Characteristics of the electrophysiological activity of muscles attached to the transverse carpal ligament in carpal tunnel syndrome

Comparative electromyographic study of the stabilizing muscles of the trapeziometacarpal joint during different rehabilitation exercises

BACKGROUND

The first dorsal interosseous (FDI) and opponens pollicis (OP) muscles are the main stabilizers of the trapeziometacarpal joint. Optimal exercise selection based on surface electromyography (sEMG) activity may help to optimize rehabilitation programs in hand therapy.

PURPOSE

To compare the sEMG activity of FDI and OP during seven typical rehabilitation exercises.

STUDY DESIGN

Cross-sectional.

METHODS

Twelve healthy participants (age: 25.92 ± 5.79, female/male: 6/6) were included. The sEMG activity of the FDI and OP was measured during seven therapeutic exercises; thumb-to-little fingertip pinch, tri-digit tip pinch, intrinsic plus fist with ball, key pinch, four- and five-finger separation with elastic resistance, and thumb-index tip pinch. Root mean square was used and adjusted to the maximum voluntary contraction (MVC) of each muscle. Friedman's test with Dunn's post-hoc was used to compare amplitude signals between exercises.

RESULTS

Intrinsic plus fist with ball showed the highest electromyographic amplitude for FDI and OP. Post-hoc analysis revealed a statistical difference between intrinsic plus fist with ball (45.67% of MVC) and four-finger separation (8.70% MVC, p < 0.0001) for the OP. While for FDI, post-hoc analysis showed statistical differences between intrinsic plus fist with ball (49.70% MVC) and four hand exercises (thumb-to-little fingertip pinch = 12.13% MVC, p < 0.0001; tri-digit tip pinch = 13.21% MVC, p < 0.0001; four-finger separation = 18.51% MVC, p = 0.004; thumb-index tip pinch = 17.66% MVC, p = 0.0028).

DISCUSSION/CONCLUSION

Intrinsic plus fist with ball exercise showed the highest sEMG activation. These findings could guide hand therapists in the optimal selection of exercises, taking into account the level of muscle activity in routine rehabilitation exercises.

Biomechanical interaction between the transverse carpal ligament and the thenar muscles

The transverse carpal ligament (TCL) serves as the origin of the thenar muscles and is integral to thenar muscle contraction anatomically and biomechanically. TCL hypertrophy has been observed in patients with carpal tunnel syndrome and is potentially caused by repetitive hand use. The purpose of this study was to investigate the biomechanical interaction between the TCL and the thenar muscles. Specifically, the morphological changes of the carpal arch, formed by the TCL, in response to thenar muscle contractions were examined during isometric tip pinch between the thumb and index finger. Ultrasound videos of the carpal tunnel were recorded from 13 healthy subjects and were synchronized with the forces measured by a pinch dynamometer. The thenar muscles' ulnar point, trapezium, and hamate were tracked by a pattern-matching program. The pinch force significantly affected the carpal arch height, width, and area (P < 0.005). As the pinch force increased from 0 to 100% maximum voluntary contraction force, the carpal arch height increased from 1.8 ± 1.0 to 2.3 ± 1.3 mm, the carpal arch width decreased from 23.9 ± 2.4 to 23.1 ± 2.4 mm, and the carpal arch area increased from 22.2 ± 13.6 to 27.3 ± 16.3 mm(2). The TCL was pulled volarly during thenar muscle contractions, providing evidence for the biomechanical interaction between the ligament and muscles. Repetitive biomechanical stimulation on the TCL from thenar muscle contractions could lead to tissue remodeling and then TCL hypertrophy. This study sheds light on the potential cause of TCL hypertrophy, which may be an etiological factor for carpal tunnel syndrome.