Mechanical and energy characteristics during shortening in isolated type-1 muscle fibres from Xenopus laevis studied at maximal and submaximal activation

 The mechanical and energy characteristics of isolated fast-twitch muscle fibres (type 1) of Xenopus laevis in isometric- and isovelocity contractions were measured at 20°C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of activation. The high stimulation frequency gave fused contractions, while at the low stimulation frequency tension fluctuated. When maximum isometric force had been reached, the fibres were shortened by 10% of the fibre length at different velocities. At 60 Hz stimulation during shortening the rate of heat production increased above the isometric rate of heat production. At 20 Hz stimulation during shortening, however, the rate of heat production was not different from the isometric rate of heat production. Mechanical efficiency was the same at the high and low level of activation. The actomyosin efficiency (i.e. the mechanical efficiency corrected for ”activation heat”) was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activated muscle. From a comparison of the present results with those obtained from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow-twitch muscle fibres. Received after revision: 13 May 1997 / Accepted: 1 July 1997     

Recruitment of fibre types and quadriceps muscle portions during repeated, intense knee-extensor exercise in humans

To investigate recruitment of slow-twitch (ST) and fast-twitch (FT) muscle fibres, as well as the involvement of the various quadriceps femoris muscle portions during repeated, intense, one-legged knee-extensor exercise, 12 healthy male subjects performed two 3-min exercise bouts at ~110% maximum thigh O₂ consumption (EX1 and EX2) separated by 6 min rest. Single-fibre metabolites were determined in successive muscle biopsies obtained from the vastus lateralis muscle (n=6) and intra-muscular temperatures were continuously measured at six quadriceps muscle sites (n=6). Creatine phosphate (CP) had decreased (P<0.05) by 27, 73 and 88% in ST fibres and 25, 71 and 89% in FT fibres after 15 and 180 s of EX1 and after 180 s of EX2, respectively. CP was below resting mean–1 SD in 15, 46, 84 and 100% of the ST fibres and 9, 48, 85 and 100% of the FT fibres at rest, after 15 and 180 s of EX1 and after 180 s of EX2, respectively. A significant muscle temperature increase (T_m) occurred within 2–4 s at all quadriceps muscle sites. T_m varied less than 10% between sites during EX1, but was 23% higher (P<0.05) in the vastus lateralis than in the rectus femoris muscle during EX2. T_m in the vastus lateralis was 101 and 109% of the mean quadriceps value during EX1 and EX2, respectively. We conclude that both fibre types and all quadriceps muscle portions are recruited at the onset of intense knee-extensor exercise, that essentially all quadriceps muscle fibres are activated during repeated intense exercise and that metabolic measurements in the vastus lateralis muscle provide a good indication of the whole-quadriceps muscle metabolism during repeated, intense, one-legged knee-extensor exercise.     

骨水泥性能及其对人工关节柄固定强度的影响

本文研究了骨水泥三种不同临床使用方法所形成的内部气泡结构状态,测定了骨水泥的抗压强度、收缩率、疲劳强度、变形积累量等物理机械性能,以及关节固定强度,用气泡状态解释了不同临床使用方法对骨水泥性能及固定强度所带来的各种影响。     

Changes of energy metabolism,myosin light chain composition,lactate dehydrogenase isozyme pattern and fibre type distribution of denervated fast-twitch muscle from rabbit after low frequency stimulation

The influence of low frequency (8–10 Hz) electrical stimulation on denervated fast-twitch muscle from rabbit was investigated. Prolonged direct stimulation of denervated muscle resulted in higher oxidative enzyme activities. Furthermore, single fibre analyses for succinate dehydrogenase showed a more uniform distribution of activity in stimulated-denervated muscle when compared to normal muscle. As was also the case following stimulation of innervated muscle, glycolytic enzymes were decreased in activity and the LDH-isozyme pattern also shifted towards heart type. No change of the myosin light chain pattern could be observed after 56 days of stimulation.This study was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 138, and by grant NI 204-2-2. H. R. is grateful to Dr. Dirk Pette for his guidance and encouragement during this work, which was done in Dr. Pette's laboratory at the University of Konstanz     

Neuromuscular function and mechanical efficiency of human leg extensor muscles during jumping exercises

The influence of prestretch amplitude on the mechanical efficiency was examined with 5 subjects, who performed 5 different series of vertical jumps, each of which differed with respect to the mechanics of the knee joint action during the prestretch (eccentric) phase of the contact on the floor. Electromyographic activity was recorded from the major extensor muscles during the entire work period of 1 min per series. In addition, expired air was collected during the test and recovery for determination of energy expenditure. Mechanical work was calculated from the vertical displacement of the body during the jumps. The results indicated that high net efficiency of 38.7% was observed in condition where amplitude of knee bending in eccentric phase was small. In large range motion the corresponding net efficiency was 30.1%. In jumps where no prestretching of extensor muscles ocurred the net efficiency was 19.7%. The high efficiency of small amplitude jumps was characterized by low myoelectrical activity of the leg extensor muscles during the positive (concentric) work phase. In addition, the small amplitude jumps had shorter transition time in the stretch-shortening cycle, high average eccentric force and high stretching speed. Therefore the results suggest that the restitution of elastic energy, which was also related to the length change and stiffness of the muscles during stretch, plays an important role in regulating the mechanical efficiency of work.     

Responses of group IV afferent units from skeletal muscle to stretch,contraction and chemical stimulation

Summary In an attempt to differentiate between nociceptive group IV muscle receptors and ergoceptive ones, the discharges of single group IV fibres from skeletal muscle in response to local pressure, sustained stretch, repetitive contraction and intra-arterial injections of bradykinin, 5-hydroxytryptamine (5-HT), potassium, phosphate, and lactate were studied in anaesthetized cats.Of the 75 fibres of the study, 5 units were activated by sustained stretch, the responses occurring with a delay. These stretch-sensitive units could not be activated by local pressure or muscular contraction. Thirteen group IV afferents raised their discharge frequency during repetitive contractions. Some of the units responded immediately with the onset of the contractions, whereas the others showed a pronounced delay.Forty-six units were tested with all or most of the above mechanical and chemical stimuli. In 32 afferents a response to at least one of the stimuli was present. Taking only these units into account, several groups of receptors could be distinguished by their different response combinations. One group was activated by pain-producing substances, but not by muscular activity and thus showed nociceptive properties. Another group showed a raised activity during muscular contractions but did not respond to the algesic agents bradykinin and 5-HT. Units belonging to this group might serve as ergoceptors. The borderline between the two groups was not sharp, a considerable number of group IV afferents was found which had both nociceptive and ergoceptive properties.     

Relationship between muscle fatigue and oxygen uptake during cycle ergometer exercise with different ramp slope increments

Summary The surface electromyogram (EMG) from active muscle and oxygen uptake ( ) were studied simultaneously to examine changes of motor unit (MU) activity during exercise tests with different ramp increments. Six male subjects performed four exhausting cycle exercises with different ramp slopes of 10, 20, 30 and 40 W · min^–1 on different days. The EMG signals taken from the vastus lateralis muscle were stored on a digital data recorder and converted to obtain the integrated EMG (iEMG). The was measured, with 20-s intervals, by the mixing chamber method. A non-linear increase in iEMG against work load was observed for each exercise in all subjects. The break point of the linear relationship of iEMG was determined by the crossing point of the two regression lines (iEMG_bp). Significant differences were obtained in the exercise intensities corresponding to maximal oxygen uptake ( ) and the iEMG_bp between 10 and 30, and 10 and 40 W · min –1 ramp exercises (P < 0.05). However, no significant differences were obtained in and corresponding to the iEMG_bp during the four ramp exercises. With respect to the relationship between and exercise intensity during the ramp increments, the -exercise intensity slope showed significant differences only for the upper half (i.e. above iEMG_bp). These results demonstrated that the and at which a nonlinear increase in iEMG was observed were not varied by the change of ramp slopes but by the exercise intensity corresponding to and the iEMG_bp was varied by the change of ramp slopes. In addition, the significant differences in the exercise intensity slopes for the upper half of the tests would suggest that the recruitment patterns of MU and/or muscle metabolic state might be considerably altered depending upon the ramp slope increments.     

Responses to intensive training and methandrostenelone administration: II. Hormonal,organ weights,muscle weights and body composition

The hormonal levels of the gonadotropins, the weight of selected organs and of the triceps surae as well as body composition were determined in Sprague Dawley rats at 3 and 6 weeks after intensive training with or without a methandrostenelone (Dianabol) supplement (0.35 mg/kg/day). The controls were sedentary rats of similar age and weight at the start of the experiment. The dianabol treated rats in the sedentary and exercise groups had a depression of plasma LH levels. There were no differences in body weight, specific gravity, lean body weight, fat or % fat between the two trained groups. Dianabol had no apparent effect on the measured parameters other than a depression of LH.     

The anaerobic recovery of frog muscle

Summary The muscle may undergo a partial recovery of its high energy phosphate stores in the absence of oxygen by the way of glycolysis (anaerobic recovery). This process has been studied in 41 pairs of frog gastrocnemii at different degrees of exhaustion induced by variable trains of supramaximal stimuli. Anaerobic recovery appears to be inadequate to replenish the fraction of muscle high energy phosphate stores (GP=ATP+PC) split as a consequence of the stimulation. The maximal amount of recovery (on the average about 5 Moles of GP per gram of fresh tissue) occurs when the muscle resting stores have been reduced to about 50%. This limitation in the extent of recovery is not a consequence of a reduced availability of glycogen but it is possibly related to the production of some metabolic intermediate, limiting the rate of the glycolytic sequence, likely the accumulation of lactic acid in the fiber. The time course of the anaerobic recovery process is characterized by at1/2 of about 2 min. The efficiency of the process, i.e. the number of the high energy phosphate bonds resynthesized by one Mole of lactic acid, appears to vary between 1.5 and 1.8, being of the same order of magnitude as the GP/L.A. ratio obtained from muscle extracts.