Control and dynamics during horizontal impulse generation

Human movement reflects continual interaction between the nervous system (control), the musculoskeletal system (dynamics), and the environment (reaction forces) operating in a closed-loop (feedback) configuration. Momentum generation during human locomotion involves control of the total body center of mass relative to the feet so that the linear and angular impulse generated during foot contact acts in a direction consistent with the task objective. The purpose of this study was to investigate how horizontal impulse (HI) is generated during the first step of the sprint start. We hypothesized the magnitude of HI generated during foot contact is influenced by the rate of horizontal ground reaction force development (RHFD), faster RHFD are associated with more anterior initial center of mass (CM) position along with increased positive work done at the ankle and knee, and initial CM position is influenced by EMG recruitment prior to touchdown used to control the leg segment. Ground reaction forces (Kistler, 1200Hz), 2D sagittal plane kinematics (NAC C2S, 200Hz), and surface EMG of eight muscles of the support leg (Konigsburg Instruments, 1200Hz) were collected simultaneously during sprint starts performed by national level, multi-event athletes. RHFD was related to force-time characteristics of HI generation, but did not significantly influence HI magnitude during foot contact. Faster RHFD was associated with smaller r-angles at touchdown, decreased shank segment angular velocity, and earlier time to peak thigh segment angular velocity during foot contact. Lower extremity NJM demand during the impact phase was influenced by initial leg segment orientation. RHFD was not found to be significantly related to lower extremity NJMW distribution, which indicates multiple NJMW distribution strategies can be used to generate the net horizontal impulse necessary to achieve the task. Initial CM position and CM trajectory during foot contact were controlled with leg segment motion, which resulted from activation of uni- and bi-articular extensor muscles of the leg. No consistent muscle activation patterns could be related to initial CM orientation or trunk-leg coordination used to control CM trajectory during contact, which indicates multiple subsystem coordination strategies may be used to achieve CM trajectory necessary for satisfying task demands.