The load-carrying capacity of the spine has been experimentally shown to increase substantially when the resultant force of all loads applied on the spine is directed in accordance to its curvature, which is called a 'follower load (FL)'. However, there have been few studies to investigate the muscle forces producing the FL owing to the difficulty of the relevant experimental measurements. This study investigated whether trunk muscle activations could be found for transmitting an external load within a range of FL direction. A two-dimensional finite element model of a lumbar spine and 117 pairs of trunk muscles was developed in the sagittal plane. An optimization technique was utilized to estimate the muscle forces generating the FL and the corresponding responses of the lumbar spine under two loading cases: the upright neutral standing posture and the posture holding 200 N in the hands. For each loading case, the muscle forces required to generate the FL could be found and the corresponding responses of the lumbar spine validated that the FL could increase the load-carrying capacity. The results showed that the FL could be produced in vivo by trunk muscles to increase the load-carrying capacity.