Latissimus Dorsi Activation During Shoulder Extension Exercises

During shoulder abduction or flexion, narrowing of the subacromial space can lead to subacromial pain syndrome. Weakness or poor coordination of the scapulothoracic and scapulohumeral muscles is among the main causes of subacromial narrowing in individuals with symptomatic rotator cuff tendinopathy. More specifically, insufficient scapular upward rotation and posterior tilt, together with an inadequate ability of the rotator cuff to counter the deltoid's superior pull on the humeral head, may cause impingement of the subacromial soft tissues during overhead dynamic tasks. In young, healthy shoulders, the cranially directed forces that occur during abduction are balanced by co-contraction of the rotator cuff, which prevents superior translation of the humeral head and protects subacromial tissues. When this stabilizing role of the rotator cuff diminishes, the deltoid attempts to compensate; however, this compensation produces a more cranially oriented force, increasing superior migration of the humeral head and the likelihood of subacromial pain.

The latissimus dorsi (LD) originates from the spinous processes of the lower thoracic vertebrae, the thoracolumbar fascia, and the iliac crest, and-together with the teres major-attaches to the medial lip of the intertubercular groove of the humerus. It contributes to shoulder adduction, internal rotation, and extension, and is an important muscle directly linking the upper limb to the trunk. In addition to the rotator cuff, the glenohumeral adductors (pectoralis major and latissimus dorsi) also help limit superior translation of the humeral head; owing to the medio-inferior direction of their tendon force vectors, they act as humeral head depressors. Osteokinematically, the LD can pull the humeral head inferiorly over the glenoid fossa, potentially helping to prevent subacromial impingement.

Various exercises have been recommended for LD rehabilitation, and surface electromyography (sEMG) studies have examined LD activity during movements such as pulldown and pullover. Numerous studies have also sought to determine maximal voluntary isometric contraction (MVIC) of the LD. Prior EMG research indicates that the highest LD MVIC levels are obtained during maximal isometric shoulder extension. A recent study using both surface and fine-wire electrodes also recorded higher LD activation during shoulder extension compared with trunk tasks. Although shoulder extension exercises are widely used at different elevation angles in clinical practice, the effects of performing extension at different angles-particularly on LD activation-have not been clearly delineated. Despite various recommendations for LD rehabilitation, it remains unclear whether different implementations produce different activation profiles, and LD activation during dynamic application of this exercise has not yet been investigated. Therefore, it is important to examine how the highest LD activity obtained during maximal isometric shoulder extension changes when the exercise is performed dynamically. Defining the LD activation profile in dynamic exercise may inform appropriate exercise selection in clinical rehabilitation and help reduce the risk of subacromial impingement.

Aim. To address this gap by examining the level of LD activation during dynamic shoulder extension relative to the reference activity determined by maximal isometric shoulder extension. The study has two specific aims: (1) to determine the EMG activation level of the latissimus dorsi during a shoulder extension exercise, and (2) to investigate the effects of performing shoulder extension at different elevation angles on latissimus dorsi activation.

• Activity Level [Time frame: Baseline (Day 1)]