Alteration in neck neuromuscular responses and upper limb proprioception in response to neck muscle fatigue

Abstract

Neck and upper limb disorders constitute two of the most frequent musculoskeletal problems that place a huge burden on the healthcare system. Neck muscles have a very high density of muscle sensory input to the central nervous system (CNS) and are known to play an important role in sensory motor integration of upper limb movements. The CNS uses the position of the head and neck in interpretation of upper limb joint position sense (JPS). Therefore, any altered neuromuscular function of the cervical extensors has the potential to impair the awareness of upper limb joint position which is critical for carrying out smooth, purposeful movements. Despite this, only a small amount of basic science research has attempted to explore the relationship between altered afferent input from the neck on both neck and upper limb neuromuscular control. Additionally, the cervical flexion relaxation ratio (FRR) is a reliable and reproducible neuromuscular marker, which has been shown to differentiate between neck pain patients and healthy controls, and presents an objective way to measure changes in neuromuscular function. Induction of fatigue provides an experimental method for altering afferent input from the neck muscles to the CNS, enabling the effects of both neck muscle function and upper limb JPS to be investigated in an experimental setting. Studies in this thesis sought to investigate whether the elbow JPS and neck FRR can be altered by fatigue of the cervical extensor muscles (CEM). This study revealed that CEM fatigue decreased the cervical FRR, by increasing the EMG activity in relaxation phase, and reduced the accuracy of elbow joint position matching in healthy individuals. Whereas, slightly expanded the FRR in subclinical neck pain patients, by increasing the EMG activity in re-extension phase. This work has important implications for our understanding of the mechanisms that the CNS uses to stabilize the neck in the face of altered afferent input, and the implications that this may have for upper limb proprioception and associated motor performance.