Pathway-Specific Neural Adaptations to Strength Training: Effects of Training Modality, Muscle and Age

Neural adaptations underpin early gains in strength training, well before hypertrophy occurs. These adaptations include increased corticospinal and reticulospinal excitability, and changes in motor unit recruitment, discharge rate, and discharge variability. Growing evidence indicates that these adaptations are training-modality specific: metronome-paced training increases corticospinal excitability, while self-paced and isometric training build comparable strength without a corticospinal signature. Whether the same modality effects hold across different muscles, or depend on which descending system ordinarily drives that muscle, is unknown, and is central to designing training and rehabilitation protocols targeted at specific neural pathways. The corticospinal and reticulospinal tracts are not evenly distributed across the body. Hand and forearm muscles receive dense direct corticospinal projections and support fine, independent movement; postural and locomotor muscles such as tibialis anterior rely more heavily on the reticulospinal tract for gross, sustained force. Every study of training-modality-specific neural adaptation to date has been conducted in the hand or forearm, leaving open whether the same corticospinal-reticulospinal dissociation generalises to muscles that depend less on the corticospinal tract in daily use. This project uses transcranial magnetic stimulation (TMS), TMS conditioned by a startling acoustic stimulus (SAS-TMS) to probe reticulospinal contribution, and high-density surface electromyography (HD-sEMG) motor unit decomposition to map these adaptations across muscles that differ systematically in their corticospinal and reticulospinal weighting. A key research gap concerns whether metronome-paced (MP), self-paced (SP), and isometric (IM) strength training engage the corticospinal and reticulospinal systems differently depending on the muscle trained. MP training may preferentially drive corticospinal adaptation in muscles built for fine motor control, such as first dorsal interosseous and the elbow flexors, while SP and IM training may drive reticulospinal and motor-unit adaptation more uniformly, particularly in muscles such as tibialis anterior that already depend on the reticulospinal tract for everyday postural and locomotor drive. This project tests that interaction directly, and asks whether it changes with age, given that corticospinal integrity is known to decline while reticulospinal function is comparatively preserved. The research is structured around four objectives. First, a systematic review will synthesise existing evidence on reticulospinal and motor-unit adaptation to strength training, coding the muscle tested in each study to establish how far current knowledge extends beyond the hand and forearm. Second, a randomised trial in young adults will compare MP, SP, and IM training across three muscles positioned along the corticospinal-reticulospinal continuum; first dorsal interosseous, elbow flexors, and tibialis anterior using an identical TMS, SAS-TMS, and HD-sEMG battery in each. Third, the same design will be run in older adults, to determine whether the modality-by-muscle pattern established in young adults is preserved, attenuated, or reorganised with age. Finally, single motor unit discharge behaviour, including persistent inward current estimation from HD-sEMG decomposition, will be related to the corticospinal and reticulospinal changes measured with TMS and SAS-TMS, connecting the non-invasive signal to its likely motoneuron-level consequence. This project will establish whether strength training-modality-specific neural adaptation is a general principle of the motor system or a property confined to the hand and forearm muscles in which it has so far been studied. The findings will inform the choice of training modality in strength and rehabilitation programs targeting specific neural pathways, particularly for restoring gait and postural strength after stroke, injury, or in older age, where the muscles involved are precisely those that have never been tested.