Forearm muscle activity differs during gripping in people with tennis elbow compared to healthy individuals

Background and aim: Lateral epicondylalgia (LE), commonly referred to as tennis elbow, is a chronic musculoskeletal injury that presents with pain over the lateral elbow that is associated with altered wrist posture during gripping. Although the underlying pathophysiology involves degenerative changes of the common extensor tendon, from which the extensor carpi radialis brevis (ECRB), extensor digitorum communis (EDC) and extensor carpi ulnaris (ECU) muscles originate, their contribution to altered wrist posture and gripping is unknown. Therefore, the aim of this study was to examine the effect of wrist posture on forearm muscle activation patterns during an isometric grip task in people with LE. Methods: Eleven individuals with LE (age range 32 to 65 years, 8 males, 10 right hand dominant) and eleven age- and sex-matched controls were recruited from the community. Surface EMG was collected from ECRB, EDC, ECU, flexor carpi radialis (FCR), flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU) and anconeus muscles. The tasks in the study were 6 s steady isometric hand grips at 15% of maximal voluntary contraction (MVC) with a neutral wrist posture, 20 degree wrist extension, and 20 degree of wrist flexion. Each muscle EMG amplitude was normalized to the same muscle's peak amplitude during MVC, at each respective wrist posture. Muscle activation was computed as the EMG root mean square in a 25 ms non-overlapping window and averaged from five grip trials at 15% MVC target grip in each wrist posture. Muscle coactivation was calculated as the proportion of each muscle's activity to the net activity of all muscles at each wrist posture. Results: Muscle activation. A main effect of group was detected for overall ECRB activation (F(1, 20) = 6.22, p = 0.021), where the LE group had significantly decreased activation compared to controls during the 15% MVC target grip force. Pairwise comparisons revealed that LE ECRB activation was decreased in wrist extension (p = 0.044) and neutral wrist (p = 0.043) posture compared to the controls. Muscle coactivation. A significant main effect of wrist posture was detected for ECRB (p = 0.048) and ECU (p = 0.019) coactivation, when both groups are combined. However, the post-hoc analyses of wrist posture revealed that ECRB, EDC and ECU coactivation differences (p < 0.05) were only present in the healthy controls, with coactivation of forearm extensors unaffected by wrist posture in LE. Conclusion: The results indicate that the forearm muscle activation is altered in LE, most notably in the ECRB which is frequently implicated in this disorder. Absence of significant differences in wrist extensors muscle coactivation between the wrist postures in LE compared to the matched healthy controls indicates the possibility of reduced motor variability due to pain related manifestations of LE. This feature is often seen in other chronic musculoskeletal conditions such as neck-shoulder pain and low back pain.