Effect of force level and training status on contractile properties following fatigue

Neuromuscular response differences to power vs strength back squat exercise in elite athletes

The study's aim was to establish the neuromuscular responses in elite athletes during and following maximal 'explosive' regular back squat exercise at heavy, moderate, and light loads. Ten elite track and field athletes completed 10 sets of five maximal squat repetitions on three separate days. Knee extension maximal isometric voluntary contraction (MIVC), rate of force development (RFD) and evoked peak twitch force (Pt) assessments were made pre- and post-session. Surface electromyography [root mean square (RMS)] and mechanical measurements were recorded during repetitions. The heavy session resulted in the greatest repetition impulse in comparison to moderate and light sessions (P < 0.001), while the latter showed highest repetition power (P < 0.001). MIVC, RFD, and Pt were significantly reduced post-session (P < 0.01), with greatest reduction observed after the heavy, followed by the moderate and light sessions accordingly. Power significantly reduced during the heavy session only (P < 0.001), and greater increases in RMS occurred during heavy session (P < 0.001), followed by moderate, with no change during light session. In conclusion, this study has shown in elite athletes that the moderate load is optimal for providing a neuromuscular stimulus but with limited fatigue. This type of intervention could be potentially used in the development of both strength and power in elite athletic populations.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Assessment of H reflex sensitivity with M wave alternation consequent to fatiguing contractions

The objective of this study was to examine the changes in H reflex sensitivity after neuromuscular fatigue associated with fluctuations of the M wave. In the maximal and submaximal voluntary contraction (MVC and SMVC) paradigms, subjects performed voluntary plantarflexion at 100% MVC and 40% MVC respectively until the limit of torque maintenance was reached. In the submaximal electrical stimulation (SMES) paradigm, the tricep surae was exhausted with sustained electrical stimulation of 40% of the maximal tolerable intensity at a 40-Hz stimulus rate. The H reflexes and maximal M waves (M(max)) of the soleus were recorded before and after the three fatigue paradigms, and the H reflex was standardized with M(max) to minimize possible bias due to fatigue-induced M wave fluctuation. The results showed a significant increase in the standardized H reflex due to the SMES paradigm in spite of M(max) potentiation. The SMVC paradigm led to a reduction in size of the standardized H reflex without modification of M(max), whereas the standardized H reflex was not mediated by the MVC paradigm, which contributed to a noticeable M(max) potentiation. The present study underscored the fact that the H reflex sensitivity and M wave amplitude were not necessarily suppressed consequent to neuromuscular fatigue, but varied with the activation history of a muscle for size-dependent efficacy of the Ia transmission pathways and postactivation potentiation.

The effects of varying time under tension and volume load on acute neuromuscular responses

The purpose of this study was to examine the effects of different methods of measuring training volume, controlled in different ways, on selected variables that reflect acute neuromuscular responses. Eighteen resistance-trained males performed three fatiguing protocols of dynamic constant external resistance exercise, involving elbow flexors, that manipulated either time-under-tension (TUT) or volume load (VL), defined as the product of training load and repetitions. Protocol A provided a standard for TUT and VL. Protocol B involved the same VL as Protocol A but only 40% concentric TUT; Protocol C was equated to Protocol A for TUT but only involved 50% VL. Fatigue was assessed by changes in maximum voluntary isometric contraction (MVIC), interpolated doublet (ID), muscle twitch characteristics (peak twitch, time to peak twitch, 0.5 relaxation time, and mean rates of force development and twitch relaxation). All protocols produced significant changes (P <or= 0.05) in the measures considered to reflect neuromuscular fatigue, with the exception of ID. Fatigue was related to an increase in either TUT or VL with greater fatigue, as reflected by MVIC and peripheral measures, being associated with differences in TUT. The lack of change in ID suggests that fatigue was more related to peripheral than central mechanisms. It was concluded that the load and contraction velocities of the repetitions have different effects on acute neuromuscular responses and should, therefore, be clearly calculated when describing training volume for dynamic constant external resistance exercise training.