 Efferent motor activity during standing. During the testing the research participants will be placed on the treadmill upright suspended in a harness by an overhead cable at the maximum load at which knee buckling and trunk collapse can be avoided i.e. BWST. All participants will stand at their optimized body weight load BWL and joint kinematics and limb load will be consistently maintained by manual assistant by trainers. All trainers are careful to provide manual assistance only when needed. A trainer positioned behind the research participant will aid in pelvis and trunk stabilization, by applying anterior forces at the pelvis andor posterior forces at the shoulders, ensuring that the trunk and pelvis are not flexed or hyper-extended. Research participants will take a break and rest at any time they feel the need to during the session. 