Hamstring strain injuries are common to sports that involve high speed running. Rehabilitation remains challenging as evidenced by the high recurrent injury rate, however little is know about how the hamstrings behave during normal running gait. In addition, there is disagreement whether the hamstrings are susceptible to injury during late swing phase when the hamstrings are active and lengthening or during stance when the contact loads are high, potentially overloading the hamstrings to cause injury. The aims for this thesis were to (1) Investigate how hamstring musculotendon mechanics during swing phase vary with sprinting speed. (2) Investigate how the "core" muscles influence hamstring stretch during sprinting. (3) Develop a least squares forward dynamics methodology for computing joint mechanics during sprinting on an un-instrumented treadmill. (4) Systematically compare biomechanical demands between stance and swing phase of the sprinting gait cycle to better understand potential injury mechanisms. Experimental measures for this study included whole body kinematics, ground reactions and electromyography (EMG) of selected lower extremity muscles, while subjects ran at speeds of 80 percent to maximal running speed on a high speed treadmill. Musculotendon actuators were represented as a series of line segments connecting origin to insertion. A Hill-type model of musculotendon contraction dynamics was assumed. Computed muscle control determined the excitations necessary to drive the experimental hip and knee kinematics. Muscle-actuated forward dynamic simulations of the sprinting gait cycle were then generated. Estimates of hamstring musculotendon stretch, force and work were obtained from the simulations of high speed running. The major findings from this research were that the hamstrings seem most likely susceptible to a strain injury during the late swing phase of high speed running and other muscles surrounding the pelvis and low back (i.e. neuromuscular coordination) have substantial influence on hamstring stretch. In addition, the hamstrings are uniquely susceptible to injury because peak loading occurs when they are lengthening and negative work is solely confined to the swing phase of gait. Training regimes with focus on improving hamstring function by utilizing lengthening contractions and neuromuscular coordination could dramatically reduce re-injury rates and be effective in preventing initial injury.
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