The effects of the level of activation and shortening velocity on energy output in type 3 muscle fibres from Xenopus laevis

 We investigated the effect of shortening velocity on the efficiency of single intact slow-twitch muscle fibres (type 3) of Xenopus laevis, at different levels of activation (10, 15, 20 and 40 Hz). Fused contractions were obtained at 40 Hz stimulation. When maximal isometric force had been reached, the fibres were shortened by 10% of the fibre length (L ₀) at 0.4, 1 and 2 L ₀/s. To investigate whether the oscillating force at low stimulation frequencies influenced power output and the rate of heat production, we also performed these experiments with the fibre bathed in dantrolene. The results with fused contractions in the presence of dantrolene were the same as with unfused contractions. At 40 Hz stimulation during shortening the rate of heat production increased above that measured during isometric contractions, while at the lower stimulation frequencies the rate of heat production was less than that during isometric contractions. Mechanical efficiency was highest at low activation, and increased more with shortening velocity than at high activation. The actomyosin efficiency (i.e. the efficiency corrected for ”activation heat”) was also highest at 10 Hz stimulation. We conclude that in slow-twitch muscle fibres of X. laevis, near the optimum shortening velocity, cross-bridge efficiency is highest for partially activated muscle. Received: 9 January 1996 / Received after revision: 24 June 1996 / Accepted: 12 July 1996     

Energetics of shortening depend on stimulation frequency in single muscle fibres from Xenopus laevis at 20° C

Single intact slow-twitch (type 3) muscle fibres from the iliofibularis muscle of Xenopus laevis were shortened at a constant velocity (0.4 L ₀/s, where L ₀ is the initial length at different levels of activation (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38±0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33±0.03 W/g dry weight). The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40±3% of the isometric force at 40 Hz. In these unfused tetani during shortening the heat production rate (0.18±0.02 W/g dry weight) significantly decreased below the isometric heat production rate (0.25±0.02 W/g dry weight). At full activation the rate of total energy production (mechanical power plus heat production rate) during shortening was 1.88±0.32 times the isometric total energy production rate. This effect, i.e. an increase in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was only 1.07±0.08 times the isometric value. These results show that the Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37±0.06).     

The mechanical and thermal properties of frog slow muscle fibres

1. A study has been made of the mechanical behaviour and the heat production of frog slow muscle fibres in iliofibularis nerve-muscle preparations at 20 degrees C.2. The slow fibre isometric tension and its rate of development increase with stimulation frequency, the increases beyond 30 Hz being relatively small. Relaxation rate also increases with stimulation frequency. The tension-length curve and maximum isometric tension (250 mN.mm(-2)) are similar to those of twitch fibres. The maximum shortening velocity is estimated to be 0.11 tonus bundle lengths per second.3. For contractions up to 60 sec at 30-50 Hz the slow fibre heat rate is steady at 6 mJ.g(-1).sec(-1). Slow fibres produce aerobic recovery heat with a time course similar to that of twitch fibres.4. The accuracy of the results is discussed, and a comparison is made with the properties of twitch fibres. It is concluded that the tension-producing reactions are thirty times slower in the slow fibres.     

The energetics of shortening amphibian cardiac muscle

An isolated amphibian cardiac muscle preparation, toad ventricular strip, was used to examine the energetics of shortening. Simultaneous measurements of force and length changes and the associated heat production were made. Both the isometric heat/stress and the enthalpy (heat + work)/load relationships were similar to those previously reported in mammalian cardiac muscle. The activation metabolism was higher in this preparation and, like its mammalian counterpart, was length dependent. The heat production measured in an isometric contraction was approximately 50% higher than that observed at the same stress level in rodent mammalian cardiac muscle. This did not affect the maximum isotonic mechanical efficiency (work / enthalpy) of the preparation which, at an afterload of 20% of the maximum stress was 18.1±1.7% (n=8). There was no evidence for a shortening heat component in this preparation during isotonic contractions. It appears therefore that the energetics of shortening amphibian cardiac muscle closely resemble the energetics of mammalian cardiac tissue.     

Efficiency and cross-bridge work output of skeletal muscle is decreased at low levels of activation

The purpose of this study was to determine how the mechanical efficiency of skeletal muscle is affected by level of activation. Experiments were performed in vitro (35 °C) using bundles of fibres from fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of mice. Measurements were made of the total work and heat produced in response to 10 brief contractions. Mechanical efficiency was the ratio of total work performed to (total heat produced + work performed). Level of activation was varied by altering stimulation frequency between 40 and 160 Hz. Efficiency did not differ significantly between the two muscle types but was significantly lower using 40 Hz stimulation (mean efficiency ± SEM, 0.092?±?0.012, n?=?12, averaged across EDL and soleus) than at any of the other frequencies (160 Hz: 0.147?±?0.007, n?=?12). Measurements of the partitioning of energy output between force-dependent and force-independent components enabled calculation of the amount of Ca²⁺ released and number of cross-bridge cycles performed during the contractions. At 40 Hz stimulation frequency, less Ca²⁺ was released than at higher frequencies and fewer cross-bridge cycles were performed. Furthermore, less work was performed in each cross-bridge cycle. It is concluded that skeletal muscles are less efficient at low levels of activation than when fully activated and this indicates that level of activation affects not only the number of cycling cross-bridges but also the ability of individual cross-bridges to perform work.     

Contractile properties, fatiguability and glycolytic metabolism in fast- and slow-twitch rat skeletal muscles of various temperatures

The influence of muscle temperature (28 and 36 degrees C) on fatiguability and glycolytic metabolism was studied during 5 min of intermittent stimulation of motor nerves of the tibialis anterior, extensor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles in the rat at 100 Hz (200 ms per s). The decline in isometric tension was not affected by muscle temperature either in fast- or in slow-twitch muscles. In fast-twitch muscles the utilization of glycogen during stimulation was the same at 28 and 36 degrees C, while in the soleus muscle it was lower at 28 degrees C. The concentration of glucose-6-phosphate immediately after stimulation was higher in the muscles at 28 degrees C than in those at 36 degrees C, whereas no difference in lactate concentration was found between the two temperature groups. These observations indicate that compared with the rate at 36 degrees C, the rate of glycogenolysis at 28 degrees C is unchanged in fast-twitch, but decreased in slow-twitch muscle. This might imply increased economy of ATP turnover during contraction in the soleus muscle at 28 degrees C.     

Dynamic properties of inferior rectus muscle of the rat

1. Isometric responses of rat inferior rectus muscle to indirect and direct stimulation were compared, and conditions were found for selective direct stimulation of twitch fibres in vitro.

2. Most of the twitch fibres were qualitatively `fast'.

3. The influence of length on isometric contractions and the relation between relative load and speed of sarcomere shortening of fast-twitch fibres were determined.

4. The isometric twitch contraction and half-relaxation times of fast-twitch inferior rectus fibres were only about one half of those of rat extensor digitorum longus fibres in the same conditions, whereas the force: velocity properties of these two fibre groups were virtually the same. These results show that the relation between intrinsic speed of shortening and duration of the active state of the contractile material is not the same for rat extraocular and hind-limb muscles.

     

Muscle strength,power and adaptations to resistance training in older people

Muscle strength and, to a greater extent, power inexorably decline with ageing. Quantitative loss of muscle mass, referred to as sarcopenia, is the most important factor underlying this phenomenon. However, qualitative changes of muscle fibres and tendons, such as selective atrophy of fast-twitch fibres and reduced tendon stiffness, and neural changes, such as lower activation of the agonist muscles and higher coactivation of the antagonist muscles, also account for the age-related decline in muscle function. The selective atrophy of fast-twitch fibres has been ascribed to the progressive loss of motoneurons in the spinal cord with initial denervation of fast-twitch fibres, which is often accompanied by reinnervation of these fibres by axonal sprouting from adjacent slow-twitch motor units (MUs). In addition, single fibres of older muscles containing myosin heavy chains of both type I and II show lower tension and shortening velocity with respect to the fibres of young muscles. Changes in central activation capacity are still controversial. At the peripheral level, the rate of decline in parameters of the surface-electromyogram power spectrum and in the action-potential conduction velocity has been shown to be lower in older muscle. Therefore, the older muscle seems to be more resistant to isometric fatigue (fatigue-paradox), which can be ascribed to the selective atrophy of fast-twitch fibres, slowing in the contractile properties and lower MU firing rates. Finally, specific training programmes can dramatically improve the muscle strength, power and functional abilities of older individuals, which will be examined in the second part of this review.     

Energy metabolism in type I and type II human muscle fibres during short term electrical stimulation at different frequencies

The degradation of adenosine triphosphate, phosphocreatine and glycogen was determined in type I and type II fibres of the human quadriceps femoris muscle during intermittent electrical stimulation at 20 and 50 Hz, (1.6 seconds stimulation, 1.6 seconds rest). Seven healthy volunteers took part in the study. Muscle biopsy samples were obtained at rest and after 10 and 20 seconds of stimulation (six and 12 contractions, respectively). The resting contents of adenosine triphosphate, phosphocreatine and glycogen were all higher (P < 0.05) in type II fibres compared to type I fibres. By the end of stimulation, whole muscle force production had declined to 84 and 77% of the initial force at 20 and 50 Hz, respectively. The phosphocreatine degradation rate after 10 and 20 seconds of stimulation was greater in type II fibres (P < 0.05) compared to type I fibres at both 20 and 50 Hz. The rates of glycogenolysis after 20 seconds stimulation in type II fibres were 3.18±1.1 and 6.31±1.39mmol glycosyl units kg^-1 s^-1. The corresponding rates in type I fibres were 0.46 ± 0.73 and 0.60 ± 0.39 mmol glycosyl units kg^-1 s^-1 which were not significantly different from zero. It is hypothesized that the decline in whole muscle force observed during electrical stimulation may be a consequence of the rapid loss of PCr stores in type II fibres.     

The force-velocity relation of the rabbit inferior oblique muscle; influence of temperature

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35°C. Isovelocity releases were used to determine the force/ velocity relation. Cooling the muscle from 35°C to 20°C increased contraction and half-relaxation times of single twitches with a temperature coefficient (Q ₁₀) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q ₁₀=1.32). Cooling decreased the maximum shortening velocity of the IO with a Q ₁₀ of 1.6 and the maximum mechanical power with a Q ₁₀ of 2.3. At 35°C, the maximum speed of shortening of the muscle (19±2 muscle lengths/s, mean ± SEM) corresponded to a maximum shortening velocity of the sarcomeres of 57±6 m/s. This value is similar to data obtained for extraocular muscles (EOM) of smaller rodents (mice and rats). In comparison with mammalian limb muscles the isometric and force-velocity properties of mammalian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of shortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscle: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.     

Energy metabolism in different human skeletal muscles during voluntary isometric contractions

Summary The energy turnover in contracting skeletal muscle was studied by measuring the rate of temperature rise during voluntary, isometric contractions and circulatory arrest in M. soleus, M. sacrospinalis and M. biceps brachii in 14 males, by thermoelements inserted in the muscles. A linear relationship between rate of temperature rise and force intensity given as per cent of maximal voluntary contraction (MVC) was demonstrated in biceps (r=0.95), but not so clearly confirmed in soleus (r=0.73). Muscle biopsies were taken from the same muscles and fibre type distribution was determined histochemically by staining for ATPase. The rate of heat production at MVC showed positive correlation to the percentage of fast twitch (FT) fibres in the muscles (r=0.90). Linear extrapolation indicates that the maximal energy turnover in human FT fibres is approximately six times that of slow twitch (ST) fibres during voluntary isometric contractions.This work was submitted by G. Bolstad as a thesis to the University of Bergen in June 1975, in partial fulfilment of the requirements for the degree of Candidatus realium     

Muscle damage induced by electrical stimulation

Electrical stimulation (ES) induces muscle damage that is characterised by histological alterations of muscle fibres and connective tissue, increases in circulating creatine kinase (CK) activity, decreases in muscle strength and development of delayed onset muscle soreness (DOMS). Muscle damage is induced not only by eccentric contractions with ES but also by isometric contractions evoked by ES. Muscle damage profile following 40 isometric contractions of the knee extensors is similar between pulsed current (75 Hz, 400 μs) and alternating current (2.5 kHz delivered at 75 Hz, 400 μs) ES for similar force output. When comparing maximal voluntary and ES-evoked (75 Hz, 200 μs) 50 isometric contractions of the elbow flexors, ES results in greater decreases in maximal voluntary contraction strength, increases in plasma CK activity and DOMS. It appears that the magnitude of muscle damage induced by ES-evoked isometric contractions is comparable to that induced by maximal voluntary eccentric contractions, although the volume of affected muscles in ES is not as large as that of eccentric exercise-induced muscle damage. It seems likely that the muscle damage in ES is associated with high mechanical stress on the activated muscle fibres due to the specificity of motor unit recruitment (i.e., non-selective, synchronous and spatially fixed manner). The magnitude of muscle damage induced by ES is significantly reduced when the second ES bout is performed 2–4 weeks later. It is possible to attenuate the magnitude of muscle damage by “pre-conditioning” muscles, so that muscle damage should not limit the use of ES in training and rehabilitation.     

Measured and modeled properties of mammalian skeletal muscle. II. The effectsof stimulus frequency on force-length and force-velocity relationships

Interactions between physiological stimulus frequencies, fascicle lengths and velocities were analyzed in feline caudofemoralis (CF), a hindlimb skeletal muscle composed exclusively of fast-twitch fibers. Split ventral roots were stimulated asynchronously to produce smooth contractions at sub-tetanic stimulus frequencies. As described previously, the peak of the sub-tetanic force-length relationship was found to shift to longer lengths with decreases in stimulus frequency, indicating a length dependence for activation that is independent of filament overlap. The sub-tetanic force-velocity (FV) relationship was affected strongly both by stimulus frequency and by length; decreases in either decreased the slope of the FV relationship around isometric. The shapes of the force transients following stretch or shortening revealed that these effects were not due to a change in the instantaneous FV relationship; the relative shape of the force transients following stretch or shortening was independent of stimulus frequency and hardly affected by length. The effects of stimulus frequency and length on the sub-tetanic FV relationship instead appear to be caused by a time delay in the length-dependent changes of activation. In contrast to feline soleus muscle, which is composed exclusively of slow-twitch fibers, CF did not yield at sub-tetanic stimulus frequencies for the range of stretch velocities tested (up to 2 L₀/s). The data presented here were used to build a model of muscle that accounted well for all of the effects described. We extended our model to account for slow-twitch muscle by comparing our fast-twitch model with previously published data and then changing the necessary parameters to fit the data. Our slow-twitch model accounts well for all previous findings including that of yielding.     

Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30 ± 0.02 to 0.34 ± 0.02 N·s⁻¹ at RL and from 0.18 ± 0.02 to 0.28 ± 0.01 N·s⁻¹ at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85 ± 0.14 to 1.14 ± 0.17 mm, and this corresponded with an increase in peak velocity (from 116 ± 18 to 136 ± 19 mm·s⁻¹). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s⁻¹). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.     

The energetic cost of activation in mouse fast-twitch muscle is the same whether measured using reduced filament overlap or N-benzyl-p-toluenesulphonamide

Aim: Force generation and transmembrane ion pumping account for the majority of energy expended by contracting skeletal muscles. Energy turnover for ion pumping, activation energy turnover (E_A), can be determined by measuring the energy turnover when force generation has been inhibited. Most measurements show that activation accounts for 25–40% of isometric energy turnover. It was recently reported that when force generation in mouse fast‐twitch muscle was inhibited using N‐benzyl‐p‐toluenesulphonamide (BTS), activation accounted for as much as 80% of total energy turnover during submaximal contractions. The purpose of this study was to compare E_A measured by inhibiting force generation by: (1) the conventional method of reducing contractile filament overlap; and (2) pharmacological inhibition using BTS. Methods: Experiments were performed in vitro using bundles of fibres from mouse fast‐twitch extensor digitorum longus (EDL) muscle. Energy turnover was quantified by measuring the heat produced during 1‐s maximal and submaximal tetanic contractions at 20 and 30 °C. Results: E _A measured using reduced filament overlap was 0.36 ± 0.04 (n = 8) at 20 °C and 0.31 ± 0.05 (n = 6) at 30 °C. The corresponding values measured using BTS in maximal contractions were 0.46 ± 0.06 and 0.38 ± 0.06 (n = 6 in both cases). There were no significant differences among these values. E_A was also no different when measured using BTS in submaximal contractions. Conclusion: Activation energy turnover is the same whether measured using BTS or reduced filament overlap and accounts for slightly more than one‐third of isometric energy turnover in mouse EDL muscle.     

Sodium channel slow inactivation and the distribution of sodium channels on skeletal muscle fibres enable the performance properties of different skeletal muscle fibre types

Na⁺ currents (I_Na) and membrane capacitance were studied with the loose patch voltage clamp technique and action potential properties were studied with a two-electrode voltage clamp on the end-plate, at the end-plate border and on extrajunctional membrane of skeletal muscle fibres. Slow inactivation regulates the available I_Na and is operative at the resting potential of both rat and human fibres. At the resting potential, slow inactivation causes a greater reduction in I_Na in fast- than in slow-twitch fibres. The relative resistance of slow-twitch fibres to slow inactivation may enable slow-twitch fibres to remain tonically active. Na⁺ channel inactivation may provide a peripheral mechanism that limits the duration that fast-twitch fibres can fire at high rates to prevent injury associated with prolonged high-frequency contraction. Consequently, slow inactivation may enable fast-twitch fibres to operate phasically at high rates or slow-twitch fibres to fire continuously at lower rates. For both fast- and slow-twitch fibres, I_Na normalized to membrane area was greatest on the end-plate, intermediate on the end-plate border and smallest on extrajunctional membrane. When normalized to membrane capacitance, I_Na was the same on the end-plate and the end-plate border and smallest on extrajunctional membrane. For a given membrane region, I_Na was larger on fast- than on slow-twitch fibres. The higher density of Na⁺ channels near the end-plate increased the safety factor for neuromuscular transmission by lowering the action potential threshold and increasing the action potential rate of rise at the end-plate.     

Slow skeletal muscles of the mouse have greater initial efficiency than fast muscles but the same net efficiency

The aim of this study was to determine whether the net efficiency of mammalian muscles depends on muscle fibre type. Experiments were performed in vitro (35°C) using bundles of muscle fibres from the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The contraction protocol consisted of 10 brief contractions, with a cyclic length change in each contraction cycle. Work output and heat production were measured and enthalpy output (work + heat) was used as the index of energy expenditure. Initial efficiency was defined as the ratio of work output to enthalpy output during the first 1 s of activity. Net efficiency was defined as the ratio of the total work produced in all the contractions to the total, suprabasal enthalpy produced in response to the contraction series, i.e. net efficiency incorporates both initial and recovery metabolism. Initial efficiency was greater in soleus (30 ± 1%; n = 6) than EDL (23 ± 1%; n = 6) but there was no difference in net efficiency between the two muscles (12.6 ± 0.7% for soleus and 11.7 ± 0.5% for EDL). Therefore, more recovery heat was produced per unit of initial energy expenditure in soleus than EDL. The calculated efficiency of oxidative phosphorylation was lower in soleus than EDL. The difference in recovery metabolism between soleus and EDL is unlikely to be due to effects of changes in intracellular pH on the enthalpy change associated with PCr hydrolysis. It is suggested that the functionally important specialization of slow-twitch muscle is its low rate of energy use rather than high efficiency.     

Muscle fibre types and their distribution in the biceps and triceps brachii of the rat and rabbit

Muscle fibre type composition and distribution in the biceps brachii (long head) and triceps brachii (long head) of the rat and rabbit were investigated using the following histochemical techniques: myosin ATPase, with preincubation at pH 10.4 and 4.35; succinate dehydrogenase (SDH) and glycogen phosphorylase. The muscle fibres were classified into slow-twitch (SO), fast-twitch glycolytic (FG), fast-twitch oxidative glycolytic (FOG and FOg) and fast-twitch oxidative fibres (FO). Significant differences in the regional distribution of muscle fibre types have been observed between the rat and the rabbit. In the rat, SO fibres were restricted to the deep regions of both biceps and triceps brachii, whereas FG fibres were located in the intermediate and superficial regions (the superficial regions contained the highest percentages of FG fibres). In the rabbit, SO and FG fibres were spread over the entire muscle, although SO and FG fibres were most abundant in the deep and superficial regions respectively. These findings indicate that the biceps and triceps brachii are more regionalised in the rat than in the rabbit.     

Correlations between MyoD, myogenin, SERCA1, SERCA2 and phospholamban transcripts during transformation of type-II to type-I skeletal muscle fibers

 Canine latissimus dorsi, composed predominantly of fast-twitch muscle fibers, were subjected to chronic 1 Hz neuromuscular stimulation for periods up to 42 days to induce changes in gene expression. This produced down regulation of SERCA1 (fast-twitch isoform of sarco(endo)plasmic reticulum Ca²⁺-ATPase), a gene product of fast-twitch muscle, and up regulation of SERCA2 (slow-twitch isoform of sarco(endo)plasmic reticulum Ca²⁺-ATPase) and phospholamban, products of genes expressed by slow-twitch muscles. To assess the involvement of MyoD and myogenin in the regulation of the expression of these genes their levels were measured during the stimulation period. The prompt, at 7 days, fall in SERCA1 mRNA preceded the fall in MyoD by about 7 days, suggesting that the decline in MyoD was not causally related to the decline in SERCA1. The prompt rise in SERCA2 mRNA at 7 days preceded the rise in myogenin by 14 days. The rise in myogenin at 21 days did correlate with the similar rise in phospholamban mRNA. Received: 13 January 1997 / Accepted: 24 March 1997     

Effects of nerve cross-union on fast-twitch and slow-graded muscle fibres in the toad

1. A method is described for resolving isometric tetanic tension developed by fast-twitch and slow-graded components of heterogeneous toad muscles. This makes use of the difference in threshold for excitation of low threshold nerve fibres which normally innervate the fast-twitch muscle fibres and high threshold nerve fibres which innervate slow-graded muscle fibres.

2. The sartorius muscle contains only fast-twitch muscle fibres whereas the posterior semitendinosus (PST) contains both fast-twitch and slow-graded muscle fibres, the latter contributing 10-15% of the maximum isometric tetanic tension.

3. Following surgical cross-union of nerve to sartorius and PST muscles, both the fast-twitch and slow-graded muscle fibre components of the PST are reinnervated by low threshold nerves originally innervating sartorius fast-twitch fibres, and sartorius fast-twitch muscle fibres are reinnervated by both low threshold and high threshold nerves formerly supplying the fast-twitch and slow-graded muscle fibre components of the PST.

4. The characteristic mechanical responses of fast-twitch muscle fibres and slow-graded muscle fibres were not transformed up to 134 and 200 days respectively following nerve cross-union.

5. PST nerve partially innervated the sartorius muscle whereas sartorius nerves completely innervated the PST muscle. Isometric tetanic tension declined markedly during repetitive indirect stimulation of cross-innervated sartorius muscles, whereas the tetanic contractions of cross-innervated PST showed a plateau of tension and resembled the response of normal muscles.

6. Normal, cross-innervated and self-innervated PST muscles gave sustained contractures in the presence of acetylcholine whereas PST muscles denervated for 120 days gave phasic contractures similar to those of normal, cross-innervated and self-innervated sartorius muscles.