Rate of force development and purely explosive contractions: from neural and muscular determinants to methods to assess central, peripheral fatigue, and asymmetries

The analysis of force-time curves has been widely investigated to assess the neuromuscular function both in rest and in fatigue conditions. Usually, the maximal voluntary force (MVF) has been used as a main outcome (i.e., pre- vs. post-fatiguing task). However, the time required to reach the MVF is close to 400/600 ms from the force onset and this is far from the time required to develop force in many real-life situations. Indeed, the ability to generate and relax muscle forces quickly (i.e., 200 ms) across various submaximal levels (i.e., neuromuscular quickness) is the basis of relevant activities of daily life, such as the stabilization of postural balance following a loss of it, and for many sports gestures, involving both upper and lower extremities, like shot put, vertical jumps, sprint both cycling and running but also in endurance running. Indeed, also human locomotion has been described as generated by an impulsive (burst-like) excitation of the muscle group with a Gaussian activation profile. Consequently, the MVF loses importance when it is related to explosive/ballistic tasks. For this reason, seems to appear more appropriate to investigate the rate of force development (RFD). Concerning that point, RFD is derived from the force/torque time curves recorded during explosive voluntary contractions from the force onset and represents an index of quickness. Over the decades, RFD has been used to investigate the acute effects of, for example, fatiguing tasks, effects on muscle damage, training, gender and age differences etc... However, although guidelines have been published to carry out valid RFD measurements, the methods used in the literature are heterogeneous. Concerning that point, Chapter 1 is dedicated to digging deep into the background of RFD and the most appropriate ways to measure it. Furthermore, space is given to some concepts of fatigue, asymmetries, and RFD-Scaling factor (RFD-SF) keeping explosive contractions as a common denominator, and then, aims and hypotheses of this thesis. Chapter 2 is dedicated to research about neural and contractile determinants of burst-like explosive isometric contractions of knee extensors. Following, Chapter 3 reports the results of a scoping review of RFD as an indicator of neuromuscular fatigue. Then, having demonstrated the validity of RFD as an indicator of neuromuscular fatigue, Chapter 4 reports the results of a study about central fatigue induced by one hundred purely explosive contractions. Chapter 5 is dedicated to investigating the upper limbs asymmetries through the RFD-SF showed that are muscle-specific and metric-dependent. In conclusion, Chapter 6 is finalised to the general discussion about the main findings, limits, and future perspectives.