About this Research Topic
Exercise-induced fatigue leads to viscoelastic, biomechanical and neuromuscular alterations which increase a muscle’s vulnerability to injury during dynamic activities. Epidemiological evidence showing a higher injury incidence in competition vs. training as well as in the latter stages of competitive matches in sports such as soccer supports the fatigue-risk model. Furthermore, greater aerobic capacity or repeated-sprint ability is associated with lower risk, at least of non-contact injury in sports. Yet, in team sports settings, injury risk screening principally involves the evaluation of peak force or peak force asymmetries in a “fresh state”, potentially neglecting the interaction between repeated high-intensity activities and fatigue-driven changes in both positive (concentric/propulsive activities) and negative (eccentric/decelerative) force production capacity. Thus, a muscle’s “strength endurance” (SE) may also be considered an important factor influencing the capacity to sustain performance during many sports activities as well as a critical but relatively under-investigated element influencing injury risk.
By way of example, while the non-fatigued hamstrings:quadriceps (H:Q) peak torque ratio is one of the most widely used and frequently investigated “risk-screening” test protocols, fatiguing simulated-sports protocols are consistently found to evoke large and practically-relevant decreases in eccentric hamstrings peak torque and the hamstrings:quadriceps (H:Q) ratio. This selective decrease in hamstring strength means that fresh-state peak torque ratios do not describe an athlete’s neuromuscular profile late in a fatiguing competition or later stages of matches when unfavorable alterations in sprinting, landing and change-of-direction mechanics are observed. Even in circumstances when (a) peak hamstring eccentric strength has been associated with a lower hamstring strain injury (HSI) risk, and (b) eccentric strengthening interventions have been shown to reduce HSI incidence, a possible contribution of both strength and SE has not been ruled out.
Furthermore, athletes with prior HSI may show deficits in SE but not in peak strength, and hamstring conditioning emphasizing SE has been found to be effective in attenuating strength decline in simulated competitions while emphasis on peak strength development has not. Therefore, in addition to overall aerobic capacity and peak muscular strength, SE may mediate the time-dependent increases in injury incidence. Hence, it could be argued that SE training should form part of the injury-minimization strategy in athletes, and SE should also be assessed in risk screening protocols.
It is however, important to note that a majority of research studies have utilized isokinetic dynamometry in strength and SE testing protocols, which is an impractical tool for systematic screening in sports environments. However, a collection of valid and reliable SE tests that can be quickly implemented without significant athlete familiarization (practice) has not yet been established. The development of easy-to-implement tests in the applied setting is therefore urgently needed for those working in the sporting environment.
We welcome the submission of research papers related to the measurement of SE and its relation to physical performance and injury, as well as the physiological mechanisms underlying SE or its changes following specific interventions. Regarding SE measurement, varied methodologies have been used in research, including single sets of 15-50 continuous isokinetic or isoinertial muscle contractions (lasting 10-90 s), multiple sets of higher-load repetitions performed with minimal inter-set rest, and the assessment of peak strength or power after fatiguing exercise bouts (e.g. repeated sprint/simulated match protocols). SE is sometimes also referred to as “work capacity” or “muscular endurance”, also describing the capacity to maintain strength during fatiguing exercise. So there is no single distinct definition available.
Regarding the physiological mechanisms underlying SE, we encourage submissions focusing on metabolic aspects, either at a molecular (e.g., monocarboxylate transporters, enzymes, mitochondrial factors), or a systemic (e.g., hemoglobin, blood lactate) level, as well as neuromuscular aspects (e.g., motor unit firing patterns, muscle synergies).
In this regard, while we will provide these examples in an effort to constrain submissions, authors should be allowed to justify their inclusion based on their own definition or protocol. We expect an emphasis on the hamstrings by virtue of the poor association between its peak and fatigued state performance, but we encourage submissions related to other muscle groups. We will consider submissions based on any sport or exercise-related activities, both from applied settings and lab studies examining:
- Physiological mechanisms underpinning changes in SE with training and detraining
- Neuromuscular effects of training aimed to improve SE
- Differences in SE at different levels of performance and between/across sports (cross-sectional)
- Relationships between SE (or changes in SE) and injury risk or injury outcomes (prospective or retrospective)
- Influence of SE training on biomechanical or neuromuscular performance risk factors/muscle recruitment strategies during fatiguing high-intensity activities
- Reliability or validity of SE tests and test methodologies
- Systematic reviews and meta-analyses targeting any of the above areas
Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.