Age-dependency in bone mass and geometry: a pQCT study on male and female master sprinters, middle and long distance runners, race-walkers and sedentary people.

OBJECTIVE: To investigate whether athletic participation allows master athletes to preserve their good bone health into old age. METHODS: Bone strength indicators of the tibia and the radius were obtained of master runners and race-walkers (n=300) competing at World and European Master Championships and of 75 sedentary controls, all aged 33-94 yrs. RESULTS: In the tibia, diaphyseal cortical area (Ar.Ct), polar moment of resistance (RPol) and trabecular bone mineral density (vBMD) were generally greater in athletes than controls at all ages. In the athletes, but not the controls, Ar.Ct, RPol (females) and trabecular vBMD were negatively correlated with age (p<0.01). Radius measures were comparable between athlete and control groups at all ages. The amalgamated data revealed negative correlations of age with Ar.Ct, RPol (females), cortical vBMD and trabecular vBMD (males; p<0.005) and positive correlations with endocortical circumference (p<0.001). CONCLUSION: This cross-sectional study found age-related differences in tibial bone strength indicators of master athletes, but not sedentary controls, thus, groups becoming more similar with advancing age. Age-related differences were noticeable in the radius too, without any obvious group difference. Results are compatible with the notion that bones adapt to exercise-specific forces throughout the human lifespan.

Variation in the determinants of power of chemically skinned human muscle fibres.

We have explored the extent to which the maximal velocity of unloaded shortening (V(max)), the force generated per unit cross-sectional area (P(0)) and the curvature of the force-velocity relationship (a/P(0) in the Hill equation) contribute to differences in peak power of chemically skinned single fibres from the quadriceps muscle of healthy young male subjects. The analysis was restricted to type I and IIA fibres that contained a single type of myosin heavy chain on electrophoretic separation. Force-velocity relationships were determined from isotonic contractions of maximally activated fibres at 15 degrees C. Mean (+/- s.d.) peak powers were 1.99 +/- 0.72 watts per litre (W L(-1)) for type I fibres and 6.92 +/- 2.41 W L(-1), for type IIA fibres. The most notable feature, however, was the very large, sevenfold, range of power outputs within a single fibre type. This wide range was a consequence of variations in each of the three components determining power: P(0), V(max) and a/P(0). Within a single fibre type, P(0) varied threefold, and V(max) and a/P(0) two- to threefold. There were no obvious relationships between P(0) and V(max) or between P(0) and a/P(0). However, there was a suggestion of an inverse relationship between a/P(0) and V(max), the effect being to reduce, somewhat, the impact of differences in V(max) on peak power. In searching for the causes of variation in peak power of fibres of the same type, it appears likely that there are two factors, one that affects P(0) and another that leads to variation in both V(max) and a/P(0).

Bone mass and geometry of the tibia and the radius of master sprinters, middle and long distance runners, race-walkers and sedentary control participants: a pQCT study.

Mechanical loading is thought to be a determinant of bone mass and geometry. Both ground reaction forces and tibial strains increase with running speed. This study investigates the hypothesis that surrogates of bone strength in male and female master sprinters, middle and long distance runners and race-walkers vary according to discipline-specific mechanical loading from sedentary controls. Bone scans were obtained by peripheral Quantitative Computed Tomography (pQCT) from the tibia and from the radius in 106 sprinters, 52 middle distance runners, 93 long distance runners and 49 race-walkers who were competing at master championships, and who were aged between 35 and 94 years. Seventy-five age-matched, sedentary people served as control group. Most athletes of this study had started to practice their athletic discipline after the age of 20, but the current training regime had typically been maintained for more than a decade. As hypothesised, tibia diaphyseal bone mineral content (vBMC), cortical area and polar moment of resistance were largest in sprinters, followed in descending order by middle and long distance runners, race-walkers and controls. When compared to control people, the differences in these measures were always >13% in male and >23% in female sprinters (p<0.001). Similarly, the periosteal circumference in the tibia shaft was larger in male and female sprinters by 4% and 8%, respectively, compared to controls (p<0.001). Epiphyseal group differences were predominantly found for trabecular vBMC in both male and female sprinters, who had 15% and 18% larger values, respectively, than controls (p<0.001). In contrast, a reverse pattern was found for cortical vBMD in the tibia, and only few group differences of lower magnitude were found between athletes and control people for the radius. In conclusion, tibial bone strength indicators seemed to be related to exercise-specific peak forces, whilst cortical density was inversely related to running distance. These results may be explained in two, non-exclusive ways. Firstly, greater skeletal size may allow larger muscle forces and power to be exerted, and thus bias towards engagement in athletics. Secondly, musculoskeletal forces related to running can induce skeletal adaptation and thus enhance bone strength.

The rate of muscle temperature increase during acute whole-body vibration exercise.

This study compared the rate of muscle temperature (Tm) increase during acute whole-body vibration (WBV), to that of stationary cycling and passive warm-up. Additionally we wanted to determine if the purported increase in counter-movement jump and peak power cycling from acute WBV could be explained by changes in muscle temperature. Eight active participants volunteered for the study, which involved a rest period of 30 min to collect baseline measures of muscle, core, skin temperature, heart rate (HR), and thermal leg sensation (TLS), which was followed by three vertical jumps and 5 s maximal cycle performance test. A second rest period of 40 min was enforced followed by the intervention and performance tests. The change in Tm elicited during cycling was matched in the hot bath and WBV interventions. Therefore cycling was performed first, proceeded by, in a random order of hot bath and acute WBV. The rate of Tm was significantly greater (P < 0.001) during acute WBV (0.30 degree C min(-1)) compared to cycle (0.15 degree C min(-1)) and hot bath (0.09 degree C min(-1)) however there was no difference between the cycle and hot bath, and the metabolic rate was the same in cycling and WBV (19 mL kg(-1) min(-1)). All three interventions showed a significant (P < 0.001) increase in countermovement jump peak power and height. For the 5 s maximal cycle test (MIC) there were no significant differences in peak power between the three interventions. In conclusion, acute WBV elevates Tm more quickly than traditional forms of cycling and passive warm-up. Given that all three warm-up methods yielded the same increase in peak power output, we propose that the main effect is caused by the increase in Tm.

Recruitment of single muscle fibers during submaximal cycling exercise.

In literature, an inconsistency exists in the submaximal exercise intensity at which type II fibers are activated. In the present study, the recruitment of type I and II fibers was investigated from the very beginning and throughout a 45-min cycle exercise at 75% of the maximal oxygen uptake, which corresponded to 38% of the maximal dynamic muscle force. Biopsies of the vastus lateralis muscle were taken from six subjects at rest and during the exercise, two at each time point. From the first biopsy single fibers were isolated and characterized as type I and II, and phosphocreatine-to-creatine (PCr/Cr) ratios and periodic acid-Schiff (PAS) stain intensities were measured. Cross sections were cut from the second biopsy, individual fibers were characterized as type I and II, and PAS stain intensities were measured. A decline in PCr/Cr ratio and in PAS stain intensity was used as indication of fiber recruitment. Within 1 min of exercise both type I and, although to a lesser extent, type II fibers were recruited. Furthermore, the PCr/Cr ratio revealed that the same proportion of fibers was recruited during the whole 45 min of exercise, indicating a rather constant recruitment. The PAS staining, however, proved inadequate to fully demonstrate fiber recruitment even after 45 min of exercise. We conclude that during cycling exercise a greater proportion of type II fibers is recruited than previously reported for isometric contractions, probably because of the dynamic character of the exercise. Furthermore, the PCr/Cr ratio method is more sensitive in determining fiber activation than the PAS stain intensity method.

Symmetry-breaking study with deformed ensembles.

A random matrix model to describe the coupling of m -fold symmetry is constructed. The particular threefold case is used to analyze data on eigenfrequencies of elastomechanical vibration of an anisotropic quartz block. It is suggested that such an experimental and theoretical study may supply a powerful means to discern the intrinsic symmetry of physical systems.

Human muscle fatigue: the significance of muscle fibre type variability studied using a micro-dissection approach.

During human locomotion the ability to generate and sustain mechanical power output is dependent on the organised variability in contractile and metabolic properties of the muscle fibres that comprise the active muscles. In studies of human exercise we have used a micro-dissection technique to obtain fragments of single muscle fibres from needle biopsies before and after exercise. Each fibre fragment is divided into two parts. One part is used to characterize the fibre type in respect of the heavy chain myosin isoform expressed. The other part of the fragment is analysed for high energy phosphate concentrations. Fibres are classified on the basis of expressing either type I, type IIA, or type IIX myosin heavy chain isoforms. It should be noted however that in the type II population many fibres co-express both IIA and the IIX isoforms and we therefore characterize these fibres on the basis of the degree of co-expression. We have used this technique to examine the time course of high energy phosphate concentration and fatigue in different fibre populations during exercise. The progressive reduction of power during maximal sprint efforts may be interpreted as the cumulative effect of metabolic depletion in successive fibre type populations from IIX to IIXa to IIAx to IIA to I. One important application of the micro-dissection technique is that PCr content may also be used as a very sensitive metabolic marker for fibre type recruitment during very short duration concentric, isometric and eccentric exercise.

Level density for deformations of the Gaussian orthogonal ensemble.

Formulas are derived for the average level density of deformed, or transition, Gaussian orthogonal random matrix ensembles. After some general considerations about Gaussian ensembles, we derive formulas for the average level density for (i) the transition from the Gaussian orthogonal ensemble (GOE) to the Poisson ensemble and (ii) the transition from the GOE to m GOEs.

Metabolic cost of lengthening, isometric and shortening contractions in maximally stimulated rat skeletal muscle.

AIM: The present study investigated the energy cost of lengthening, isometric and shortening contractions in rat muscle (n = 19). METHODS: With electrical stimulation the rat medial gastrocnemius muscle was maximally stimulated to perform 10 lengthening, isometric and shortening contractions (velocity 25 mm s(-1)) under experimental conditions (e.g. temperature, movement velocity) that resemble conditions in human movement. RESULTS: Mean +/- SD force-time-integral of the first contraction was significantly different between the three protocols, 2.4 +/- 0.2, 1.7 +/- 0.2 and 1.0 +/- 0.2 N s, respectively (P < 0.05). High-energy phosphate consumption was not significantly different between the three modes of exercise but a trend could be observed from lengthening (7.7 +/- 2.7 micromol approximately P muscle(-1)) to isometric (8.9 +/- 2.2 micromol approximately P muscle(-1)) to shortening contractions (10.4 +/- 1.6 micromol approximately P muscle(-1)). The ratio of high-energy phosphate consumption to force-time-integral was significantly lower for lengthening [0.3 +/- 0.1 micromol approximately P (N s)(-1)] and isometric [0.6 +/- 0.2 micromol approximately P (N s)(-1)] contractions compared with shortening [1.2 +/- 0.2 micromol approximately P (N s)(-1)] contractions (P < 0.05). CONCLUSION: The present results of maximally stimulated muscles are comparable with data in the literature for voluntary human exercise showing that the energy cost of force production during lengthening exercise is approximately 30% of that in shortening exercise. The present study suggests that this finding in humans probably does reflect intrinsic muscle properties rather than effects of differential recruitment and/or coactivation.

Metabolically assessed muscle fibre recruitment in brief isometric contractions at different intensities.

This study investigated the recruitment of type I, IIA and IIAX fibres after seven isometric contractions at 40, 70 and 100% maximal voluntary knee extension torque (MVC, 1 s on/1 s off). Biopsies of the vastus lateralis muscle were collected from seven subjects at rest and immediately post-exercise. Fibre fragments were dissected from the freeze-dried samples and characterized as type I, IIA and IIAX using mATPase staining. Phosphocreatine (PCr) and creatine (Cr) content were measured in the remaining part of characterized fibres. A decline in the ratio of PCr to Cr (PCr/Cr) was used as an indication of activation. The mean peak torques were, respectively, 39 (2), 72 (2) and 87 (6)% MVC. Cumulative distributions of type I and IIA fibres were significantly shifted to lower PCr/Cr ratios at all intensities (Kolmogorov-Smirnov test, P<0.05). The cumulative distribution of type IIAX fibres showed a significant leftward shift only at 87% MVC ( P<0.05). A hierarchical order of fibre activation with increasing intensity of exercise was found, with some indication of rate coding for type I and IIA fibres. Evidence for activation of type IIAX fibres was only found at 87% MVC.

Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions.

Voluntary activation levels during lengthening, isometric, and shortening contractions (angular velocity 60 degrees/s) were investigated by using electrical stimulation of the femoral nerve (triplet, 300 Hz) superimposed on maximal efforts. Recruitment of fiber populations was investigated by using the phosphocreatine-to-creatine ratio (PCr/Cr) of single characterized muscle fibers obtained from needle biopsies at rest and immediately after a series of 10 lengthening, isometric, and shortening contractions (1 s on/1 s off). Maximal voluntary torque was significantly higher during lengthening (270 +/- 55 N.m) compared with shortening contractions (199 +/- 47 N.m, P < 0.05) but was not different from isometric contractions (252 +/- 47 N.m). Isometric torque was higher than torque during shortening (P < 0.05). Voluntary activation level during maximal attempted lengthening contractions (79 +/- 8%) was significantly lower compared with isometric (93 +/- 5%) and shortening contractions (92 +/- 3%, P < 0.05). Mean PCr/Cr values of all fibers from all subjects at rest were 2.5 +/- 0.6, 2.0 +/- 0.7, and 2.0 +/- 0.7, respectively, for type I, IIa, and IIax fibers. After 10 contractions, the mean PCr/Cr values for grouped fiber populations (regardless of fiber type) were all significantly different from rest (1.3 +/- 0.2, 0.7 +/- 0.3, and 0.8 +/- 0.6 for lengthening, isometric, and shortening contractions, respectively; P < 0.05). The cumulative distributions of individual fiber populations after either contraction mode were significantly different from rest (P < 0.05). Curves after lengthening contractions were less shifted compared with curves from isometric and shortening contractions (P < 0.05), with a smaller shift for the type IIax compared with type I fibers in the lengthening contractions. The results indicate a reduced voluntary drive during lengthening contractions. PCr/Cr values of single fibers indicated a hierarchical order of recruitment of all fiber populations during maximal attempted lengthening contractions.

Changes in PCr/Cr ratio in single characterized muscle fibre fragments after only a few maximal voluntary contractions in humans.

AIM: This methodological study investigated the number of brief maximal voluntary isometric contractions (MVC) needed to show evidence of fibre activation, as indicated by changes in the phosphocreatine to creatine (PCr/Cr) ratio. METHODS: Subjects performed series of four, seven and/or 10 MVC (1 s on, 1 s off) of the m. quadriceps (60 degrees -flexion angle). Biopsy samples of the m. vastus lateralis were taken at rest and immediately post-exercise. Single muscle fibres were dissected from the freeze-dried samples and classified as types I, IIA or IIAX, using mATPase stainings. Fragments of characterized fibres were analysed for PCr and Cr content. Analyses of variance were performed to investigate changes in PCr/Cr per fibre group over time, followed by Bonferroni post-hoc test (P < 0.01). The fifth percentile of resting values of each fibre group was determined. RESULTS: Mean PCr/Cr ratio after four, seven and 10 MVCs were significantly lower for all fibre groups (P < 0.01). The mean decreases were 44, 64 and 76%, respectively. However, only after seven and 10 contractions PCr/Cr ratios of all, but three type I and two type IIAX fibres, individual fibres were below the fifth percentile. CONCLUSION: In very short duration exercise, involving seven brief maximal voluntary contractions, changes in the PCr/Cr ratio indicated activation of different characterized muscle fibre fragments. The results suggest that this approach may be useful for investigating the pattern of fibre type activation in exercise of very short duration.

Effect of antagonist muscle fatigue on knee extension torque.

The effect of hamstring fatigue on knee extension torque was examined at different knee angles for seven male subjects. Before and after a dynamic flexion fatigue protocol (180 degrees s(-1), until dynamic torque had declined by 50%), maximal voluntary contraction extension torque was measured at four knee flexion angles (90 degrees, 70 degrees, 50 degrees and 30 degrees ). Maximal torque generating capacity and voluntary activation of the quadriceps muscle were determined using electrical stimulation. Average rectified EMG of the biceps femoris was determined. Mean dynamic flexion torque declined by 48+/-11%. Extensor maximal voluntary contraction torque, maximal torque generating capacity, voluntary activation and average rectified EMG at the four knee angles were unaffected by the hamstring fatigue protocol. Only at 50 degrees knee angle was voluntary activation significantly lower (15.7%) after fatigue ( P<0.05). In addition, average rectified EMG before fatigue was not significantly influenced by knee angle. It was concluded that a fatigued hamstring muscle did not increase the maximal voluntary contraction extension torque and knee angle did not change coactivation. Three possible mechanisms may explain the results: a potential difference in recruited fibre populations in antagonist activity compared with the fibres which were fatigued in the protocol, a smaller loss in isometric torque generating capacity of the hamstring muscle than was expected from the dynamic measurements and/or a reduction in voluntary activation.

Variability in fibre properties in paralysed human quadriceps muscles and effects of training.

A spinal cord injury usually leads to an increase in contractile speed and fatigability of the paralysed quadriceps muscles, which is probably due to an increased expression of fast myosin heavy chain (MHC) isoforms and reduced oxidative capacity. Sometimes, however, fatigue resistance is maintained in these muscles and also contractile speed is slower than expected. To obtain a better understanding of the diversity of these quadriceps muscles and to determine the effects of training on characteristics of paralysed muscles, fibre characteristics and whole muscle function were assessed in six subjects with spinal cord lesions before and after a 12-week period of daily low-frequency electrical stimulation. Relatively high levels of MHC type I were found in three subjects and this corresponded with a high degree of fusion in 10-Hz force responses (r=0.88). Fatigability was related to the activity of succinate dehydrogenase (SDH) (r=0.79). Furthermore, some differentiation between fibre types in terms of metabolic properties were present, with type I fibres expressing the highest levels of SDH and lowest levels of alpha-glycerophosphate dehydrogenase. After training, SDH activity increased by 76+/-26% but fibre diameter and MHC expression remained unchanged. The results indicate that expression of contractile proteins and metabolic properties seem to underlie the relatively normal functional muscle characteristics observed in some paralysed muscles. Furthermore, training-induced changes in fatigue resistance seem to arise, in part, from an improved oxidative capacity.

In situ rat fast skeletal muscle is more efficient at submaximal than at maximal activation levels.

The influence of stimulation frequency on efficiency (= total work output/high-energy phosphate consumption) was studied using in situ medial gastrocnemius muscle tendon complexes of the rat. The muscles performed 20 repeated concentric contractions (2/s) at 34 degrees C. During these repeated contractions, the muscle was stimulated via the severed sciatic nerve with either 60, 90, or 150 Hz. The muscle was freeze-clamped immediately after these contractions, and high-energy phosphate consumption was determined by measuring intramuscular chemical change relative to control muscles. The average values (+/-SD) of efficiency calculated for 60, 90, and 150 Hz were 18.5 +/- 1.5 (n = 7), 18.6 +/- 1.5 (n = 9), and 14.7 +/- 1.3 mJ/micromol phosphate (n = 9). The results indicate that the efficiency of the muscles that were submaximally activated (60 or 90 Hz) was higher (+26%, P < 0.05) than that of those maximally activated (150 Hz). Additional experiments showed that the low efficiency at maximal activation levels is unlikely to be the result of a higher energy turnover by the Ca2+ -ATPase relative to the total energy turnover. Therefore, alternative explanations are discussed.

Effects of training on contractile properties of paralyzed quadriceps muscle.

Effects of two different training regimens on the contractile properties of the quadriceps muscle were studied in six individuals with spinal cord injury. Each subject had both limbs trained with the two regimens, consisting of stimulation with low frequencies (LF) at 10 HZ or high frequencies (HF) at 50 HZ; one limb of each subject was stimulated with the LF protocol and the other with the HF regimen. Twelve weeks of daily training increased tetanic tension by approximately 20%, which was not significantly different between training regimens. Interestingly, after HF but not LF training, the unusual high forces at the low frequency range of the force-frequency relationship decreased, possibly due to a reduced activation per impulse. After LF but not HF training, force oscillation amplitudes declined (by 33%) as relaxation tended to slow, which may have opposed possible effects of reduced activation as seen after HF training. Finally, fatigue resistance also increased rapidly after LF training (by 43%) but not after HF training. These results indicate that different types of training may selectively change different aspects of function in disused muscles.

Submaximal exercise capacity and maximal power output in polio subjects.

OBJECTIVES: To compare the submaximal exercise capacity of polio subjects with postpoliomyelitis syndrome (PPS) and without (non-PPS) with that of healthy control subjects, to investigate the relationship of this capacity with maximal short-term power and quadriceps strength, and to evaluate movement economy. DESIGN: Cross-sectional survey. SETTING: University hospital. PARTICIPANTS: Forty-three polio subjects (25 PPS, 18 non-PPS) and 12 control subjects. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Power output, oxygen uptake, and heart rate were measured in an incremental submaximal cycle ergometry test. Maximal short-term power was measured in 5-second all-out efforts. Knee extensor strength was measured on a chair dynamometer. RESULTS: The mean submaximal power +/- standard deviation at 80% of heart rate reserve of 83.8 +/- 29.9 watts in the polio subjects was significantly less than the mean submaximal power of 142.1 +/- 30.4 watts in the control group. However, expressed as a percentage of the maximal short-term power, submaximal power did not differ between the groups. Strength and maximal short-term power correlated significantly (p < .005) with submaximal power (r = .64 and .76, respectively). The oxygen uptake was higher than theoretically expected for the given submaximal power output in polio subjects, and appeared to increase with increasing asymmetry in strength and power between legs. No differences were found between PPS and non-PPS subjects. CONCLUSION: The submaximal work capacity of polio subjects was severely reduced, mainly in association with the reduced muscle capacity. And, because of a reduced movement economy, their energy cost was elevated. Although muscle loads in activities such as walking and climbing stairs differ from cycling, they also may require elevated relative levels of effort, predisposing subjects to premature fatigue in sustained activity.

Muscle oxygen uptake and energy turnover during dynamic exercise at different contraction frequencies in humans.

1. It has been established that pulmonary oxygen uptake is greater during cycle exercise in humans at high compared to low contraction frequencies. However, it is unclear whether this is due to more work being performed at the high frequencies and whether the energy turnover of the working muscles is higher. The present study tested the hypothesis that human skeletal muscle oxygen uptake and energy turnover are elevated during exercise at high compared to low contraction frequency when the total power output is the same. 2. Seven subjects performed single-leg dynamic knee-extensor exercise for 10 min at contraction frequencies of 60 and 100 r.p.m. where the total power output (comprising the sum of external and internal power output) was matched between frequencies (54 +/- 5 vs. 56 +/- 5 W; mean +/- S.E.M.). Muscle oxygen uptake was determined from measurements of thigh blood flow and femoral arterial - venous differences for oxygen content (a-v O(2) diff). Anaerobic energy turnover was estimated from measurements of lactate release and muscle lactate accumulation as well as muscle ATP and phosphocreatine (PCr) utilisation based on analysis of muscle biopsies obtained before and after each exercise bout. 3. Whilst a-v O(2) diff was the same between contraction frequencies during exercise, thigh blood flow was higher (P < 0.05) at 100 compared to 60 r.p.m. Thus, muscle V(O2) was higher (P < 0.05) during exercise at 100 r.p.m. Muscle V(O2) increased (P < 0.05) by 0.06 +/- 0.03 (12 %) and 0.09 +/- 0.03 l min(-1) (14 %) from the third minute to the end of exercise at 60 and 100 r.p.m., respectively, but there was no difference between the two frequencies. 4. Muscle PCr decreased by 8.1 +/- 1.7 and 9.1 +/- 2.0 mmol (kg wet wt)(-1), and muscle lactate increased to 6.8 +/- 2.1 and 9.8 +/- 2.5 mmol (kg wet wt)(-1) during exercise at 60 and 100 r.p.m., respectively. The total release of lactate during exercise was 48.7 +/- 8.8 and 64.3 +/- 10.6 mmol at 60 and 100 r.p.m. (not significant, NS). The total anaerobic ATP production was 47 +/- 8 and 61 +/- 12 mmol kg(-1), respectively (NS). 5. Muscle temperature increased (P < 0.05) from 35.8 +/- 0.3 to 38.2 +/- 0.2 degrees C at 60 r.p.m. and from 35.9 +/- 0.3 to 38.4 +/- 0.3 degrees C at 100 r.p.m. Between 1 and 7 min muscle temperature was higher (P < 0.05) at 100 compared to 60 r.p.m. 6. The estimated mean rate of energy turnover during exercise was higher (P < 0.05) at 100 compared to 60 r.p.m. (238 +/- 16 vs. 194 +/- 11 J s(-1)). Thus, mechanical efficiency was lower (P < 0.05) at 100 r.p.m. (24 +/- 2 %) compared to 60 r.p.m. (28 +/- 3 %). Correspondingly, efficiency expressed as work per mol ATP was lower (P < 0.05) at 100 than at 60 r.p.m. (22.5 +/- 2.1 vs. 26.5 +/- 2.5 J (mmol ATP)(-1)). 7. The present study showed that muscle oxygen uptake and energy turnover are elevated during dynamic contractions at a frequency of 100 compared with 60 r.p.m. It was also observed that muscle oxygen uptake increased as exercise progressed in a manner that was not solely related to the increase in muscle temperature and lactate accumulation.

Phosphocreatine and ATP content in human single muscle fibres before and after maximum dynamic exercise.

The recovery of high-energy phosphate levels in single human skeletal muscle fibres following short-term maximal (all-out) exercise was investigated. Three male volunteers exercised maximally for 25 s on an isokinetic cycling ergometer. Muscle biopsy samples from the vastus lateralis were collected at rest, immediately post-exercise and at 1.5 min of recovery. The subjects also performed a second exercise bout 1.5 min after the first, on a separate occasion. Single muscle fibres were dissected, characterized and assigned to one of four groups according to their myosin heavy chain (MyHC) isoform content; namely, type I, IIA, IIAx and IIXa (the latter two groups containing either less or more than 50% IIX MyHC). Fibres were analysed for adenosine 5'-triphosphate (ATP), inosine-5'-monophosphate (IMP), phosphocreatine (PCr) and creatine (Cr) levels. Type I fibres had a lower Cr content than type II fibres (P<0.01). Within type II fibres resting [PCr] increased with increasing MyHC IIX isoform content (r=0.59, P<0.01). Post-exercise [PCr] was very low in all fibre groups (P<0.01 versus rest) while great reductions in ATP were also observed (P<0.01 versus rest), especially in the type II fibre groups. [PCr] at 1.5 min of recovery was still lower compared to rest for all fibre groups (P<0.01) especially in the IIAx and IIXa fibres.

Metabolism changes in single human fibres during brief maximal exercise.

Changes in high-energy phosphate levels in single human skeletal muscle fibres after 10 s of maximal (all-out) dynamic exercise were investigated. Muscle biopsies from vastus lateralis of two volunteers were collected at rest and immediately post exercise. Single muscle fibres were dissected from dry muscle and were assigned into one of four groups according to their myosin heavy chain (MyHC) isoform content: that is type I, IIA, IIAx and IIXa (the latter two groups containing either less or more than 50% IIX MyHC). Fragments of characterised fibres were analysed by HPLC for ATP, inosine-monophosphate (IMP), phosphocreatine (PCr) and creatine levels. After 10 s of exercise, PCr content ([PCr]) declined by approximately 46, 53, 62 and 59 % in type I, IIA, IIAx and IIXa fibres, respectively (P < 0.01 from rest). [ATP] declined only in type II fibres, especially in IIAx and IIXa fibres in which [IMP] reached mean values of 16 +/- 1 and 18 +/- 4 mmol (kg dry mass)(-1), respectively. While [PCr] was reduced in all fibre types during the brief maximal dynamic exercise, it was apparent that type II fibres expressing the IIX myosin heavy chain isoform were under a greatest metabolic stress as indicated by the reductions in [ATP].