Neural mechanisms of the repeated bout effect: corticospinal, reticulospinal, and motor-unit adaptations to eccentric exercise

Unaccustomed eccentric exercise can induce exercise-induced muscle damage, typically characterised by transient reductions in maximal force, increased muscle soreness, restricted range of motion, and altered neuromuscular function. When the same eccentric stimulus is repeated days or weeks later, these adverse responses are markedly attenuated. This robust protective adaptation is referred to as the repeated bout effect. Although the repeated bout effect has traditionally been attributed to adaptations within skeletal muscle and connective tissue, emerging evidence indicates that the nervous system may also adapt following an initial bout of damaging eccentric exercise. Studies using high-density electromyography have demonstrated changes in motor-unit discharge behaviour, discharge variability, recruitment thresholds, and force steadiness during and after repeated bouts of eccentric exercise. However, it remains unresolved whether these neural adaptations contribute directly to the protective effect, reflect compensatory responses to existing muscle damage, or emerge secondary to reduced soreness and accelerated functional recovery. This PhD project will investigate how descending motor drive and motor-unit behaviour are modified across repeated bouts of eccentric exercise. The biceps brachii will be used as the experimental model because it is highly susceptible to eccentric exercise-induced muscle damage and is well suited to the investigation of upper-limb descending motor control. Importantly, the biceps brachii also provides a valuable model for examining reticulospinal contributions to movement, which may be particularly relevant during upper-limb and forceful motor tasks compared with the distal or lower-limb muscles that have been more commonly studied. The project will integrate advanced neurophysiological techniques to assess adaptations across multiple levels of the motor system. Transcranial magnetic stimulation and motor-evoked potential recruitment curves will be used to quantify corticospinal excitability. Startling auditory stimulation will be used to probe reticulospinal contributions to motor output. High-density electromyography and motor-unit decomposition will be used to examine how changes in descending drive are expressed at the level of individual motor units. Together, these approaches will enable corticospinal, reticulospinal, and motor-unit adaptations to be assessed before, during, and after repeated bouts of damaging eccentric elbow-flexor exercise. The research will address three core objectives. First, it will characterise corticospinal and reticulospinal responses to eccentric exercise-induced muscle damage in the biceps brachii. Second, it will determine whether these responses are altered following a repeated bout, when muscle damage and force loss are expected to be attenuated. Third, it will examine whether changes in descending drive are associated with alterations in motor-unit recruitment, discharge behaviour, and the recovery of muscle function. By integrating measures of muscle damage, descending pathway excitability, and single motor-unit behaviour, this project will provide a multi-level assessment of neural adaptation during the repeated bout effect. The findings are expected to advance current understanding of how the nervous system adapts to damaging exercise and may inform the development of future exercise, rehabilitation, and strength-training strategies.