Note: This work is currently under manuscript preparation so data will not be presented. Preliminary work for this study was presented at the 44th Annual Meeting for the American Society of Biomechanics.
Overview: Moment arms are often linked to locomotor specialization due to their influence on torque generation, muscle length changes, and muscle shortening velocity. For example, animals adapted for sprinting (cheetas, greyhounds, human sprinters) were found to have small moment arms in their limbs, while animals known for digging tend to have larger moment arms. Small moment arms impair torque generation, but reduce muscle fiber strain and shortening velocity, which are beneficial for power production and rapid limb swing. Large moment arms, meanwhile, amplify a given muscle force to increase torque generation, and thus, are better suited to situations where strength is more important than speed. However, whether moment arms are immutable in response to exercise is largely unknown. Moment arm length is largely determined by joint structure, whose components of muscle and bone are known to be plastic in reponse to loading. Therefore, the purpose of this study was to investigate moment arm adaptability in response to loading history during growth.
In this study, 24 guineafowl were split evenly into 3 groups at 2 weeks-old. An exercise (EXE) group was stored in a large pen that allowed to ample opportunity to run aroud and jump to perches. A sedentary (SED) group was house in a small pen with a low ceiling to discourage movement, including jumping. The third group, called the botox (BTX) group, were house similarly to the SED birds, but also received bilateral intramuscular injections of botox into their plantarflexors to induce paralysis (i.e. muscle unloading). EXE and SED birds received injections of a saline solution instead. Injections were started at 7-8 weeks of age, and were given every 5 weeks for 20 weeks until birds had reached ~6 months of age. Birds were then sacrificed and the left hindlimb was isolated, with all musculature except plantarflexor tendon removed. Moment arms were measured using a tendon travel protocol, which is based on the concept of virtual work. This method assumes that joint work and muscle work are equal, so that moment arms are the ratio of the linear displacment of the dnon to the joint angle displacment (force is equal). Retroreflective markers were attached to the bones in order to use 3D motion capture to track joint movement. A linear transducer was attached to the tendon to measure linear displacement and to keep the tendon under constant tension. Each bird limb underwent 3 trials of 8-10 flexion/extension cycles. Linear vs. angular displacement plots were generated from data compiled across all 3 trials, and a 3rd order polynomial was fit to the data to calculate moment arms. A linear mixed model was used to test for group differences, as well as any group x angle interactions. Moment arm analyses were performed in MATLAB, while statistical analyses were run in R.
