Shoulder muscle activity after latissimus dorsi transfer in an active elevation

Area of Science:

• Orthopedics
• Biomechanics
• Musculoskeletal Rehabilitation

Background:

• Latissimus dorsi transfer (LDT) is used for massive irreparable rotator cuff tears, often resulting in increased scapulothoracic (ST) contribution to shoulder elevation.
• The specific shoulder muscles and their recruitment timing responsible for this altered ST contribution post-LDT remain unclear.

Purpose of the Study:

• To identify shoulder muscles and their recruitment timing contributing to increased ST contribution and shoulder elevation after LDT for massive irreparable posterosuperior rotator cuff tears (MIRT).
• To compare shoulder kinematics and muscle activation between LDT-treated and asymptomatic contralateral shoulders.

Main Methods:

• Thirteen patients with MIRT and preoperative pseudoparalysis underwent LDT and were assessed after a minimum 1-year follow-up.
• Three-dimensional electromagnetic tracking measured maximum active elevation of the shoulder (MAES) in LDT and asymptomatic contralateral shoulders (ACS).
• Surface electromyography (EMG) recorded activation (% EMG max) and timing for latissimus dorsi (LD), deltoid, teres major, trapezius (upper, middle, lower), and serratus anterior muscles during MAES.

Main Results:

• No significant difference in overall thoracohumeral motion was found between LDT and ACS shoulders during MAES.
• Glenohumeral motion was significantly lower in LDT shoulders compared to ACS.
• Latissimus dorsi (LD) muscle activation (% EMG max) did not differ, but deltoid and upper trapezius showed significantly higher activation in LDT shoulders.
• Middle trapezius showed higher activation during scapular abduction in LDT shoulders.
• No differences in lower trapezius, serratus anterior, or teres major activation, nor in the timing of recruitment for any muscle, were observed between groups.

Conclusions:

• Post-LDT for MIRT, the latissimus dorsi (LD) muscle retains its native activation levels during maximum active elevation of the shoulder (MAES).
• Increased scapulothoracic (ST) contribution is likely due to the native LD function combined with augmented activation of the deltoid and trapezius muscles.
• This compensatory muscle activation pattern may increase glenohumeral joint reaction forces, potentially enhancing active elevation in previously pseudoparalytic shoulders.