Biomechanical effects of load-carrying in adolescent idiopathic scoliosis: a subject-specific musculoskeletal modelling study

Load carrying is a common activity during adolescence, yet its biomechanical consequences for the structurally asymmetric spine in adolescent idiopathic scoliosis (AIS) remain poorly understood. This study investigated how load magnitude, carrying height, and side of application influence spinal loading and paraspinal muscle forces in AIS. A subject-specific musculoskeletal model of a 16-year-old female with a 59° right-convex thoracic curve was used to simulate bilateral and unilateral load carrying at the hand, chest, and above-shoulder levels, with loads corresponding to 5%, 10%, and 15% of body weight. Intervertebral compressive and lateral forces and erector spinae (ES) and multifidus (MF) muscle forces were evaluated. Spinal loading increased progressively with load magnitude and carrying height. At 15% body weight, compressive forces at the curve apex increased from 511-566 N at hand level to 721-831 N above the shoulder. Unilateral carrying produced distinct side-dependent responses: convex-side carrying generated greater compressive forces, whereas concave-side carrying generated greater lateral forces, which were concentrated around the curve apex. Paraspinal muscles also exhibited muscle-specific responses. ES forces primarily reflected the imposed external asymmetry, with unilateral carrying eliciting predominantly contralateral recruitment, whereas MF maintained a characteristic convex-side dominance concentrated around the curve apex. These findings demonstrate that load carrying in AIS produces a coupled biomechanical response governed by both the external loading configuration and the pre-existing structural asymmetry of the spine, highlighting the importance of considering loading direction and muscular adaptation alongside load magnitude when evaluating everyday carrying activities in AIS.