The evaluation of rate of force development during rapid contractions has recently become quite popular for characterising explosive strength of athletes, elderly individuals and patients. The main aims of this narrative review are to describe the neuromuscular determinants of rate of force development and to discuss various methodological considerations inherent to its evaluation for research and clinical purposes. Rate of force development (1) seems to be mainly determined by the capacity to produce maximal voluntary activation in the early phase of an explosive contraction (first 50–75 ms), particularly as a result of increased motor unit discharge rate; (2) can be improved by both explosive-type and heavy-resistance strength training in different subject populations, mainly through an improvement in rapid muscle activation; (3) is quite difficult to evaluate in a valid and reliable way. Therefore, we provide evidence-based practical recommendations for rational quantification of rate of force development in both laboratory and clinical settings.

/pdf/rate-of-force-development-physiological-and-methodological-5a4kwox9nh.pdf

Rate of force development: physiological and methodological considerations

Purpose: To establish the reliability of various measures obtained during single and repeated countermovement jump (CMJ) performance in an elite athlete population. Methods: Two studies, each involving 15 elite Australian Rules Football (ARF) players were conducted where subjects performed two days, separated by one week, of AM and PM trials of either a single (CMJ1) or 5 repeated CMJ (CMJ5). Each trial was conducted on a portable force-plate. The intraday, interday, and overall typical error (TE) and coefficient of variation (CV%) were calculated for numerous variables in each jump type. Results: A number of CMJ1 and CMJ5 variables displayed high intraday, interday, and overall reliability. In the CMJ1 condition, mean force (CV 1.08%) was the most reliable variable. In the CMJ5, flight time and relative mean force displayed the highest repeatability with CV of 1.88% and 1.57% respectively. CMJ1Mean force was the only variable with an overall TE < smallest worthwhile change (SWC). Conclusion: Selected variables obtained during CMJ1 and CMJ5 performance can be used to assess the impact of both acute and chronic training and competition. Variables derived from the CMJ5 may respond differently than their CMJ1 counterparts and should provide insights into differential mechanisms of response and adaptation.

/pdf/reliability-of-measures-obtained-during-single-and-repeated-r2tdkecw89.pdf

Reliability of measures obtained during single and repeated countermovement jumps

5 – Functional Evaluation of the Elbow

Validation of shear wave elastography in skeletal muscle.

Test-retest reliability.

An increase in the period over which a muscle generates force can lead to the generation of greater force and, therefore, for example in jumping, to greater jump height. The aim of this study was to examine the effect of squat depth on maximum vertical jump performance. We hypothesized that jump height would increase with increasing depth of squat due to the greater time available for the generation of muscular force. Ten participants performed jumps from preferred and deep squat positions. A computer model simulated jumps from the different starting postures. The participants showed no difference in jump height in jumps from deep and preferred positions. Simulated jumps produced similar kinematics to the participants' jumps. The optimal squat depth for the simulated jumps was the lowest position the model was able to jump from. Because jumping from a deep squat is rarely practised, it is unlikely that these jumps were optimally coordinated by the participants. Differences in experimental vertical ground reaction force patterns also suggest that jumps from a deep squat are not optimally coordinated. These results suggest there is the potential for athletes to increase jump performance by exploiting a greater range of motion.

/pdf/the-influence-of-squat-depth-on-maximal-vertical-jump-3k0cb31mba.pdf

The influence of squat depth on maximal vertical jump performance

Elbow strength and endurance in patients with a ruptured distal biceps tendon.

Hamstring strain injury is a frequent and serious injury in competitive and recreational sports. While Nordic hamstring (NH) eccentric strength training is an effective hamstring injury-prevention method, the protective mechanism of this exercise is not understood. Strength training increases muscle strength, but also alters muscle architecture and stiffness; all three factors may be associated with reducing muscle injuries. The purpose of this study was to examine the effects of NH eccentric strength training on hamstring muscle architecture, stiffness, and strength. Twenty healthy participants were randomly assigned to an eccentric training group or control group. Control participants performed static stretching, while experimental participants performed static stretching and NH training for 6 weeks. Pre- and post-intervention measurements included: hamstring muscle architecture and stiffness using ultrasound imaging and elastography, and maximal hamstring strength measured on a dynamometer. The experimental group, but not the control group, increased volume (131.5 vs. 145.2 cm3, p < 0.001) and physiological cross-sectional area (16.1 vs. 18.1 cm2, p = 0.032). There were no significant changes to muscle fascicle length, stiffness, or eccentric hamstring strength. The NH intervention was an effective training method for muscle hypertrophy, but, contrary to common literature findings for other modes of eccentric training, did not increase fascicle length. The data suggest that the mechanism behind NH eccentric strength training mitigating hamstring injury risk could be increasing volume rather than increasing muscle length. Future research is, therefore, warranted to determine if muscle hypertrophy induced by NH training lowers future hamstring strain injury risk.

/pdf/the-effect-of-nordic-hamstring-strength-training-on-muscle-19r2iuit34.pdf

The effect of Nordic hamstring strength training on muscle architecture, stiffness, and strength.

Introduction: Idiopathic inflammatory myopathies (IIMs, or myositis) represent a group of autoimmune diseases that result in decreased muscle strength and/or endurance. Non-invasive tools to assess muscle may improve our understanding of the clinical and functional consequences of myopathies and their response to treatment. In this study we examine magnetic resonance elastography (MRE), a non-invasive technique that assesses the shear modulus (stiffness) of muscle, in IIM subjects. Methods: Nine subjects with active myositis completed the MRE protocol. Participants lay in a positioning device, and scans of the vastus medialis (VM) were taken in the relaxed state and at two contraction levels. Manual inversion was used to estimate the stiffness. Results: A significant reduction in muscle stiffness was seen in myositis subjects compared with healthy controls during the “relaxed” condition. Discussion: The use of non-invasive technologies such as MRE may provide greater understanding of the pathophysiology of IIM and improve assessment of treatment efficacy. Muscle Nerve, 2011

/pdf/evaluation-of-muscles-affected-by-myositis-using-magnetic-2ep3zmsvjr.pdf

Zachary J. Domire

Papers

The influence of squat depth on maximal vertical jump performance

Elbow strength and endurance in patients with a ruptured distal biceps tendon.

The effect of Nordic hamstring strength training on muscle architecture, stiffness, and strength.

Evaluation of muscles affected by myositis using magnetic resonance elastography.

Stem diameter and micromotion of press fit radial head prosthesis: A biomechanical study