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.

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.

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.

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.

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°/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.

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.

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.

Peripheral vascular changes after electrically stimulated cycle training in people with spinal cord injury

OBJECTIVE

To test whether a short period of training leads to adaptations in the cross-sectional area of large conduit arteries and improved blood flow to the paralyzed legs of individuals with spinal cord injury (SCI).

DESIGN

Before-after trial.

SETTING

Rehabilitation center, academic medical center.

PARTICIPANTS

Nine men with spinal cord lesions.

INTERVENTION

Six weeks of cycling using a functional electrically stimulated leg cycle ergometer (FES-LCE).

MAIN OUTCOME MEASURES

Longitudinal images and simultaneous velocity spectra were measured in the common carotid (CA) and femoral (FA) arteries using quantitative duplex Doppler ultrasound examination. Arterial diameters, peak systolic inflow volumes (PSIVs), mean inflow volumes (MIVs), and a velocity index (VI), representing the peripheral resistance, were obtained at rest. PSIVs and VI were obtained during 3 minutes of hyperemia following 20 minutes of FA occlusion.

RESULTS

Training resulted in significant increases in diameter (p < .01), PSIVs (p < .01), and MIVs (p < .05), and reduced VI (p < .01) of the FA, whereas values in the CA remained unchanged. Postocclusive hyperemic responses were augmented, indicated by significantly higher PSIVs (p <.01) and a trend toward lower VI.

CONCLUSION

Six weeks of FES-LCE training increased the cross-sectional area of large conduit arteries and improved blood flow to the paralyzed legs of individuals with SCI.

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].

In vivo specific tension of human skeletal muscle

In this study, we estimated the specific tensions of soleus (Sol) and tibialis anterior (TA) muscles in six men. Joint moments were measured during maximum voluntary contraction (MVC) and during electrical stimulation. Moment arm lengths and muscle volumes were measured using magnetic resonance imaging, and pennation angles and fascicular lengths were measured using ultrasonography. Tendon and muscle forces were modeled. Two approaches were followed to estimate specific tension. First, muscle moments during electrical stimulation and moment arm lengths, fascicular lengths, and pennation angles during MVC were used (data set A). Then, MVC moments, moment arm lengths at rest, and cadaveric fascicular lengths and pennation angles were used (data set B). The use of data set B yielded the unrealistic specific tension estimates of 104 kN/m(2) in Sol and 658 kN/m(2) in TA. The use of data set A, however, yielded values of 150 and 155 kN/m(2) in Sol and TA, respectively, which agree with in vitro results from fiber type I-predominant muscles. In fact, both Sol and TA are such muscles. Our study demonstrates the feasibility of accurate in vivo estimates of human muscle intrinsic strength.

Reproducibility of contractile properties of the human paralysed and non-paralysed quadriceps muscle

This study assessed the reproducibility of electrically evoked, isometric quadriceps contractile properties in eight people with spinal cord injury (SCI) and eight able-bodied (AB) individuals. Over all, the pooled coefficients of variation (CVps) in the SCI group were significantly lower (ranging from 0.03 to 0.15) than in the AB group (ranging from 0.08 to 0.21) (P<0.05). Furthermore, in all subjects, the variability of force production increased as stimulation frequency decreased (P<0.01). In subjects with SCI, variables of contractile speed are clearly less reproducible than tetanic tension or resistance to fatigue. Contractile properties of quadriceps muscles of SCI subjects were significantly different from that of AB subjects. Muscles of people with SCI were less fatigue resistant (P<0.05) and produced force-frequency relationships that were shifted to the left, compared with AB controls (P<.01). In addition, fusion of force responses resulting from 10 Hz stimulation was reduced (P<.05) and speed of contraction (but not relaxation) was increased (P<0.05), indicating an increased contractile speed in paralysed muscles compared with non-paralysed muscles. These results correspond with an expected predominance of fast glycolytic muscle fibres in paralysed muscles. It is concluded that quadriceps dynamometry is a useful technique to study muscle function in non-paralysed as well as in paralysed muscles. Furthermore, these techniques can be reliably used, for example, to assess therapeutic interventions on paralysed muscles provided that expected differences in relative tetanic tension and fatigue resistance are larger than approximately 5% and differences in contractile speed are larger than approximately 15%.

In vivo measurement-based estimations of the human Achilles tendon moment arm

The aim of the present study was to estimate and compare in vivo measurement-based Achilles tendon moment arm lengths at rest and during isometric plantarflexion maximum voluntary contraction (MVC) using the centre-of-rotation (COR) and the tendon-excursion (TE) methods. Both methods were based on morphometric analysis of sagittal-plane magnetic resonance images of the foot. Using the COR method, moment arms were obtained at ankle angles from 15 degrees of dorsiflexion to 30 degrees of plantarflexion in steps of 15 degrees, digitizing the perpendicular distance from a moving centre of rotation in the tibio-talar joint to the Achilles tendon action line. The TE method was based on measurement of calcaneal displacement along the tibial axis during 15 degrees rotations of the ankle joint, from 30 degrees of dorsiflexion to 45 degrees of plantarflexion. The two methods gave similar estimations at rest varying from 4.3 to 5.6 cm. Using the COR method, the Achilles tendon moment arm during MVC was larger by 1-1.5 cm (22-27%, P < 0.01) than the respective resting value. In contrast, no difference (P > 0.05) was found between the resting and MVC moment arm estimations of the TE method. The disagreement in moment arms during MVC may be attributed to differences in the assumptions made between the two methods. The TE method has more limitations than the COR method and its estimations during MVC should be treated with caution. Resting Achilles tendon moment arm estimations of the COR method should be multiplied by 1.22-1.27 when maximal isometric plantarflexion joint moments, musculotendon forces and stresses are predicted using modelling.

Total power output generated during dynamic knee extensor exercise at different contraction frequencies

A novel approach has been developed for the quantification of total mechanical power output produced by an isolated, well-defined muscle group during dynamic exercise in humans at different contraction frequencies. The calculation of total power output comprises the external power delivered to the ergometer (i.e., the external power output setting of the ergometer) and the "internal" power generated to overcome inertial and gravitational forces related to movement of the lower limb. Total power output was determined at contraction frequencies of 60 and 100 rpm. At 60 rpm, the internal power was 18+/- 1 W (range: 16-19 W) at external power outputs that ranged between 0 and 50 W. This was less (P<0.05) than the internal power of 33+/-2 W (27-38 W) at 100 rpm at 0-50 W. Moreover, at 100 rpm, internal power was lower (P<0.05) at the higher external power outputs. Pulmonary oxygen uptake was observed to be greater (P<0.05) at 100 than at 60 rpm at comparable total power outputs, suggesting that mechanical efficiency is lower at 100 rpm. Thus a method was developed that allowed accurate determination of the total power output during exercise generated by an isolated muscle group at different contraction frequencies.

Altered contractile properties of the quadriceps muscle in people with spinal cord injury following functional electrical stimulated cycle training

STUDY DESIGN

A longitudinal training study.

OBJECTIVES

To assess if contractile speed and fatigability of paralysed quadriceps muscles in individuals with spinal cord injury (SCI) can be altered by functional electrical stimulation leg cycle ergometry (FES-LCE) training.

SETTINGS

The Sint Maartenskliniek rehabilitation centre and the University of Nijmegen, Nijmegen, the Netherlands.

METHODS

Contractile properties of the quadriceps muscle were studied in seven people with motor-complete SCI who participated in a FES-LCE training program. Subjects trained for 30 min, three times per week for 6 weeks. Contractile speed and fatigue characteristics of electrically stimulated isometric contractions were compared before and after 6 weeks of FES-LCE.

RESULTS

Fatigue resistance improved following FES-LCE training as indicated by the higher forces maintained in response to repetitive electrical stimulation. In contrast with an improved fatigue resistance, the maximal rate of force rise was unaffected, the speed of relaxation increased and the fusion of a 10 Hz force signal decreased. Furthermore, the force-frequency relationship shifted to the right at low stimulation frequencies, indicated by a decline in the ratio of 1 and 100 Hz force responses as well as the ratio of 10 and 100 Hz force responses.

CONCLUSION

FES-LCE training can change the physiological properties of the quadriceps muscle in people with SCI. Even after a short period of training, the stimulated muscles become more resistant to fatigue. Furthermore, the increased speed of relaxation and associated decreased fusion and altered force-frequency relationship following training may be related to adaptations in the calcium handling processes, which reflect an early response of long-term disused muscles.

Influence of muscle temperature on the contractile properties of the quadriceps muscle in humans with spinal cord injury

Low muscle temperature in paralysed muscles of individuals with spinal cord injury may affect the contractile properties of these muscles. The present study was therefore undertaken to assess the effects of increased muscle temperature on the isometric contractile properties of electrically stimulated paralysed quadriceps muscles. When muscle temperature at a depth of 3 cm was increased from approximately 32 degrees C to approximately 36 degrees C by ultra-short-wave application, the half-relaxation time shortened and low-frequency force responses became less fused, but the maximal rate of increase in force remained unchanged. Heating had no effect upon either force decline or slowing of relaxation during fatiguing contractions. The force-frequency relationship of the paralysed quadriceps muscle was shifted to the right after the muscle was heated. Despite this shift, however, the relationship still resembled that in muscles of non-paralysed individuals, probably due to the unexplained high twitch forces. These results indicate that reduced muscle temperature in spinal-cord-injured individuals may lead to an underestimation of the changes in contractile properties in terms of relaxation rate or the degree of fusion with low-frequency stimulation. In addition, the force-frequency relationship of paralysed muscles does not accurately reflect the magnitude of these changes, even when the muscle is heated, and should therefore be treated with caution.

Human muscle power generating capability during cycling at different pedalling rates

The effect of different pedalling rates (40, 60, 80, 100 and 120 rev min-1) on power generating capability, oxygen uptake (O2) and blood lactate concentration [La]b during incremental tests was studied in seven subjects. No significant differences in O2,max were found (mean +/- S.D., 5.31 +/- 0.13 l min-1). The final external power output delivered to the ergometer during incremental tests (PI,max) was not significantly different when cycling at 60, 80 or 100 rev min-1 (366 +/- 5 W). A significant decrease in PI,max of 60 W was observed at 40 and 120 rev min-1 compared with 60 and 100 rev min-1, respectively (P < 0.01). At 120 rev min-1 there was also a pronounced upward shift of the O2-power output (O2-P) relationship. At 50 W O2 between 80 and 100 rev min-1 amounted to +0.43 l min-1 but to +0.87 l min-1 between 100 and 120 rev min-1. The power output corresponding to 2 and 4 mmol l-1 blood lactate concentration (P[La]2 and P[La]4 ) was also significantly lower (> 50 W) at 120 rev min-1 (P < 0.01) while pedalling at 40, 60, 80 and 100 rev min-1 showed no significant difference. The maximal peak power output (PM, max) during 10 s sprints increased with pedalling rate up to 100 rev min-1. Our study indicates that with increasing pedalling rate the reserves in power generating capability increase, as illustrated by the PI,max/PM,max ratio (54.8, 44.8, 38.1, 34.6, 29.2%), the P[La]4/PM,max ratio (50.4, 38.9, 31.0, 27.7, 22.9%) and the P[La]2/PM,max ratio (42.8, 33.5, 25.6, 23.1, 15.6%) increases. Taking into consideration the O2,max, the PI,max and the reserve in power generating capability we concluded that choosing a high pedalling rate when performing high intensity cycling exercise may be beneficial since it provides greater reserve in power generating capability and this may be advantageous to the muscle in terms of resisting fatigue. However, beyond 100 rev min-1 there is a decrease in external power that can be delivered for an given O2 with an associated earlier onset of metabolic acidosis and clearly this will be disadvantageous for sustained high intensity exercise.

Effects of high-frequency initial pulses and posttetanic potentiation on power output of skeletal muscle

The effects of high-frequency initial pulses (HFIP) and posttetanic potentiation on mechanical power output during concentric contractions were examined in the in situ medial gastrocnemius of the rat with an intact origin on the femur and blood supply. Stimulation of the muscle was performed via the severed sciatic nerve. In the experiments, HFIP or the potentiating tetanus was followed by a stimulation of 80, 120, or 200 Hz. The results showed that both HFIP and the tetanus increased power output at high contraction velocities (>75 mm/s) when followed by a train of 80 or 120 Hz (200 Hz resulted in no effects). Mechanical power output was increased maximally by HFIP to 120 and 168% by the tetanus. Furthermore, when HFIP or the tetanus were followed by a train of 80 Hz, the peak power in the power-velocity curve tended to be shifted to a higher velocity.

Changes in the tibialis anterior tendon moment arm from rest to maximum isometric dorsiflexion: in vivo observations in man

OBJECTIVE

In the present study, we examined the hypothesis that the tibialis anterior tendon moment arm increases during maximum isometric dorsiflexion as compared with rest.

BACKGROUND

In musculoskeletal modelling applications, moment arms from passive muscles at rest are assumed representative of those measured during isometric muscle contraction. The validity of this assumption is questionable in musculotendon actuators enclosed by retinacular systems as in tibialis anterior.

DESIGN AND METHODS

Sagittal-plane magnetic resonance images of the right ankle were taken in six subjects at rest and during maximum isometric dorsiflexion at six ankle angles between dorsiflexion and plantarflexion having the body placed in the supine position and the knee flexed at 90 degrees. Instant centres of rotation in the tibio-talar joint, tibialis anterior tendon action lines and moment arms were identified in the sagittal plane at ankle angles of -15 degrees, 0 degrees,+15 degrees and +30 degrees at rest and during maximum isometric dorsiflexion.

RESULTS

At any given ankle angle, the tibialis anterior tendon moment arm during maximum isometric dorsiflexion increased by 0.9-1.5 cm (P<0.01) compared with rest. This was attributed to a displacement of both tibialis anterior tendon action line by 0.8-1.2 cm (P<0.01) and all instant centres of rotation by 0.3-0.4 cm (P<0. 01) distally in relation to their corresponding resting positions.

CONCLUSIONS AND IMPLICATIONS

The assumption that the tibialis anterior tendon moment arm does not change from rest to maximum isometric dorsiflexion is invalid. Erroneous tendon forces, muscle stresses and joint moments by as much as 30% would be calculated using resting tibialis anterior tendon moment arms in the moment equilibrium equation around the ankle joint during maximum isometric dorsiflexion.

RELEVANCE

A substantial increase in the tibialis anterior tendon moment arm occurs from rest to maximum isometric dorsiflexion. This needs to be taken into consideration when using planimetric musculoskeletal modelling for analysing maximal static ankle dorsiflexion loads.

Temperature effect on the rates of isometric force development and relaxation in the fresh and fatigued human adductor pollicis muscle

The purpose of the present study was to investigate the effect of temperature on the rates of isometric force development and relaxation in electrically activated fresh and fatigued human adductor pollicis muscle. Following immersion of the lower arm for 20 min in water baths of four different temperatures, muscle temperatures were approximately 37, 31, 25 and 22 C. Maximal isometric force was reduced by 16.8 +/- 1.5 % at 22 C. The stimulation frequency-force and -rate of force development relationships were shifted to the left at lower temperatures. Q10 values for the maximal rates of force development and relaxation, and the times for 100 to 50 % and 50 to 25 % force relaxation, were about 2.0 between 37 and 25 C and about 3.8 between 25 and 22 C. However, the time for 50 to 25 % force relaxation had a relatively high Q10 value between 25 and 22 C (6.9) and this parameter also appeared to be more sensitive to fatigue compared to the other indices of relaxation. Nevertheless, the effect of fatigue on all parameters decreased with cooling over the entire (37-22 C) temperature range.

The measurement of force/velocity relationships of fresh and fatigued human adductor pollicis muscle

The purpose of the study was to obtain force/velocity relationships for electrically stimulated (80 Hz) human adductor pollicis muscle (n = 6) and to quantify the effects of fatigue. There are two major problems of studying human muscle in situ; the first is the contribution of the series elastic component, and the second is a loss of force consequent upon the extent of loaded shortening. These problems were tackled in two ways. Records obtained from isokinetic releases from maximal isometric tetani showed a late linear phase of force decline, and this was extrapolated back to the time of release to obtain measures of instantaneous force. This method gave usable data up to velocities of shortening equivalent to approximately one-third of maximal velocity. An alternative procedure (short activation, SA) allowed the muscle to begin shortening when isometric force reached a value that could be sustained during shortening (essentially an isotonic protocol). At low velocities both protocols gave very similar data (r2 = 0.96), but for high velocities only the SA procedure could be used. Results obtained using the SA protocol in fresh muscle were compared to those for muscle that had been fatigued by 25 s of ischaemic isometric contractions, induced by electrical stimulation at the ulnar nerve. Fatigue resulted in a decrease of isometric force [to 69 (3)%], an increase in half-relaxation time [to 431 (10)%], and decreases in maximal shortening velocity [to 77 (8)%] and power [to 42 (5)%]. These are the first data for human skeletal muscle to show convincingly that during acute fatigue, power is reduced as a consequence of both the loss of force and slowing of the contractile speed.

Contractile properties of the quadriceps muscle in individuals with spinal cord injury

Selected contractile properties and fatigability of the quadriceps muscle were studied in seven spinal cord-injured (SCI) and 13 able-bodied control (control) individuals. The SCI muscles demonstrated faster rates of contraction and relaxation than did control muscles and extremely large force oscillation amplitudes in the 10-Hz signal (65 +/- 22% in SCI versus 23 +/- 8% in controls). In addition, force loss and slowing of relaxation following repeated fatiguing contractions were greater in SCI compared with controls. The faster contractile properties and greater fatigability of the SCI muscles are in agreement with a characteristic predominance of fast glycolytic muscle fibers. Unexpectedly, the SCI muscles exhibited a force-frequency relationship shifted to the left, most likely as the result of relatively large twitch amplitudes. The results indicate that the contractile properties of large human locomotory muscles can be characterized using the approach described and that the transformation to faster properties consequent upon changes in contractile protein expression following SCI can be assessed. These measurements may be useful to optimize stimulation characteristics for functional electrical stimulation and to monitor training effects induced by electrical stimulation during rehabilitation of paralyzed muscles.

RNA content differs in slow and fast muscle fibers: implications for interpretation of changes in muscle gene expression

Quantification of a specific muscle mRNA per total RNA (e.g., by Northern blot analysis) plays a crucial role in assessment of developmental, experimental, or pathological changes in gene expression. However, total RNA content per gram of a particular fiber type may differ as well. We have tested this possibility in the distinct fiber types of adult rat skeletal muscle. Sections of single fibers were hybridized against 28S rRNA as a marker for RNA content. Quantification of the hybridization showed that the 28S rRNA content decreases in the order I>IIA>IIX>IIB, where Type I fibers show a five- to sixfold higher expression level compared to Type IIB fibers. Results were verified with an independent biochemical determination of total RNA content performed on pools of histochemically defined freeze-dried single fibers. In addition, the proportion of myosin heavy chain (MHC) mRNA per microgram of total RNA was similar in slow and fast fibers, as demonstrated by Northern blot analysis. Consequently, Type I fibers contain five- to sixfold more MHC mRNA per microgram of tissue than IIB fibers. These differences are not reflected in the total fiber protein content. This study implies that proper assessment of mRNA levels in skeletal muscle requires evaluation of total RNA levels according to fiber type composition.

Improved high-performance liquid chromatographic assay for the determination of "high-energy" phosphates in mammalian skeletal muscle. Application to a single-fibre study in man

A sensitive and reproducible method for the determination of adenine nucleotides (ATP, IMP) and creatine compounds [creatine (Cr), phosphocreatine (PCr)] in freeze-dried single human muscle fibre fragments is presented. The method uses isocratic reversed-phase high-performance liquid chromatography of methanol extracts. Average retention times (min) of ATP, IMP and PCr, Cr from standard solutions were 10.6+/-0.42, 2.11+/-0.06 (n=6) and 10.5+/-0.31 and 1.19+/-0.02 (n=9), respectively. Detection limits in extracts from muscle fibre fragments were 2.0, 1.0, 3.0 and 2.0 mmol/kg dm, respectively. The assay was found successful for analysis of fibre-fragments weighing > or = 1 microg.

Human power output during repeated sprint cycle exercise: the influence of thermal stress

Thermal stress is known to impair endurance capacity during moderate prolonged exercise. However, there is relatively little available information concerning the effects of thermal stress on the performance of high-intensity short-duration exercise. The present experiment examined human power output during repeated bouts of short-term maximal exercise. On two separate occasions, seven healthy males performed two 30-s bouts of sprint exercise (sprints I and II), with 4 min of passive recovery in between, on a cycle ergometer. The sprints were performed in both a normal environment [18.7 (1.5) degrees C, 40 (7)% relative humidity (RH; mean SD)] and a hot environment [30.1 (0.5) degrees C, 55 (9)% RH]. The order of exercise trials was randomised and separated by a minimum of 4 days. Mean power, peak power and decline in power output were calculated from the flywheel velocity after correction for flywheel acceleration. Peak power output was higher when exercise was performed in the heat compared to the normal environment in both sprint I [910 (172) W vs 656 (58) W; P < 0.01] and sprint II [907 (150) vs 646 (37) W; P < 0.05]. Mean power output was higher in the heat compared to the normal environment in both sprint I [634 (91) W vs 510 (59) W; P < 0.05] and sprint II [589 (70) W vs 482 (47) W; P < 0.05]. There was a faster rate of fatigue (P < 0.05) when exercise was performed in the heat compared to the normal environment. Arterialised-venous blood samples were taken for the determination of acid-base status and blood lactate and blood glucose before exercise, 2 min after sprint I, and at several time points after sprint II. Before exercise there was no difference in resting acid-base status or blood metabolites between environmental conditions. There was a decrease in blood pH, plasma bicarbonate and base excess after sprint I and after sprint II. The degree of post-exercise acidosis was similar when exercise was performed in either of the environmental conditions. The metabolic response to exercise was similar between environmental conditions; the concentration of blood lactate increased (P < 0.01) after sprint I and sprint II but there were no differences in lactate concentration when comparing the exercise bouts performed in a normal and a hot environment. These data demonstrate that when brief intense exercise is performed in the heat, peak power output increases by about 25% and mean power output increases by 15%; this was due to achieving a higher pedal cadence in the heat.

Disability and functional assessment in former polio patients with and without postpolio syndrome

OBJECTIVES

To compare perceived health problems and disability in former polio subjects with postpolio syndrome (PPS) and those without postpolio syndrome (non-PPS), and to evaluate perceived health problems, disability, physical performance, and muscle strength.

DESIGN

Cross-sectional survey; partially blinded data collection.

SUBJECTS

One hundred three former polio subjects, aged 32 to 60yrs. This volunteer sample came from referrals and patient contacts. Criterion for PPS: new muscle weakness among symptoms.

MAIN OUTCOME MEASURES

Nottingham Health Profile (NHP), adapted D-code of the International Classification of Impairments, Disabilities and Handicaps, performance test, and muscle strength assessment.

RESULTS

PPS subjects (n = 76) showed higher scores (p < .001) than non-PPS subjects (n = 27) within the NHP categories of physical mobility, energy, and pain. On a 16-item Polio Problems List, 78% of PPS subjects selected fatigue as their major problem, followed by walking outdoors (46%) and climbing stairs (41%). The disabilities of PPS subjects were mainly seen in physical and social functioning. No differences in manually tested strength were found between patient groups. PPS subjects needed significantly more time for the performance test than non-PPS subjects and their perceived exertion was higher. Perceived health problems (NHP-PhysMobility) correlated significantly with physical disability (r = .66), performance-time (r = .54), and muscle strength (r = .38). With linear regression analysis, 54% of the NHP-PhysMobility score could be explained by the performance test (time and exertion), presence of PPS, and muscle strength, whereas strength itself explained only 14% of the NHP-PhysMobility score.

CONCLUSIONS

PPS subjects are more prone to fatigue and have more physical mobility problems than non-PPS subjects. In former polio patients, measurements of perceived health problems and performance tests are the most appropriate instruments for functional evaluation.

Differences in human antagonistic ankle dorsiflexor coactivation between legs; can they explain the moment deficit in the weaker plantarflexor leg?

The present study examined the hypothesis that the antagonistic ankle dorsiflexor coactivation level during maximum isometric voluntary plantarflexion (MVC) is a function of ankle angle. Six male subjects generated plantarflexion and dorsiflexion MVC trials at ankle angles of -15 deg (dorsiflexed direction), 0 deg (neutral position), +15 deg (plantarflexed direction) and +30 deg having the knee flexed at an angle of 90 deg. In all contractions surface EMG measurements were taken from tibialis anterior and soleus which were considered representative muscles of all dorsiflexors and plantarflexors, respectively. Antagonistic dorsiflexor coactivation was expressed as normalized EMG and moment. Calculations of the antagonistic dorsiflexor moment were based on the tibialis anterior EMG-dorsiflexor moment relationship from contractions at 50, 40, 30, 20 and 10 % of the dorsiflexion MVC moment. In both legs dorsiflexor coactivation level followed an open U-shaped pattern as a function of ankle angle. Differences of 9 and 14 % (P < 0.05) were found in the measured net plantarflexion MVC moment between legs at ankle angles of -15 and +30 deg, respectively. No difference (P > 0.05) was found in the calf circumference between legs. Differences were found in the antagonistic dorsiflexor coactivation between legs at ankle angles of -15 and +30 deg. In the weaker leg the antagonistic EMG measurements were higher by 100 and 45 % (P < 0.01) and the estimated antagonistic moments were higher by 70 and 43 % (P < 0.01) compared with the weaker leg at -15 and +30 deg, respectively. This finding was associated with a decreased range of motion (ROM) in the weaker leg (14 %, P < 0.01), such that no difference (P > 0.05) was found in dorsiflexor antagonistic coactivation between legs at end-range ankle angles. The findings of the study (i) have to be taken into consideration when estimating musculoskeletal loads in the lower extremity, (ii) imply that stretching training can result in a stronger plantarflexion at end-range ankle angles through inhibition of the dorsiflexors, and (iii) imply a neural drive inadequacy during a plantarflexion MVC at end-range angles.

In vivo measurements of the triceps surae complex architecture in man: implications for muscle function

1. The objectives of this study were to (1) quantify experimentally in vivo changes in pennation angle, fibre length and muscle thickness in the triceps surae complex in man in response to changes in ankle position and isometric plantarflexion moment and (2) compare changes in the above muscle architectural characteristics occurring in the transition from rest to a given isometric plantarflexion intensity with the estimations of a planimetric muscle model assuming constant thickness and straight muscle fibres. 2. The gastrocnemius medialis (GM), gastrocnemius lateralis (GL) and soleus (SOL) muscles of six males were scanned with ultrasonography at different sites along and across the muscle belly at rest and during maximum voluntary contraction (MVC) trials at ankle angles of -15 deg (dorsiflexed direction), 0 deg (neutral position), +15 deg (plantarflexed direction) and +30 deg. Additional images were taken at 80, 60, 40 and 20% of MVC at an ankle angle of 0 deg. 3. In all three muscles and all scanned sites, as ankle angle increased from -15 to +30 deg, pennation increased (by 6-12 deg, 39-67%, P < 0.01 at rest and 9-16 deg, 29-43%, P < 0.01 during MVC) and fibre length decreased (by 15-28 mm, 32-34%, P < 0.01 at rest and 8-10 mm, 27-30%, P < 0.05 during MVC). Thickness in GL and SOL increased during MVC compared with rest (by 5-7 mm, 36-47%, P < 0.01 in GL and 6-7 mm, 38-47%, P < 0.01 in SOL) while thickness of GM did not differ (P > 0.05) between rest and MVC. 4. At any given ankle angle the model underestimated changes in GL and SOL occurring in the transition from rest to MVC in pennation angle (by 9-12 deg, 24-38%, P < 0.01 in GL and 9-14 deg, 25-28%, P < 0.01 in SOL) and fibre length (by 6-15 mm, 22-39%, P < 0.01 in GL and 6-8 mm, 23-24%, P < 0.01 in SOL). 5. The findings of the study indicate that the mechanical output of muscle as estimated by the model used may be unrealistic due to errors in estimating the changes in muscle architecture during contraction compared with rest.

Changes in Achilles tendon moment arm from rest to maximum isometric plantarflexion: in vivo observations in man

1. The purpose of the present study was to examine the effect of a plantarflexor maximum voluntary contraction (MVC) on Achilles tendon moment arm length. 2. Sagittal magnetic resonance (MR) images of the right ankle were taken in six subjects both at rest and during a plantarflexor MVC in the supine position at a knee angle of 90 deg and at ankle angles of -30 deg (dorsiflexed direction), -15 deg, 0 deg (neutral ankle position), +15 deg (plantarflexed direction), +30 deg and +45 deg. A system of mechanical stops, support triangles and velcro straps was used to secure the subject in the above positions. Location of a moving centre of rotation was calculated for ankle rotations from -30 to 0 deg, -15 to +15 deg, 0 to +30 deg and +15 to +45 deg. All instant centres of rotation were calculated both at rest and during MVC. Achilles tendon moment arms were measured at ankle angles of -15, 0, +15 and +30 deg. 3. At any given ankle angle, Achilles tendon moment arm length during MVC increased by 1-1.5 cm (22-27 %, P < 0.01) compared with rest. This was attributed to a displacement of both Achilles tendon by 0.6-1.1 cm (P < 0.01) and all instant centres of rotation by about 0.3 cm (P < 0.05) away from their corresponding resting positions. 4. The findings of this study have important implications for estimating loads in the musculoskeletal system. Substantially unrealistic Achilles tendon forces and moments generated around the ankle joint during a plantarflexor MVC would be calculated using resting Achilles tendon moment arm measurements.

Shortening-induced force depression in human adductor pollicis muscle

1. The effects of single isovelocity shortening contractions on force production of the electrically stimulated human adductor pollicis muscle were investigated in seven healthy male subjects. 2. Redeveloped isometric force immediately following isovelocity shortening was always depressed compared with the isometric force recorded at the same muscle length but without preceding shortening. The maximal isometric force deficit (FD) was (mean +/- S.E.M.) 37 +/- 2% after 38 deg of shortening at 6.1 deg s-1. 3. The FD was positively correlated with angular displacement (r2 > 0.98) and decreased with increasing velocity of the shortening step. Stimulation at 20 Hz instead of 50 Hz reduced absolute force levels during the contractions to about 73% and the FD was decreased to a similar extent. Eighty-nine per cent of the velocity-related variation in the FD could be explained by the absolute force levels during shortening. 4. FD was largely abolished by allowing the muscle to relax briefly (approximately 200 ms), a time probably too short for significant metabolic recovery. 5. At all but the highest velocities there was a linear decline in force during the latter part of the isovelocity shortening phase, suggesting that the mechanisms underlying FD were active during shortening. 6. Our results show that shortening-induced force deficit is a significant feature of human muscle working in situ and is proportional to the work done by the muscle-tendon complex. This finding has important implications for experimental studies of force-velocity relationships in the intact human.

Comparison of the molecular, antigenic and ATPase determinants of fast myosin heavy chains in rat and human: a single-fibre study

Combined methodologies of histochemistry, immunohistochemistry, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), reverse transcriptase polymerase chain reaction (RT-PCR) and a histochemical method specific for myofibrillar ATPase (mATPase) of the type IIX myosin heavy chain (MyHC) isoform were used to study human and rat single fibres to examine the homology between type II MyHC isoform-based fibres of both species. We demonstrate that human type II fibres exhibit antigenic mATPase and 3'-untranslated region (3'-UTR) sequence determinants homologous to the IIA and IIX but not the IIB MyHC isoforms of the rat. Both immunolabelling with anti-MyHC monoclonal antibodies and the mATPase method used with frozen sections confirmed that all human type II fibres express type IIA and/or type IIX MyHC. Quantitative immunohistochemistry failed to recognize human fibres with antigenic characteristics corresponding to hybrid IIXB MyHC-based fibres. Ca2+-stimulated maximum myosin ATPase activity, determined by quantitative histochemistry, revealed that human IIX fibres (with an optical density or OD = 0.707) display enzyme activity which is comparable to that of the rat type IIX (OD = 0.687) but lower than that of the rat type IIB fibres (OD = 0.836). The results do not support the notion that MyHC IIB is expressed in human limb muscles, even in hybrid fibres. We conclude that human type II fibres have been misclassified in numerous previous publications and that this has important implications in attempts to compare the physiological characteristics of fibre types, particularly when animal models are used.

Myosin heavy chain isoform expression and high energy phosphate content in human muscle fibres at rest and post-exercise

1. The relationship between myosin heavy chain (MyHC) isoforms and high energy phosphate content was studied in human muscle fibres at rest and following maximal dynamic exercise lasting 25 s. 2. Single fibre fragments were characterized as type I, type IIA, type IIX or type IIAX. These latter fibres were subdivided into five groups on the basis of the proportion of MyHC IIX isoform present. 3. Resting ATP concentration in type I fibres was 10% lower than in type II fibres (P < 0.05), but no differences were found amongst type IIA, IIX and IIAX fibre groups. Phosphocreatine (PCr) content was lower in type I than in type II fibres (P < 0.01) and, amongst type II fibres, increased progressively with the amounts of MyHC IIX expressed. 4. After 25 s of maximal dynamic exercise ATP concentration was reduced in all fibres. The decrease was approximately 25% in type I fibres and between 47 and 66% in the type II subgroups. 5. Post-exercise PCr content was low in all fibre types. Fibre groups with the lowest post-exercise PCr also had the lowest ATP and the highest inosine monophosphate contents. delta PCr (rest to post-exercise) was smallest in type I fibres and showed a progressive increase in the type II fibre groups as the proportion of the faster IIX myosin heavy chain isoform increased.

In vivo IIX and IIB fiber recruitment in gastrocnemius muscle of the rat is compartment related

The purpose of the present study was to investigate to what extent fast-twitch IIX and IIB fiber recruitment was related to the natural existing muscle compartments (subvolumes of muscle innervated by different primary nerve branches) in rat medial gastrocnemius. Three groups (n = 6) of rats trotted on a motor-driven treadmill (20 degrees incline) at different speeds. A fourth group served as controls, and a fifth group received in situ electrical stimulation of all medial gastrocnemius muscle fibers. Postexercise glycogen levels (periodic acid-Schiff staining intensities) were made. Running caused more and in situ stimulation caused less glycogen breakdown in the proximal IIX and IIB fibers compared with the fibers of the same type in the most distal compartment. Furthermore, the boundaries of the most distal compartment could often be recognized in the periodic acid-Schiff-stained cross sections. It was concluded that during running the proximal IIX and IIB fibers were recruited to a greater extent (and at lower treadmill speeds) compared with the distal IIX and IIB fibers, respectively.

Fast-twitch muscle unit properties in different rat medial gastrocnemius muscle compartments

1. The effect of muscle unit (MU) localization on physiological properties was investigated within the fast-twitch fatigue-resistant (FR) and fast-fatigable (FF) MU populations of rat medial gastrocnemius (MG) muscle. Single MG MUs were functionally isolated by microdissection of the ventral roots. FR and FF MU properties of the most proximal and distal muscle compartments were compared. The most proximal and distal compartment are subvolumes of the MG innervated by the most proximal and distal primary nerve branch, respectively. A subsample of the isolated units was glycogen depleted and muscle cross sections were stained for glycogen and myosin-adenosinetriphosphatase. 2. It was shown that proximal FF and FR units reached optimum length for force production at shorter muscle lengths compared with the distal FR and FF units. 3. The fast MUs of the proximal compartment had small territories that were located close to and/or within the mixed region (containing type I, IIA, IIX, and IIB fibers) of the muscle. The fast MUs of the distal compartment had greater territories that were located in the more superficial muscle part (containing only type IIX and IIB fibers) and in some cases spanned the entire area of the distal muscle compartment. 4. FR and FF MUs consisted of muscle fibers identified histochemically as type IIX and IIB, respectively. 5. Within each of the FR and FF MU populations, MUs that were located in the most proximal muscle compartment were more resistant to fatigue compared with the units located in the most distal compartment. 6. Cross-sectional fiber areas were smaller for the proximal FR and FF fibers, but specific force did not differ among units. Consequently, when account was taken of the innervation ratio, the proximal FR and FF units produced less force than distal units of the same type. Tetanic forces were 87 +/- 27 (SD) mN (proximal FR), 154 +/- 53 (SD) mN (distal FR), 142 +/- 25 (SD) mN (proximal FF), and 229 +/- 86 (SD) mN (distal FF). 7. The present findings suggest that with increasing demand placed on rat MG during in vivo locomotion, recruitment is likely to proceed from proximal to distal muscle parts within the FR and FF MU populations.

Repeated force production and metabolites in two medial gastrocnemius muscle compartments of the rat

The most proximal and distal motor nerve branches in the rat medial gastrocnemius innervate discrete muscle compartments dominated by fast-twitch oxidative and fast-twitch glycolytic fibers, respectively. The functional consequences of the difference in oxidative capacity between these compartments were investigated. Wistar rats were anesthetized with pentobarbital sodium (90 mg/kg ip). Changes in force of both compartments during 21 isometric contractions (train duration 200 ms, stimulation frequency 120 Hz, 3 s between contractions) were studied in situ with and without blood flow. Without blood flow, force and phosphocreatine declined to a greater extent in the proximal than the distal compartment compared with the run with intact flow. After the protocol without blood flow, when flow was restored, the time constants for force recovery (which were closely associated to the recovery of phosphocreatine) were 37 +/- 7 (SD) (proximal compartment) and 148 +/- 20 s (distal compartment). It was concluded that the proximal compartment had a four times higher oxidative capacity and, therefore, a superior ability for repeated force production.

Non-linear relationship between O2 uptake and power output at high intensities of exercise in humans

1. A slow component to pulmonary oxygen uptake (VO2) is reported during prolonged high power exercise performed at constant power output at, or above, approximately 60% of the maximal oxygen uptake. The magnitude of the slow component is reported to be associated with the intensity of exercise and to be largely accounted for by an increased VO2 across the exercising legs. 2. On the assumption that the control mechanism responsible for the increased VO2 is intensity dependent we hypothesized that it should also be apparent in multi-stage incremental exercise tests with the result that the VO2-power output relationship would be curvilinear. 3. We further hypothesized that the change in the VO2-power output relationship could be related to the hierarchical recruitment of different muscle fibre types with a lower mechanical efficiency. 4. Six subjects each performed five incremental exercise tests, at pedalling rates of 40, 60, 80, 100 and 120 rev min-1, over which range we expected to vary the proportional contribution of different fibre types to the power output. Pulmonary VO2 was determined continuously and arterialized capillary blood was sampled and analysed for blood lactate concentration ([lactate]b). 5. Below the level at which a sustained increase in [lactate]b was observed pulmonary VO2 showed a linear relationship with power output; at high power outputs, however, there was an additional increase in VO2 above that expected from the extrapolation of that linear relationship, leading to a positive curvilinear VO2-power output relationship. 6. No systematic effect on the magnitude or onset of the 'extra' VO2 was found in relation to pedalling rate, which suggests that it is not related to the pattern of motor unit recruitment in any simple way.

The mATPase histochemical profile of rat type IIX fibres: correlation with myosin heavy chain immunolabelling

In the present study we report a novel histochemical method which, by sequential pre-incubations in alkaline and acidic media, selectively differentiates muscle fibres expressing myosin heavy chain IIX, on the basis of a specific profile for myofibrillar actomyosin ATPase (mATPase) activity. The enzyme reactions were tested for specificity by means of anti-myosin heavy chain monoclonal antibodies, which were characterized on Western blots of muscle homogenates. Enzyme histochemical reactions with the traditional pH buffers were compared to those of the new method and, in conjunction with the immunoreactions, used to confirm the relationship between MyHC expression and the distinct profiles for mATPase. Immunohistochemical reactions demonstrated that the new method only differentiates those fibres expressing myosin heavy chain IIX. The method revealed a continuum in which the intermediate staining intensities corresponded to hybrid fibres expressing myosin heavy chain IIX in combination with either the IIA or IIB forms. Quantitative histochemistry and immunohistochemistry (by image analysis), used to examine the relationship between staining intensities for mATPase and amounts of myosin heavy chain IIX expression, revealed that the new method discriminates well between hybrid fibres expressing variable amounts of the IIX isoform (r2 = 0.93).