We investigated the capacity for torque development and muscle activation at the onset of fast voluntary isometric knee extensions at 30, 60, and 90 degrees knee angle. Experiments were performed in subjects (n = 7) who had high levels (>90%) of activation at the plateau of maximal voluntary contractions. During maximal electrical nerve stimulation (8 pulses at 300 Hz), the maximal rate of torque development (MRTD) and torque time integral over the first 40 ms (TTI40) changed in proportion with torque at the different knee angles (highest values at 60 degrees ). At each knee angle, voluntary MRTD and stimulated MRTD were similar (P < 0.05), but time to voluntary MRTD was significantly longer. Voluntary TTI40 was independent (P > 0.05) of knee angle and on average (all subjects and angles) only 40% of stimulated TTI40. However, among subjects, the averaged (across knee angles) values ranged from 10.3 +/- 3.1 to 83.3 +/- 3.2% and were positively related (r2 = 0.75, P < 0.05) to the knee-extensor surface EMG at the start of torque development. It was concluded that, although all subjects had high levels of voluntary activation at the plateau of maximal voluntary contraction, among subjects and independent of knee angle, the capacity for fast muscle activation varied substantially. Moreover, in all subjects, torque developed considerably faster during maximal electrical stimulation than during maximal voluntary effort. At different knee angles, stimulated MRTD and TTI40 changed in proportion with stimulated torque, but voluntary MRTD and TTI40 changed less than maximal voluntary torque.

Objectives: The aims of this study were (1) to quantify and compare the load of a professional football team's training days and matches and (2) to compare training of nonstarters the day after the match with regular training of starters and nonstarters. Methods: On-field training load during in-season training days (categorized as days before match day, i.e., MD minus) and 3 friendly matches were recorded using alocal positioning measurement system. Results: Mixed linear models showed lower load when training approached match day. Relative to match values (100%), training values for running (52 -20%; MD-4 -MD-1) and high-speed running (38 -15%) were lower than for total distance (67 -35%), and all considerably lower than match values. On average, medium and high accelerations and decelerations during training were more similar to match values (90 -39%). Load during nonstarters training was lower than during regular training for almost all variables on MD-4 and several high-intensity variables on MD-3 and MD-2. Conclusions: The results highlight that acceleration and deceleration measures complement more commonly used external load variables based on distance and speed. Furthermore, nonstarters are potentially under-loaded compared to starters, especially in terms of (high-speed) running.

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. 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. 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. FD was largely abolished by allowing the muscle to relax briefly (approximately 200 ms), a time probably too short for significant metabolic recovery. 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. 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.

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. Keywords:

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: 5x1 min vibration (frequency 30 Hz, amplitude 8 mm) with 2 min rest in between. During vibration, subjects stood barefoot on the vibration platform with their knees at an angle of 110 degrees. At 90 s 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 3 h. 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 2 weeks ( 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.