other patient groups there was a decrease. ff782bc1db