Regional Activation of the Latissimus Dorsi Muscle by High-Density Surface Electromyography During Isometric Shoulder and Trunk Exercises

Abstract

Background:

The latissimus dorsi (LD) comprises 2 neuromuscular regions—the thoracic and lumbar-pelvic-costal (LPC)—that may exhibit distinct activation patterns during shoulder and trunk exercises. A detailed understanding of these regional differences is crucial for optimizing therapeutic interventions due to their potential impact on muscle imbalances and rehabilitation outcomes in people with musculoskeletal conditions.

Hypothesis:

Increasing contraction levels are expected to amplify differences between the thoracic and LPC regions, with the thoracic region demonstrating greater activation, particularly during shoulder extension and internal rotation tasks.

Study Design:

Descriptive laboratory study.

Level of Evidence:

Level 5.

Methods:

A total of 20 healthy participants (15 men and 5 women) were recruited. High-density surface electromyography (HD-sEMG) electrode grids were positioned over the LD. Participants performed isometric contractions for shoulder extension, adduction, internal rotation, and lateral trunk bending at 10%, 30%, 50%, 70%, and 100% of their maximum voluntary contraction (MVC). The variables assessed included sEMG amplitude for each region and the spatial distribution of muscle activation (x- and y-axis barycenter).

Results:

During shoulder extension, the thoracic region showed greater sEMG amplitudes than the LPC region at 50% (P = 0.02), 70% (P = 0.01), and 100% MVC (P < 0.01). Furthermore, shoulder extension generated greater amplitudes than all other exercises across both regions (P < 0.05). Barycenter analysis revealed a significant cephalomedial shift in the x-axis during both shoulder extension and internal rotation.

Conclusion:

The thoracic region is preferentially recruited during isometric shoulder extension and internal rotation. This spatial redistribution of muscle activity suggests a functional specialization within the LD, which has important implications for the development of targeted shoulder rehabilitation strategies.

Clinical Relevance:

Exercises emphasizing shoulder extension may preferentially activate the thoracic region of the LD, thereby enhancing scapular stability while reducing strain on the rotator cuff tendons.