A. de Haan

Short-term effects of whole-body vibration on maximal voluntary isometric knee extensor force and rate of force rise

Abstract. Whole-Body vibration (WBV) may lead to muscle contractions via reflex activation of the primary muscle spindle (Ia) fibres. WBV has been reported to increase muscle power in the short term by improved muscle activation. The present study set out to investigate the acute effects of a standard WBV training session on voluntary activation during maximal isometric force production (MVC) and maximal rate of force rise (MRFR) of the knee extensors. Twelve students underwent a single standard WBV training session: 5×1xa0min vibration (frequency 30xa0Hz, amplitude 8xa0mm) with 2xa0min rest in between. During vibration, subjects stood barefoot on the vibration platform with their knees at an angle of 110°. At 90xa0s following vibration, maximal voluntary knee extensor force was reduced to 93 (5)% [mean (SD), P<0.05] of baseline value and recovered within the next 3xa0h. Voluntary activation remained significantly depressed (2–4%). Neither the electrically induced MRFR nor voluntary MRFR were significantly affected by WBV. In addition, six WBV training sessions in 2xa0weeks (n=10) did not enhance either voluntary muscle activation during MVC [99 (2)% of the baseline value] or voluntary MRFR [98 (9)% of the baseline value]. It is concluded that in the short term, WBV training does not improve muscle activation during maximal isometric knee extensor force production and maximal rate of force rise in healthy untrained students.

The effects of 11 weeks whole body vibration training on jump height, contractile properties and activation of human knee extensors

The purpose of the present study was to investigate whether 11xa0weeks of whole body vibration (WBV) training applied in a way that is commonly seen in practice, i.e. without additional loads, would improve muscle activation and/or contractile properties of the knee extensor muscles and counter movement jump height in healthy subjects. Ten subjects belonging to the experimental group trained three times a week and stood bare-foot with a 110xa0° knee angle on a vibration platform (30xa0Hz, 8xa0mm amplitude). They underwent five to eight sets of 1-min vibration with 1xa0min rest in between. Ten control subjects followed the same training programme but stood (110xa0° knee angle) beside the platform. Before, during and following the training period the subjects were tested. Values [mean (SEM)] obtained in the last test were expressed as percentages of the baseline value and presented for control and experimental groups. Quadriceps femoris isometric muscle force [105.4 (6.2)%, 99.9 (2.0)%; P=0.69], voluntary activation [107.1 (6.0)%, 101.1 (2.3)%; P=0.55] and maximal rate of voluntary force rise [95.4 (6.0)%, 103.3 (7.7)%; P=0.57] did not improve. The maximal rate of force rise during electrical stimulation was increased [102.3 (4.5)%, 123.6 (7.5)%; P=0.02]. Counter movement jump height was not affected by WBV [103.7 (1.8)%, 103.0 (2.8)%; P=0.71]. In conclusion, 11 weeks of standard two-legged WBV training without additional training loads did not improve functional knee extensor muscle strength in healthy young subjects.

The force-velocity relationship of human adductor pollicis muscle during stretch and the effects of fatigue

1 We have examined the force‐velocity characteristics of tetanically activated human adductor pollicis working in vivo, in the fresh and fatigued states. 2 The increase in force in response to stretch was divided into two major components. The first, steady, component persisted after the stretch and is concluded not to be a function of active cycling cross‐bridges because it was not affected by either the velocity of the stretch or the level of muscle activation. 3 The origin of the second, transient, component of the increased force seen during stretch is consistent with cross‐bridge activity since it increased with increasing velocity of stretch and was proportional to the level of activation. 4 It is likely that both components of the stretch response make a significant contribution to muscle performance when acting to resist a force. For the fastest stretch used, the contributions of cross‐bridge and non‐cross‐bridge mechanisms were equal. For the slowest stretch, lasting 10 s and over the same distance, the force response was attributed almost entirely to non‐cross‐bridge mechanisms. 5 As a result of acute fatigue (50 % isometric force loss) there were only small reductions in the non‐cross‐bridge component of the force response to stretch, while the cross‐bridge component decreased in absolute terms. 6 The transient component of the stretch response increased as a result of fatigue, relative to the isometric force, while the force during shortening decreased. The results are consistent with a decrease in cross‐bridge turnover in fatigued muscle.

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.

The influence of stimulation frequency on force-velocity characteristics of in situ rat medial gastrocnemius muscle.

Force (and power)‐velocity characteristics were determined at different stimulation frequencies in th situ rat muscle with nerve stimulation at 36 degrees C. In isometric contractions (duration, 150 ms), maximal force is generated at approximately 120 Hz. In contrast, in the high velocity (250 mm s‐1) shortening contractions, frequencies of approximately 400 Hz were needed to obtain maximal dynamic force, while 120 Hz elicited only approximately 26% of the maximum. At the highest velocity measured, power production was significantly different (P < 0.05) among frequencies of 80, 120, 200 and 400 Hz, suggesting that maximal shortening velocity should be assessed using very high stimulation frequencies. However, the results further indicate that lower frequencies may be adequate in exercise studies that investigate fatigue and changes in power output during series of repetitive contractions.

Calcium regulation and muscle disease.

Changes in intracellular Ca2+-concentration play an important role in the excitation–contraction–relaxation cycle of skeletal muscle. In this review we describe various inheritable muscle diseases to highlight the role of Ca2+ -regulatory mechanisms. Upon excitation the ryanodine receptor releases Ca2+ in the cytosol. During and after contraction the sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) pumps Ca2+ back in the SR resulting in relaxation. An abnormal change in the intracellular Ca2+ -concentration results in defective muscle contraction and/or relaxation, which is the cause of various muscle diseases. Malignant hyperthermia (MH) and central core disease (CCD) are both caused by mutations in the ryanodine receptor but show different clinical phenotypes. In MH an acute increase of Ca2+ results in excessive muscle contraction causing rigidity, while in CCD a chronic rise of cytosolic Ca2+ is seen, leading to mitochondrial damage, disorganization of myofibrils and muscle weakness. In Brody disease and also in mitochondrial myopathies, SERCA functions sub optimal causing a prolonged physiological Ca2+ -elevation leading to slowing of relaxation. Defective actin–myosin interactions, as in nemaline myopathy and also in mitochondrial myopathies due to ATP-shortage, cause Ca2+-hyposensitivity and slowness of contraction. Information of Ca2+ -kinetics in these inherited muscular diseases improves our understanding of the role of calcium in the physiology and pathophysiology of the skeletal muscle cell.

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

Abstract The purpose of the study was to obtain force/velocity relationships for electrically stimulated (80u2009Hz) human adductor pollicis muscle (nu2009=u20096) and to quantify the effects of fatigue. There are two major problems of studying human muscle inu2009situ; 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 (r2u2009=u20090.96), 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 25u2009s 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.

Non-invasive continuous core temperature measurement by zero heat flux

Reliable continuous core temperature measurement is of major importance for monitoring patients. The zero heat flux method (ZHF) can potentially fulfil the requirements of non-invasiveness, reliability and short delay time that current measurement methods lack. The purpose of this study was to determine the performance of a new ZHF device on the forehead regarding these issues. Seven healthy subjects performed a protocol of 10 min rest, 30 min submaximal exercise (average temperature increase about 1.5 °C) and 10 min passive recovery in ambient conditions of 35 °C and 50% relative humidity. ZHF temperature (T(zhf)) was compared to oesophageal (T(es)) and rectal (T(re)) temperature. ΔT(zhf)-T(es) had an average bias ± standard deviation of 0.17 ± 0.19 °C in rest, -0.05 ± 0.18 °C during exercise and -0.01 ± 0.20 °C during recovery, the latter two being not significant. The 95% limits of agreement ranged from -0.40 to 0.40 °C and T(zhf) had hardly any delay compared to T(es). T(re) showed a substantial delay and deviation from T(es) when core temperature changed rapidly. Results indicate that the studied ZHF sensor tracks T(es) very well in hot and stable ambient conditions and may be a promising alternative for reliable non-invasive continuous core temperature measurement in hospital.

Voluntary drive-dependent changes in vastus lateralis motor unit firing rates during a sustained isometric contraction at 50% of maximum knee extension force

The purpose of the present study was to relate the expected inter-subject variability in voluntary drive of the knee extensor muscles during a sustained isometric contraction to the changes in firing rates of single motor units. Voluntary activation, as established with superimposed electrical stimulation was high (range: 91–99%, n=8) during a short maximal contraction, but was lower (range: 69–100%) in most subjects at the point of force failure during a sustained (49.1±10.1xa0s) fatiguing contraction at 50% of maximum force. On a different experimental day the firing behaviour of 27 single motor units was recorded with wire electrodes in the vastus lateralis muscle, 24 of which could be monitored from the time of recruitment to the point of force failure (53.6±9.8xa0s). Motor unit firing behaviour differed considerably among subjects. During the second half of the sustained, fatiguing contraction the changes in firing rate firing rate variability of early recruited units ranged from −10% to +100% and from −50% to +160% respectively among subjects. There were significant positive linear relations between voluntary activation, on the one hand, and rectified surface electromyogram (rsEMG, r=0.82), the changes in motor unit firing rate (r=0.49) and firing rate variability (r=0.50) towards the point of force failure on the other. The present data suggest that differences in voluntary drive that appear among subjects during fatigue may be an important determinant of motor unit firing behaviour.

Mind your step: Metabolic energy cost while walking an enforced gait pattern

The energy cost of walking could be attributed to energy related to the walking movement and energy related to balance control. In order to differentiate between both components we investigated the energy cost of walking an enforced step pattern, thereby perturbing balance while the walking movement is preserved. Nine healthy subjects walked three times at comfortable walking speed on an instrumented treadmill. The first trial consisted of unconstrained walking. In the next two trials, subject walked while following a step pattern projected on the treadmill. The steps projected were either composed of the averaged step characteristics (periodic trial), or were an exact copy including the variability of the steps taken while walking unconstrained (variable trial). Metabolic energy cost was assessed and center of pressure profiles were analyzed to determine task performance, and to gain insight into the balance control strategies applied. Results showed that the metabolic energy cost was significantly higher in both the periodic and variable trial (8% and 13%, respectively) compared to unconstrained walking. The variation in center of pressure trajectories during single limb support was higher when a gait pattern was enforced, indicating a more active ankle strategy. The increased metabolic energy cost could originate from increased preparatory muscle activation to ensure proper foot placement and a more active ankle strategy to control for lateral balance. These results entail that metabolic energy cost of walking can be influenced significantly by control strategies that do not necessary alter global gait characteristics.