Human grip is commonly used in sport and recreational activities and involves the coordinated activation of forearm muscles. Lateral epicondylalgia (LE) is a common musculoskeletal disorder that involves pathology of the tendon of extensor carpi radialis brevis (ECRB) at its insertion onto the lateral epicondyle of the humerus. LE presents clinically as pain provoked by gripping activities and has a major impact on function. This results in days off work and time off sport. Despite considerable research into the pathophysiology of LE, it remains poorly understood whether forearm muscles activity is altered during gripping in individuals with LE. This thesis aimed to comprehensively understand the extent of current knowledge and provide new data of activity of the wrist/forearm muscles in LE.
Study 1 clarifies the current understanding of muscle activity in LE by way of systematic review with quantitative analysis. This review identified that studies of individuals with LE revealed altered motor unit morphology, abnormal patterns of muscle activity during resisted wrist extension and tennis strokes, fewer cortical peaks and less distance between cortical representation of muscles (mapped with transcranial magnetic stimulation), and delayed activation of wrist extensor muscles during gripping, when contrasted to pain-free controls. The latter finding was also present on the asymptomatic limb. Importantly, this systematic review identified limited understanding of gripping despite the clinical importance of this task in provocation of LE. The only study that had investigated muscle activity during this task had severe limitations that compromise the interpretation of the results. Limitations included the use of surface electromyography (which does not record from individual muscles), and performance of sustained contraction with high grip force (50% maximum voluntary contraction), which is likely to be provocative of pain in some individuals with LE.
On the basis of some evidence of sensorimotor changes in the asymptomatic limb of individuals with unilateral LE, Study 2 was conducted as a systematic review with meta-analysis to determine the extent bilateral differences in sensorimotor function between individuals with unilateral tendinopathy and pain-free controls. Study 2 identified evidence for several sensory and motor system differences, and evidence for tendon pathology in the asymptomatic limb of individuals with other unilateral tendinopathies (Achilles tendinopathy) but not LE when contrasted to pain-free controls.
On the foundation that tendon pathology was present in asymptomatic limb of individuals with Achilles tendinopathy, Study 3 investigated whether there was a greater prevalence of tendon pathology of the asymptomatic common extensor tendon of individuals with LE than pain-free controls. Unlike the findings in the Achilles tendon, Study 3 did not identify greater tendon pathology in the asymptomatic limb of individuals with unilateral LE than that present in the tendons of pain-free individuals.
As no studies had explored specific muscle activation during a low level grip, Study 4 compared the contribution of forearm muscle activity to gripping between the symptomatic and asymptomatic sides in individuals with unilateral LE and pain-free individuals. This study highlighted altered muscle activity in LE characterised by a lower contribution of ECRB in the symptomatic arm and a greater contribution of extensor digitorum communis (EDC) and flexor digitorum profundus (FDP) to total EMG bilaterally, in individuals with LE than pain-free individuals. Modified distribution of activity between the synergist wrist extensor muscles ECRB and EDC has potential to impact on the functional performance of grip. The strategy used by individuals with might compromise efficient grip as finger flexor muscles (FDP) would need to overcome finger extensor muscle activity (EDC). Although potentially negative for grip, the different motor pattern could have consequences for ECRB pathology by unloading the muscle. This could provide a benefit of pain reduction, but also potential negative effects from stress shielding of the tendon.
Study 5 investigated potential differences in muscle synergies (defined as groups of muscles working together to simplify central nervous system processing) between individuals with LE and pain-free individuals. For both groups, analysis with non-negative matrix factorization identified two muscle synergies involved in control of forearm muscles during repetitive gripping. Comparison between groups indicated the variance accounted for by the muscle synergies was lower in the LE group than pain-free individuals, which suggests more variation in muscle activation pattern for that group.
The studies outlined in this thesis identified differences in the control of forearm muscles between individuals with and without LE. The findings imply that EDC provides a greater contribution and ECRB a lesser contribution to the gripping task in individuals with LE than pain-free controls. Given the cross-sectional study design it is not possible to clarify whether the differences preceded or followed LE, but in both cases rehabilitation of muscle activation is likely to be beneficial for recovery. Clinical interventions to modify forearm muscle activity may provide clinical benefit, but future research is needed to determine whether altered muscle activity can be changed and whether this improves outcomes.