The latissimus dorsi, commonly known as the lats, is the broadest muscle of the back, stretching from the lower thoracic and lumbar spine, the iliac crest, and the lower ribs, up to the humerus bone in the upper arm. This muscle is primarily responsible for the powerful movements of arm extension, adduction (pulling the arm toward the body), and internal rotation, making it fundamental for pulling motions like rowing or climbing. When one lat muscle appears visibly larger or more developed than the other, this muscular asymmetry is rarely a sign of serious health concern. It typically reflects a difference in how the two sides of the body are used or trained.
Behavioral and Training Habits That Create Imbalance
The most frequent cause of lat size difference stems from the preferential use of one side during everyday activities and structured exercise. Almost everyone possesses a dominant side, which is naturally stronger and more coordinated for tasks such as carrying a heavy bag, reaching, or throwing. This constant, asymmetrical engagement of the dominant-side lat leads to slight, chronic overdevelopment compared to the non-dominant side, creating a functional imbalance over time.
These daily habits are often compounded by errors in training form, particularly during bilateral exercises where both sides work simultaneously, such as pull-ups or barbell rows. When fatigue sets in during these movements, the stronger lat will involuntarily compensate for the weaker side, taking on a greater percentage of the load to complete the repetition. This compensatory action perpetuates the imbalance, as the stronger side receives more growth stimulus while the weaker side continues to be undertrained.
A disparity in the mind-muscle connection, the conscious focus on contracting the target muscle, is another factor. Individuals often find it easier to “feel” or neurologically activate their dominant lat during a set. If the weaker side has a poor connection, the muscle fibers do not receive optimal neural drive, resulting in sub-maximal activation and reduced potential for hypertrophy. This lack of focused intent can further hinder the development of the smaller lat.
Underlying Structural and Anatomical Contributors
While training habits are the primary cause, some asymmetries can be traced to structural and anatomical variations. Skeletal alignment issues, such as mild, undiagnosed scoliosis, can shift the origin and insertion points of the latissimus dorsi muscles. This mechanical shift places one lat at a biomechanical disadvantage, forcing it to work from a suboptimal length-tension relationship, which can hinder its development relative to the other side.
Uneven hip or shoulder height, often related to leg length discrepancy or postural habits, also creates an asymmetrical base of support for the lats. When the torso is not perfectly aligned, the muscles attaching the arm to the spine and pelvis are forced to contract unevenly to stabilize the body during movement. This chronic uneven stabilization leads to a different workload and, consequently, a size disparity.
Previous injuries can also play a role, particularly those involving the shoulder, rib cage, or the nerve pathways that innervate the lats. Nerve pathways controlling the latissimus dorsi can be compromised by injuries, leading to reduced muscle activation or atrophy on the affected side. Furthermore, slight congenital or genetic variations in the muscle belly or tendon insertion point can create a perceived size difference that is difficult to completely eliminate through training alone. If the asymmetry is sudden, rapidly worsening, or accompanied by pain, a medical professional should be consulted to rule out nerve impingement or underlying structural issues.
Actionable Strategies for Correcting Asymmetry
The most effective strategy for correcting lat asymmetry is to temporarily prioritize unilateral training movements. Replacing bilateral exercises like barbell rows with single-arm variations, such as dumbbell rows or single-arm cable pulldowns, ensures that each lat receives an independent and measurable workload. This eliminates the opportunity for the dominant side to compensate for the weaker side.
When performing unilateral work, it is important to always begin the set with the smaller or weaker lat, as this is when focus and energy levels are highest. The goal is to perform a controlled, high-quality set with the weaker side, and then only match that performance—in terms of weight and repetitions—with the stronger side. The stronger side should never exceed the performance of the weaker side, preventing the gap from widening further.
A targeted adjustment to training volume can encourage the smaller lat to catch up. This involves adding one or two extra sets exclusively for the underdeveloped lat after the standard work has been completed. This slight increase in volume and time under tension provides a greater stimulus for hypertrophy on the targeted side.
Integrating specific mobility and activation drills into the warm-up is important for improving the mind-muscle connection with the smaller lat. Exercises like light, high-repetition single-arm pullovers or using an internal cue like “squeeze the elbow toward the hip” can enhance neural drive. Focusing on a slow, controlled tempo, especially during the eccentric phase of the lift, ensures that the targeted lat is fully engaged throughout the movement.