Impact of neck muscle fatigue on upper limb sensorimotor integration

Abstract

Neck muscles have a high density of sensory receptors which project to the central nervous system, and have important role in sensory motor integration (SMI). The cerebellum is important for SMI and it is known to undergo neuroplastic changes in response to performing a novel motor acquisition task. Alterations in neck sensory input impacts motor output to upper limb muscles, cerebellar disinhibition, and performance accuracy in response to novel motor acquisition. Despite this, the impact of cervical extensor muscle (CEM) fatigue on cerebellar-motor cortex plasticity and SMI pathways in response to a novel motor acquisition task has not been yet investigated. Study one used short latency somatosensory evoked potentials (SEPs) to investigate the differential effects of CEM fatigue on motor learning and retention; and on sensorimotor processing from distal hand muscles. CEM fatigue impaired upper limb motor learning performance in conjunction with differential changes in SEP peak amplitudes related to SMI. Study two used a paired pulse cerebellar-motor cortex transcranial magnetic stimulation (TMS) technique to determine whether CEM fatigue alters cerebellar disinhibition in response to novel motor skill acquisition. Neck fatigue led to a lessened capacity for cerebellar disinhibition coupled with diminished motor learning relative to a control group. Study three used an eye-hand tracking protocol to investigate the effect of CEM fatigue on accuracy of pointing to both visual and hidden targets. CEM fatigue reduced the accuracy of upper limb tracking to a hidden target. Study four used short- and medium-latency SEPs to determine the impact of CEM fatigue on motor performance accuracy and retention of proximal upper limb muscles, as well as neural processing changes in response to novel motor acquisition using proximal upper limbs muscles. CEM fatigue had minimal impact on proximal upper-limb motor performance accuracy, but lead to differential changes in both short- and medium- latency SEP peak amplitudes related to SMI. Overall, this thesis suggesting that SMI areas including the cerebellum are impacted by CEM fatigue, likely because the altered afferent input from the neck due to fatigue alters body schema, impacting awareness of upper limb position sense, resulting in decreased upper limb performance accuracy.