David R. Coleman

EFFECT OF CONTRACTION MODE OF SLOW-SPEED RESISTANCE TRAINING ON THE MAXIMUM RATE OF FORCE DEVELOPMENT IN THE HUMAN QUADRICEPS

This study examined the effects of slow‐speed resistance training involving concentric (CON, n = 10) versus eccentric (ECC, n = 11) single‐joint muscle contractions on contractile rate of force development (RFD) and neuromuscular activity (EMG), and its maintenance through detraining. Isokinetic knee extension training was performed 3 · week−1 for 10 weeks. Maximal isometric strength (+11.2%) and RFD (measured from 0–30/50/100/200 ms, respectively; +10.5%–20.5%) increased after 10 weeks (P < 0.01–0.05); however, there was no effect of training mode. Peak EMG amplitude and rate of EMG rise were not significantly altered with training or detraining. Subjects with below‐median normalized RFD (RFD/MVC) at 0 weeks significantly increased RFD after 5‐ and 10‐weeks training, which was associated with increased neuromuscular activity. Subjects who maintained their higher RFD after detraining also exhibited higher activity at detraining. Thus, only subjects with a lesser ability to rapidly attain their maximum force before training improved RFD with slow‐speed resistance exercise. Muscle Nerve, 2008

Changes in muscle force-length properties affect the early rise of force in vivo.

Changes in contractile rate of force development (RFD), measured within a short time interval from contraction initiation, were measured after a period of strength training that led to increases in muscle fascicle length but no measurable change in neuromuscular activity. The relationship between training‐induced shifts in the moment–angle relation and changes in RFD measured to 30 ms (i.e., early) and 200 ms (i.e., late) from the onset of isometric knee extension force were examined; shifts in the moment–angle relation were used as an overall measure of changes in quadriceps muscle fascicle length. A significant proportion of the variance in RFD measured only in the initial contraction phase (0–30 ms) could be explained by shifts in the moment–angle relation (r = −0.66–0.71; R2 = 0.44–0.50). Training‐induced increases in muscle fascicle length may lead to a reduced or complete lack of adaptive gains in contractile RFD, especially in the early contraction phase. Muscle Nerve 39: 512–520, 2009

Leg stiffness in human running: Comparison of estimates derived from previously published models to direct kinematic-kinetic measures.

It is not presently clear whether mathematical models used to estimate leg stiffness during human running are valid. Therefore, leg stiffness during the braking phase of ground contact of running was calculated directly using synchronous kinematic (high-speed motion analysis) and kinetic (force platform) analysis, and compared to stiffness calculated using four previously published kinetic models. Nineteen well-trained male middle distance runners (age=21.1±4.1yr; VO(2max)=69.5±7.5mlO(2)kg(-1)min(-1)) completed a series of runs of increasing speed from 2.5 to 6.5ms(-1). Leg stiffness was calculated directly from kinetic-kinematic analysis using both vertical and horizontal forces to obtain the resultant force in the line of leg compression (Model 1). Values were also estimated using four previously published mathematical models where only force platform derived and anthropometric measures were required (Models 2-5; Morin et al., 2005, Morin et al., 2011, Blum et al., 2009, Farley et al., 1993, respectively). The greatest statistical similarity between leg stiffness values occurred with Models 1 and 2. The poorest similarity occurred when values from Model 4 were compared with Model 1. Analyses suggest that the poor correlation between Model 1 other models may have resulted from errors in the estimation in change in leg length during the braking phase. Previously published mathematical models did not provide accurate leg stiffness estimates, although Model 2, used by Morin et al. (2005), provided reasonable estimates that could be further improved by the removal of systematic error using a correction factor (K=1.0496K(Model2)).

Lack of effect of moderate-duration static stretching on plantar flexor force production and series compliance.

BACKGROUND The effects of an acute bout of moderate-duration static stretching on plantar flexor force production, series compliance of the muscle-tendon unit, and levels of neuromuscular activation were examined. METHODS Eighteen active individuals (9 men and 9 women) performed four 45-s static plantar flexor stretches and a time-matched control of no stretch (where subjects remained seated in the dynamometer for 4 min with no stretch being performed). Measures of peak isometric moment, rate of force development, neuromuscular activation (interpolated twitch technique and electromyography), twitch force characteristics, passive moment during stretch, and tendon elongation during maximal voluntary contractions were taken before and after the stretching. FINDINGS Despite a significant stress-relaxation response during stretch (9.3%, P<0.01) there were no significant differences in peak isometric moment (P=0.35; effect size 0.13), rate of force development (P=0.93; effect size 0.01), neuromuscular activation (interpolated twitch: P=0.86; electromyography: P=0.09; effect size 0.02), or tendon elongation (P=0.61; effect size 0.07) after stretching. Twitch characteristics were also unchanged after stretching, although there was a reduction in the rate of twitch torque relaxation (RR(t); P<0.01). INTERPRETATION The acute bout of moderate-duration static stretching did not impair the force generating capacity of the plantar flexors or negatively affect muscle-tendon mechanical properties. Static stretching may not always have detrimental consequences for force production. Thus, clinicians may be able to apply moderate-duration stretches to patients without risk of reducing muscular performance.