Introduction Human grip involves coordinated activity of the wrist extensor muscles to overcome finger flexor torque and maintain an optimal wrist position3. Lateral epicondylalgia (LE) is characterised by pain at the common extensor tendon, particularly the extensor carpi radialis brevis (ECRB) portion, during grip and is associated with motor system impairments. These impairments include reduced wrist extensor muscle activity during gripping (ECRB and extensor carpi radialis longus (ECRL))1; abnormal biomechanics characterised by less wrist extension during grip2; and widespread weakness of the upper limb muscles except for metacarpophalangeal joint extension (extensor digitorum communis (EDC)).1 On this basis, this study aimed to test the hypothesis that people with LE would use redistributed pattern of muscle activity involving greater relative contribution of EDC and lesser contribution of ECRB.
Methods Intramuscular electromyography (EMG) electrodes were used to record muscle activity from ECRB, ECRL, EDC, flexor digitorum superficialis and profundus (FDS and FDP), and flexor carpi radialis (FCR), in 15 participants with LE and 15 age-matched pain-free controls. Participants performed a pain-free gripping task at 20% maximum grip strength with the wrist unconstrained and the upper limb in four positions; shoulder neutral with elbow flexed to 90° and shoulder flexed to 90° with elbow extended, both with forearm pronated and neutral. In order to evaluate the contribution of individual muscles EMG amplitudes for individual muscles were expressed as a proportion of the sum of EMG amplitudes for all muscles (e.g. ECRB EMG/sum of all EMG). Expression of each muscle’s activity as a proportion of total activity allowed evaluation of our hypothesis.
Results People with LE demonstrated a greater contribution in EDC and FDP (mean difference 10.4% [95% CI 2.9 to 17.9], and 11.1% [2.5 to 19.7], respectively), and a lower contribution in ECRB than controls (–14% [–24.5 to –3.5]), with gripping in all positions (see Figure 1). For both groups, EDC contributed more activity when the forearm was pronated than neutral (2.9%, [0.6–5.2]), and FDS contributed significantly more activity with forearm in neutral than pronated (–3.9% [–5.5 to –2.4]). FCR contributed significantly more activity with elbow in flexed than extended (4.9% [1.8–8.1]).
Discussion These findings indicate that the relative contribution of EDC to gripping is greater and that of ECRB is less in people with LE. This implies a greater contribution than ECRB to control the wrist position during gripping in people with LE than pain-free individuals (i.e. to overcome the finger flexor torque). This apparent redistribution of muscle activity might provide short-term benefit to limit contraction of ECRB and reduce pain provocation. Subtle differences in muscle activity with changes in limb position can be explained by changes in the orientation relative to gravity. The findings of this study demonstrate differences in the coordination of the forearm flexors and extensor muscles in people with and without LE. These results may suggest consideration of individual muscle activation could be important for design of physical rehabilitation.
References 1 Alizadehkhaiyat, et al. J Orthop Res. 2007;25:1651–1657
2 Bisset, et al. Arch Phys Med. 2006;87:490–495
3 Snijders, et al. MSSE. 1987;19:518–523