Electromechanical behaviour of human muscles in vertical jumps

Summary The relationships of muscle structure to the potentiation of myoelectrical activity and to the use of prestretching in five lower limb muscles were studied in different vertical jumping conditions. The subjects for the study were six male students, divided according to the muscle fiber distribution in m. vastus lateralis into fast and slow groups. The subjects performed vertical jumps (1) from a static squatting position (SJ), (2) with a preliminary counter movement (CMJ) and (3) after dropping (DJ) from five different heights. Myoelectrical (EMG) activity was recorded from mm. gluteus maximus, vastus lateralis, vastus medialis, rectus femoris and gastrocnemius in each jumping condition and integrated (IEMG) for the eccentric and concentric phases of contact. EMG activity showed potentiation during the eccentric phase of movement when compared to the concentric phase. The fast and slow groups did not differ significantly in this respect, whereas in DJ conditions the relative (% from SJ) height of rise of the center of gravity was greater in the slow than in the fast group. The result indicated that the utilization of elastic energy during jumping was possible better in subjects having a high percentage of slow twitch muscle fibres in their vastus lateralis muscles.     

Leap of faith: voluntary emersion behaviour and physiological adaptations to aerial exposure in a non-aestivating freshwater fish in response to aquatic hypoxia

Lowland stream fauna in areas of intensive agriculture are increasingly under threat from anthropogenic activities leading to eutrophication and subsequent hypoxia. Survival of hypoxic episodes depends upon a combination of behavioural and physiological adaptations. Responses of inanga (Galaxias maculatus: Galaxiidae) to aquatic hypoxia were investigated in the laboratory. Contrary to expectation inanga did not display behaviour that might reduce energy expenditure during oxygen limitation, with swimming activity slightly, but significantly elevated relative to normoxia. Instead, as dissolved oxygen concentrations decreased, the fish moved higher in the water column, increased their swimming speed and exhibited aquatic surface respiration. Physiological changes such as enhanced opercular frequency were also noted. As hypoxia deepened inanga started to leap out of the water, emersing themselves on a floating platform. Once emersed, fish exhibited an enhanced oxygen consumption rate compared to hypoxic fish. Thus inanga appear better adapted to escape hypoxia (a behavioural adaptation) rather than tolerate it (physiological adaptation). The emersion strategy used for inanga in response to severe hypoxia is in agreement with their ability to take up more oxygen from the air than from hypoxic water and therefore may justify the potentially increased risks of desiccation and predation associated with leaving the water.     

Electrical properties of normal,denervated and organ-cultured slow fibres of toad cruralis muscles

Two types of fibres were characterized in the cruralis muscle of the toad using electrophysiological techniques: the slow and the fast fibres. Five to ten slow fibres were easily identified on the inner face of this muscle. The cruralis slow fibres developed the ability to produce action potentials 40 days after the sciatic nerve was transected at the hip level, while the slow fibres of the pyriformis muscle showed a latent period of 17 days after the same surgical treatment. However, when in addition to this procedure the nerve was transected at the point where it enters the muscle but without damaging the slow fibres, the latency was about 20 days. The slow fibres of the cruralis muscle maintained in organ culture developed the ability to produce action potentials in 24 days. During the winter the slow fibres of in vivo denervated cruralis and pyriformis muscles did not develop the ability to produce regenerative responses. Moreover organ-cultured cruralis muscles taken from winter toads showed this same inability.These results further support the idea that the excitability of slow fibres is under the control of a neural factor rather than of activity. The seasonal dependence points to the fact that the metabolic state of the muscle is of crucial importance in determining the development of excitability of slow fibres.This work was supported by grants from Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET).     

Hypertrophic response of human skeletal muscle to strength training in hypoxia and normoxia

The purpose of this study was to test the hypothesis that work-induced skeletal muscle hypertrophy may be reduced by training in chronic hypobaric hypoxia compared to normoxia. Five healthy males [mean age 34.4 (SEM 2.2) years] performed strength training of the elbow flexors for 1 month, at altitude (A) (5050 m) and with the same absolute loads at sea level (SL), 8 months later. The EF cross-sectional area (CSA), determined at mid-arm by nuclear magnetic resonance imaging, increased by 11.3 (SEM 3.7)% (P < 0.05) at A and 17.7 (SEM 4.5) % (P < 0.05) at SL. Isometric maximal voluntary contraction (MVC) increased by 9.5 (SEM 2.6)% (P<0.05) at A and 13.6 (SEM 2.4)% (P<0.05) at SL. The CSA and MVC changes in A were significantly smaller than at SL (P<0.05). Muscle specific tension did not change in either condition. No changes in muscle plus bone or MVC of the untrained, controlateral arm were observed. Thus, although there was no indication of muscle wasting at A, the hypertrophic response of skeletal muscle when trained in chronic hypoxia seemed to be significantly lower than that produced in normoxia. This effect could have arisen either from a direct depression of protein synthesis and/or hormonal changes provoked by hypoxia.     

Erythropoietin acute reaction and haematological adaptations to short, intermittent hypobaric hypoxia

This study aimed to determine whether brief hypoxic stimuli in a hypobaric chamber are able to elicit erythropoietin (EPO) secretion, and to effectively stimulate erythropoiesis in the short term. In two different experiments, a set of haematological, biochemical, haemorheological, aerobic performance, and medical tests were performed in two groups of healthy subjects. In the first experiment, the mean plasma concentration of EPO ([EPO]) increased from 8.7 to 13.5 mU · ml⁻¹ (55.2%; P < 0.01) after 90 min of acute exposure at 540 hPa, and continued to rise until a peak was attained 3 h after the termination of hypoxia. In the second experiment, in which subjects were exposed to a simulated altitude of up to 5500 m (504 hPa) for 90 min, three times a week for 3 weeks, all haematological indicators of red cell mass increased significantly, reaching the highest mean values at the end of the programme or during the subsequent 2 weeks, including packed cell volume (from 42.5 to 45.1%; P < 0.01), red blood cell count (from 4.55 × 10⁶ to 4.86 × 10⁶ · l⁻¹; P < 0.01), reticulocytes (from 0.5 to 1.4%; P < 0.01), and haemoglobin concentration (from 14.3 to 16.2 g · dl⁻¹; P < 0.01), without an increase in blood viscosity. Arterial blood oxygen saturation during hypoxia was improved (from 60% to 78%; P < 0.05). Our most relevant finding is the ability to effectively stimulate erythropoiesis through brief intermittent hypoxic stimuli (90 min), in a short period of time (3 weeks), leading to a lower arterial blood desaturation in hypoxia. The proposed mechanism for these haematological and functional adaptations is the repeated triggering effect of EPO production caused by the intermittent hypoxic stimuli. Accepted: 15 December 1999     

低氧诱导因子-1在调控骨骼肌缺氧时能量代谢发生适应性变化的机制研究进展

低氧诱导因子-1(HIF-1)是一种调控组织细胞氧稳态的关键性核转录因子,广泛存在于哺乳动物和人体内,其表达和活性受到细胞氧浓度的严密调控。它能在生理性和病理性缺氧缺血的情况下,通过调控细胞能量代谢、血管发生、红细胞生成、细胞生存、细胞增殖和凋亡等生物学效应,使细胞适应低氧环境得以生存或者走向凋亡。本文中我们主要概述了HIF-1的结构功能,及其在缺氧时调控骨骼肌能量代谢方面发生适应性变化的机制及研究进展。     

Force-time characteristics and fiber composition in human leg extensor muscles

Summary Recording of the force-time (f-t) characteristics of muscular contraction expresses the rate at which tension is developed. To further understand the problems involved in force production during voluntary contraction, the f-t curve was registered during maximal voluntary isometric extension of both legs performed in the sitting position with the knee angle at 107 degrees. 38 athletes representing various sport events, five pairs of monozygous, and ten pairs of dizygous twins were used as subjects. The reference group consisted of eight normal men. The data disclosed that the time to produce certain force levels showed good trial-to-trial and satisfactory day-to-day reproducibility below tension levels of 0.9×P ₀. At these force levels the time of tension development was positively (p<0.05) related to the per cent distribution of slow twitch fibers in the vastus lateralis muscle. This result is consistent with animal experiments concerning the mechanical characteristics of slow and fast muscles. In addition, it was observed that the athletic groups had f-t curves different from the other subjects. Genetic factors had only slight influence on the f-t measurement.     

Human erythropoietin response to hypocapnic hypoxia,normocapnic hypoxia,and hypocapnic normoxia

This study investigated the human erythropoietin (EPO) response to short-term hypocapnic hypoxia, its relationship to a normoxic or hypoxic increase of the haemoglobin oxygen affinity, and its suppression by the addition of CO₂ to the hypoxic gas. On separate days, eight healthy male subjects were exposed to 2 h each of hypocapnic hypoxia, normocapnic hypoxia, hypocapnic normoxia, and normal breathing of room air (control experiment). During the control experiment, serum-EPO showed significant variations (ANOVAP = 0.047) with a 15% increase in mean values. The serum-EPO measured in the other experiments were corrected for these spontaneous variations in each individual. At 2 h after ending hypocapnic hypoxia (10% O₂ in nitrogen), mean serum-EPO increased by 28% [baseline 8.00 (SEM 0.84) U · 1⁻¹, post-hypoxia 10.24 (SEM 0.95) U · 1⁻¹, P = 0.005]. Normocapnic hypoxia was produced by the addition of CO₂ (10% Co₂ with 10% O₂) to the hypoxic gas mixture. This elicited an increased ventilation, unaltered arterial pH and haemoglobin oxygen affinity, a lower degree of hypoxia than during hypocapnic hypoxia, and no significant changes in serum-EPO (ANOVAP > 0.05). Hypocapnic normoxia, produced by hyperventilation of room air, elicited a normoxic increase in the haemoglobin oxygen affinity without changing serum-EPO. Among the measured blood gas and acid-base parameters, only the partial pressures of oxygen in arterial blood during hypocapnic hypoxia were related to the peak values of serum-EPO (r = −0.81,P = 0.01). The present human EPO responses to hypoxia were lower than those which have previously been reported in rodents and humans. In contrast with the earlier rodent studies, it was found that human EPO production could not be triggered by short-term increases in pH and haemoglobin oxygen affinity per se, and the human EPO response to hypoxia could be suppressed by concomitant normocapnia without acidosis.     

The effects of different patterns of muscle activity on capillary density,mechanical properties and structure of slow and fast rabbit muscles

Summary When rabbit fast muscles were chronically stimulated at a frequency naturally occurring in nerves to slow muscles (10 Hz), there was a transformation towards a slow muscle type such as an increase of capillary density, increased activity of the oxidative enzyme, succinic dehydrogenase, and a decrease of muscle fibre diameters. After 28 days the intensity and distribution of SDH and the capillary density were similar to those of soleus.The increases in capillary density preceded the changes in activity of SDH; there was a significantly greater capillary/muscle fibre ratio and number of capillaries/mm² in muscles stimulated for only 4 days at which time no change could be detected in SDH. These changes were induced by slow frequency stimulation only, and not by an overall increase of activity.Stimulation of fast muscles for 4 days at a higher frequency naturally occurring in the nerves to fast muscles (short bursts of tetani), with the same total number of stimuli as that used in slow frequency stimulation did not produce any changes in capillary density, activity of SDH or contraction times. No changes were observed in either fast or slow muscles stimulated with short bursts of tetani (and lower total number of impulses) up to 28 days.Activation of fast muscles at 5 Hz continuously or 10 Hz intermittently also caused an increase in capillary density.It is therefore concluded that only low frequency activation of fast muscles brings about a transformation of the muscle fibres towards a slow type and that the first noticeable change is an increase in the capillary density.     

Mechanical characteristics and fiber composition of human leg extensor muscles

Summary To investigate the influence of skeletal muscle fiber composition on the mechanical performance of human skeletal muscle under dynamic conditions, 34 physical education students with differing muscle fiber composition (M. vastus lateralis) were used as subjects to perform maximal vertical jumps on the force-platform. Two kinds of jumps were performed: one from a static starting position (SJ), the other with a preliminary counter-movement (CMJ). The calculated mechanical parameters included height of rise of center of gravity (h), average force (¯F), net impulse (NI) and average mechanical power (W). It was observed that the percentage of fast twitch fibers was significantly related (p< 0.05-0.01) to these variables in SJ condition and also to h and NI of the positive work phase in CMJ. It is concluded that skeletal muscle fiber composition also determines performance in a multijoint movement. The result is explainable through the differences in the mechanical characteristics of the motor units and their respective muscle fibers.     

Lactate in blood,mixed skeletal muscle,and FT or ST fibres during cycle exercise in man

The relationship between muscle and blood lactate levels during progressively step-wise incrementing cycle exercise has been investigated in 10 male subjects. Steps between power outputs during exercise were 50 W and each stage, from loadless pedalling until voluntary exhaustion, lasted 4 min. Blood samples and biopsies (m. vastus lateralis) were taken for lactate determination at each power output beginning with the exercise intensity perceived by the subject as being “rather moderate”. The ratio muscle: blood lactate was greater than one at all power outputs and increased most markedly at the power output closest to that eliciting 4 mmol × I^-1 blood lactate (W_OBLA). At W_OBLA. blood lactate was positively correlated to muscle lactate concentrations which covaried widely among subjects (mean 8.3. range 4.5–14.4 mmol × kg^-l wet weight). Muscle fibres from the W_OBLA biopsy in 6 subjects were dissected out and identified as fast twitch (FT) or slow twitch (ST). No significant difference in lactate concentration was observed between pools of FT or ST fibres.     

Contractile properties of human skeletal muscle in childhood and adolescence

Summary Using a combination of single maximal stimuli and maximum voluntary contractions, a comparison has been made of muscle properties in pre- and post-pubertal male subjects. In the dorsiflexor and plantarflexor muscles of the ankle, the twitch and maximum voluntary torques were approximately twice as large in the older subjects; the mean height and mean weight increased by factors of 1.20 and 1.86 respectively. The only other muscle parameter that changed, as a function of age, was the contraction time of the ankle dorsiflexors; the mean value was significantly longer in the older subjects. In the younger subjects, there were already clear differences between the dorsiflexor and plantarflexor muscles, the former developing smaller torques and having shorter contraction and half-relaxation times, greater post-activation potentiation and more susceptibility to fatigue. Even in the youngest subject, motor unit activation was complete in the ankle dorsiflexors; although this was not always true of the plantarflexors, the difference between the two subject groups was not significant.Supported by the Muscular Dystrophy Association of Canada     

Miniature end-plate potentials and sensitivity to acetylcholine in the fast and slow limb muscles of hibernating golden hamsters

Summary During hibernation reduction of frequency of the miniature end-plate potential (m.e.p.p.) was observed. This parameter was reduced in the extensor digitorum longus (EDL) muscle to 16% and in the soleus muscle to 7% of values found in awake animals. The ACh-sensitive area of the individual muscle fibres in both types of muscle increased about three times during hibernation.     

A comparison of contractile properties in human arm and leg muscles

Summary Isometric twitch properties have been compared in two pairs of opposing human limb muscles; these were the brachial biceps and triceps, and the anterior tibial and plantarflexor muscles. All four muscles were examined in each of 24 healthy subjects (16 men and 8 women). The brachial triceps had the shortest contraction and half-relaxation times and the greatest twitch potentiation, while the plantarflexors had the most prolonged twitches and least potentiation; the anterior tibial and brachial biceps muscles had similar charateristics. Susceptibility to fatigue was less in the plantarflexors than in the other three muscles. When muscles were assessed without reference to their anatomical sites, a significant relationship was noted between contraction time and potentiation, but not between either of these features and fatiguability. There was no evidence that muscles were uniformly ‘faster’ or ‘slower’ in some subjects than in others. Supported by financial aid from the Muscular Dystrophy Association of Canada     

Relations between the effect of tetanus toxin on the neuromuscular transmission and histological functional properties of various muscles of the rat

Summary Various doses of tetanus toxin were injected into three hind leg and two fore leg muscles of the rat. The neuromuscular transmission was tested by recording the mass action potential of the muscles elicited by a single electrical stimulus to the motor nerve after strong symptoms of local tetanus had developed. The muscle responses were depressed and blocked at lower toxin doses in the fast tibialis anterior than in the mixed gastrocnemius latemlis, while blocking of the slow soleus required the highest dose. The extensor carpi radialis and the flexor carpi ulnaris muscles showed medium sensitivity. In all five muscles the contraction time was measured and correlated with its individual minimal blocking dose. The more phasic (i.e., the faster) the muscle, the more sensitive its neuromuscular transmission was to tetanus toxin. The proportional distribution of red, white, and intermediate fibres, which are associated with specific end-plate types, was evaluated for the five muscles. The percentage of white fibres in the muscles displayed a very good negative correlation with the blocking dose. The relation between structures of end-plates and effects of tetanus toxin were analysed and it is suggested that the differences in sensitivity to tetanus toxin in the neuromuscular transmission in the five muscles is determined by a differential distribution of endplates with varying sensitivities to this toxin due to structural properties.This study is a part of a doctoral dissertation submitted by one of the authors (H.K.) to the Faculty of Medicine, University of Göttingen. Some of the results were presented at the 48th and 49th Congr. of German Physiol. Soc. (Kretzschmar et al., 1977, 1978) and at the 5th Internat. Conf. on Tetanus (Kretzschmar et al., 1979)     

Twitch contraction properties of plantar flexor muscles in pre- and post-pubertal boys and men

This study compared electrically evoked twitch contraction characteristics of the plantar flexor muscles in pre-pubertal (11-year-old) and post-pubertal (16-year old) boys, and young (19- to 23-year-old) men. The posterior tibial nerve was stimulated by supramaximal square-wave pulses of 1 ms duration at rest and after brief (5 s) isometric maximal voluntary contraction (MVC) of the plantar flexor muscles, i.e. during post-activation potentiation. Men had higher MVC force than boys and post-pubertal boys higher than pre-pubertal boys. Pre-pubertal boys had lower peak twitch forces (P _t) at rest and when potentiated compared with post-pubertal boys and men, whereas no significant differences were found between post-pubertal boys and men. Pre-pubertal boys had higher ratios of P _t at rest and potentiated P _t to MVC force than post-pubertal boys and men. No age-related differences were obtained in post-activation potentiation, rest and potentiated twitch contraction and half-relaxation time, and MVC force relative to body mass. The main findings of the study were that puberty is characterized by increased muscle force-generating capacity with no change in twitch potentiation and time-course characteristics, and that twitch force-generating capacity develops in an adult-like pattern after puberty. Accepted: 10 April 2000     

Age- and training-related changes in the collagen metabolism of rat skeletal muscle

Summary The effects of ageing and life-long endurance training on the collagen metabolism of skeletal muscle were evaluated in a longitudinal study. Wistar rats performed treadmill running 5 days a week for 2 years. The activities of collagen biosynthesis enzymes, prolyl-4-hydroxylase and galactosylhydroxylysyl glucosyltransferase, were highest in the muscles of the youngest animals, decreased up to the age of 2 months and from then on remained virtually unchanged. The enzyme activity in young animals was higher in the slow collagenous soleus muscle than in the rectus femoris muscle. The enzyme activity in the soleus muscle was higher for older trained rats than older untrained rats. The relative proportion of type I collagen increased and that of type III collagen decreased with age, suggesting a more marked contribution by type I collagen to the agerelated accumulation of total muscular collagen. The results show that collagen biosynthesis decreases with maturation and that life-long endurance training maintains a higher level of biosynthesis in slow muscles.     

Ventilatory response to acute hypoxia in transgenic mice over-expressing erythropoietin: effect of acclimation to 3-week hypobaric hypoxia

We used transgenic mice constitutively over-expressing erythropoietin ("tg6" mice) and wild-type (wt) mice to investigate whether the high hematocrit (hct), consequence of Epo over-expression affected: (1) the normoxic ventilation (V (E)) and the acute hypoxic ventilatory response (HVR) and decline (HVD), (2) the increase in ventilation observed after chronic exposure to hypobaric hypoxia (430mmHg for 21 days), (3) the respiratory "blunting", and (4) the erythrocythemic response induced by chronic hypoxia exposure. V (E) was found to be similar in tg6 and wt mice in normoxia (FIO2=0.21). Post-acclimation V (E) was significantly elevated in every time point in wt mice at FIO2=0.10 when compared to pre-acclimation values. In contrast, tg6 mice exhibited a non-significant increase in V (E) throughout acute hypoxia exposure. Changes in V (E) are associated with adjustments in tidal volume (V(T)). HVR and HVD were independent of EE in tg6 and wt mice before chornic hypoxia exposure. HVR was significantly greater in wt than in tg6 mice after chronic hypoxia. After acclimation, HVD decreased in tg6 mice. Chronic hypoxia exposure caused hct to increase significantly in wt mice, while only a marginal increase occurred in the tg6 group. Although pre-existent EE does not appear to have an effect on HVR, the observation of alterations on V(T) suggests that it may contribute to time-dependent changes in ventilation and in the acute HVR during exposure to chronic hypoxia. In addition, our results suggest that EE may lead to an early "blunting" of the ventilatory response.