Bilateral Deficit in Exercise and Sport: A Narrative Review.

Uysal, AA, Bereket Yücel, S, Stone, MH, and Carroll, K. Bilateral deficit in exercise and sport: A narrative review. J Strength Cond Res 40(7): 843-873, 2026-The primary purpose of this review was to conduct a comprehensive, up-to-date discussion on bilateral deficit (BLD) and to explore the underlying mechanisms of BLD in various conditions, such as during isometric and dynamic exercises. The secondary aim of the investigators was to examine the relationship between BLD and athletic performance. Although the physiological mechanisms of BLD are not fully understood, interhemispheric inhibition has been considered to be the primary neurological cause. In addition, biomechanical factors associated with BLD include postural differences, changes in movement patterns, variations in the contribution of synergist muscles, and differences in force output due to different shortening velocities between unilateral and bilateral actions. Bilateral deficit is more commonly detected in dynamic than in isometric contractions, with 50 of 63 dynamic contraction studies reporting BLD compared with 45 of 80 in isometric studies. In addition to the studies reporting the presence of BLD, 6 dynamic and 14 isometric contraction studies reported both BLD and no BLD within the same study, depending on the movement angle, limb side, and different subject categories. Eleven studies were investigations of the effect of unilateral or bilateral training on BLD; 10 of these reported statistically significant effects of training specificity. In other words, bilateral training reduced BLD, and unilateral training increased BLD. Nevertheless, there have been contradictory results in the literature on the relationship between BLD and athletic performance. Future studies may equalize external loads in each limb and increase stability of the multijoint exercises to assess the relationship between BLD and athletic performance.