Roger C. Woledge

Changes in maximal voluntary force of human adductor pollicis muscle during the menstrual cycle.

1. Muscle strength of the adductor pollicis (AP) was studied throughout the menstrual cycle to determine whether any variation in force is similar to the known cyclical changes in ovarian hormones. Three groups of young women were studied: trained regularly menstruating athletes (trained), untrained regularly menstruating (untrained) and trained oral contraceptive pill users (OCU). In addition a group of untrained young men was studied as controls. 2. Maximum voluntary force (MVF) of AP was measured over a maximum period of 6 months. Ovulation was detected by luteinizing hormone measurements or change in basal body temperature. There was a significant increase in MVF (about 10%) during the follicular phase of the menstrual cycle when oestrogen levels are rising, in both the trained and untrained groups. This was followed by a similar in MVF around the time of ovulation. Neither the OCU nor the male subjects showed cyclical changes in MVF.

Muscle activity and acceleration during whole body vibration: Effect of frequency and amplitude

BACKGROUND Whole body vibration may improve muscle and bone strength, power and balance although contradictory findings have been reported. Prolonged exposure may result in adverse effects. We investigated the effects of high (5.5 mm) and low (2.5mm) amplitude whole body vibration at various frequencies (5-30 Hz) on muscle activity and acceleration throughout the body. METHODS Surface electromyographic activity was recorded from 6 leg muscles in 12 healthy adults (aged 31.3 (SD 12.4) years). The average rectified acceleration of the toe, ankle, knee, hip and head was recorded from 15 healthy adults (36 (SD 12.1) years) using 3D motion analysis. FINDINGS Whole body vibration increased muscle activity 5-50% of maximal voluntary contraction with the greatest increase in the lower leg. Activity was greater with high amplitude at all frequencies, however this was not always significant (P<0.05-0.001). Activation tended to increase linearly with frequency in all muscles except gluteus maximus and biceps femoris. Accelerations throughout the body ranged from approximately 0.2 to 9 g and decreased with distance from the platform. Acceleration at the head was always < 0.33 g. The greatest acceleration of the knee and hip occurred at approximately 15 Hz and thereafter decreased with increasing frequency. INTERPRETATION Above the knee at frequencies > 15 Hz acceleration decreased with distance from the platform. This was associated with increased muscle activity, presumably due to postural control and muscle tuning mechanisms. The minimal acceleration at the head reduces the likelihood of adverse reactions. The levels of activation are unlikely to cause hypertrophy in young healthy individuals but may be sufficient in weak and frail people.

Effects of whole body vibration on motor unit recruitment and threshold

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.

In mice, the muscle weakness due to age is absent during stretching.

1. The contractile force was compared in isolated soleus muscles from young (2.5‐8 months old) and aged (28‐31 months old) mice. Force was measured at 25 degrees C during isometric tetanic contractions during isovelocity stretching and shortening contractions. 2. The normalized isometric force was lower by 13.3% in muscles from aged mice. Muscles from young and aged mice produced 0.951 +/‐ 0.031 N mg‐1 (n = 12) and 0.824 +/‐ 0.048 N mg‐1 (n = 9) respectively. The relaxation time, from 90 to 10% of the tetanic force, of muscles from aged mice was 102.1 +/‐ 3.7 ms (n = 6), which was longer than that for muscles from young mice, 84.4 +/‐ 3.8 ms (n = 6) (means +/‐ S.E.M.). 3. The force during shortening was also reduced in muscles from aged animals by the same proportion as the isometric force. Therefore the force during shortening relative to the isometric force was the same in muscles from young and aged mice. 4. During rapid stretching soleus muscles from aged mice produced a similar force to those from young mice. Therefore stretch can remove the weakness in muscles of aged mice. 5. These changes in muscles from aged mice are similar to those produced when inorganic phosphate (Pi) levels are raised, in skinned rabbit psoas fibres, or during fatigue or with low intracellular pH (pHi), in frog muscle. It is possible therefore that the force loss due to ageing may be due to a higher Pi level or a lower pHi.

Energy storage during stretch of active single fibres from frog skeletal muscle

Heat production and force were measured during tetani of single muscle fibres from anterior tibialis of frog. During stimulation fibres were either kept under isometric conditions, or were stretched or allowed to shorten (at constant velocity) after isometric force had reached its plateau value. The energy change was evaluated as the sum of heat and work (work = integral of force with respect to length change). Net energy absorption occurred during stretch at velocities greater than about 0.35 L0 s−1 (L0 is fibre length at resting sarcomere length 2.10 μm). Heat produced by 1 mm segments of the fibre was measured simultaneously and separately; energy absorption is not an artefact due to patchy heat production. The maximum energy absorption, 0.092 ± 0.002 P0L0 (mean ±s.e.m., n= 8; where P0 is isometric force at L0), occurred during the fastest stretches (1.64 L0 s−1) and amounted to more than half of the work done on the fibre. Energy absorption occurred in two phases. The amount in the first phase, 0.027 ± 0.003 P0L0 (n= 32), was independent of velocity beyond 0.18 L0 s−1. The quantity absorbed in the second phase increased with velocity and did not reach a limiting value in the range of velocities used. After stretch, energy was produced in excess of the isometric rate, probably from dissipation of the stored energy. About 34 % (0.031 P0L0/0.092 P0L0) of the maximum absorbed energy could be stored elastically (in crossbridges, tendons, thick, thin and titin filaments) and by redistribution of crossbridge states. The remaining energy could have been stored in stretching transverse, elastic connections between myofibrils.

Heart rate and its variability change after the menopause

Resting heart rate and heart rate variability of 33 postmenopausal women were compared with those of 50 premenopausal women of comparable activity level, none of whom had used hormone replacement therapy. Heart rate was measured as the mean of at least 600 consecutive R‐R intervals obtained from electrocardiograph (ECG) records, and its variability as the standard deviation of these intervals. Activity levels were assessed by a scale modified from the Allied Dunbar National Fitness Survey (1992). There was a significant reduction in both mean R‐R interval and the standard deviation in the postmenopausal women who had experienced their last menstrual period (LMP) 1 year or more prior to the observations being made, but no observable changes during the first year post menopause.

Inferring crossbridge properties from skeletal muscle energetics

Work is generated in muscle by myosin crossbridges during their interaction with the actin filament. The energy from which the work is produced is the free energy change of ATP hydrolysis and efficiency quantifies the fraction of the energy supplied that is converted into work. The purpose of this review is to compare the efficiency of frog skeletal muscle determined from measurements of work output and either heat production or chemical breakdown with the work produced per crossbridge cycle predicted on the basis of the mechanical responses of contracting muscle to rapid length perturbations. We review the literature to establish the likely maximum crossbridge efficiency for frog skeletal muscle (0.4) and, using this value, calculate the maximum work a crossbridge can perform in a single attachment to actin (33 x 10(-21) J). To see whether this amount of work is consistent with our understanding of crossbridge mechanics, we examine measurements of the force responses of frog muscle to fast length perturbations and, taking account of filament compliance, determine the crossbridge force-extension relationship and the velocity dependences of the fraction of crossbridges attached and average crossbridge strain. These data are used in combination with a Huxley-Simmons-type model of the thermodynamics of the attached crossbridge to determine whether this type of model can adequately account for the observed muscle efficiency. Although it is apparent that there are still deficiencies in our understanding of how to accurately model some aspects of ensemble crossbridge behaviour, this comparison shows that crossbridge energetics are consistent with known crossbridge properties.

Angiotensin-I converting enzyme genotype-dependent benefit from hormone replacement therapy in isometric muscle strength and bone mineral density.

Low bone mineral density (BMD) and muscle weakness are major risk factors for postmenopausal osteoporotic fracture. Hormone replacement therapy (HRT) reverses the menopausal decline in maximum voluntary force of the adductor pollicis and reduces serum angiotensin-I converting enzyme (ACE) levels. The insertion (I) allele of the ACE gene polymorphism is associated with lower ACE activity and improved muscle efficiency in response to physical training. Therefore, we examined whether the presence of the I allele in postmenopausal women would affect the muscle response to HRT. Those taking HRT showed a significant gain in normalized muscle maximum voluntary force slope, the rate of which was strongly influenced by ACE genotype (16.0 +/- 1.53%, 14.3 +/- 2.67%, and 7.76 +/- 4.13%, mean +/- SEM for II, ID, and DD genotype, respectively; P = 0.017 for gene effect, P = 0.004 for I allele effect). There was also a significant ACE gene effect in the response of BMD to HRT in Wards triangle (P = 0.03) and a significant I allele effect in the spine (P = 0.03), but not in the neck of femur or total hip. These data suggests that low ACE activity associated with the I allele confers an improved muscle and BMD response in postmenopausal women treated with HRT.

Molar enthalpy change for hydrolysis of phosphorylcreatine under conditions in muscle cells

The enthalpy change for the hydrolysis of phosphorylcreatine (PCr) by hydrochloric acid or by alkaline phosphatase was observed at 0, 25, and 37 degrees C. The value for delta H is -44 kJ mol-1 under alkaline, Mg2+-free conditions and is almost independent of temperature, ionic strength, and concentration of reactants. In muscle the reaction is accompanied by a transfer of protons from the buffers (largely histidine) to orthophosphate, release of Mg2+ from PCr, and binding of Mg2+ to orthophosphate. Measurements are reported of the heats of these processes. The calculated value of the overall heat of hydrolysis of PCr (including these processes) at pH 7, pMg 3 is -35 kJ mol-1.

Neither changes in phosphorus metabolite levels nor myosin isoforms can explain the weakness in aged mouse muscle.

1. The contractile force, phosphorus metabolite levels, intracellular pH and myosin isoforms were compared in isolated soleus and extensor digitorum longus (EDL) muscles from young (6 month old) and aged (28 month old) mice, at 23 degrees C. 2. The isometric force per unit cross‐sectional area was significantly lower by 21 +/‐ 5% in soleus muscles from aged mice compared to those from young mice (mean +/‐ S.E.M., n = 11 and 9 respectively). 3. The EDL muscle contained twice as much total creatine and phosphocreatine as the soleus, 1.7 times as much ATP, and 0.4 times the inorganic phosphate (Pi) per unit weight. The intracellular pH and free ADP levels were not significantly different between these muscle types. 4. There was no significant difference in resting metabolite levels between young and old EDL or soleus despite the difference in mechanical strength. 5. Examination of the expression of myosin isoforms by non‐denaturing gel electrophoresis has shown that the percentage of each isoform does not change with respect to age; thus, if there is an atrophic process occurring, it is not fibre type specific. 6. We have determined that neither the Pi levels nor the intracellular pH can explain the differences seen in muscle strength with age. There is also no correlation between muscle weakness and any of the other metabolites responsible for energy transduction (phosphocreatine, ATP or ADP).