The purpose of this research was to determine consequences of repeated impacts during running. Part I. This study investigated midsole hardness influences on mechanics and hematology during a prolonged downhill run. Twenty-four males ran downhill (−12%, 3.4 m·s−1, 30 minutes) wearing soft, medium or hard midsoles (40, 55, or 70 Shore A). Mean peak tibial acceleration (PTA) was calculated every five minutes. Plasma free hemoglobin (PfHb), hemoglobin (Hb), hematocrit (Hct), and creatine kinase (CK) were analyzed pre- and post-exercise. PTA was initially less (p < 0.05) and tended to be less (p = 0.057) In the soft versus the hard group. Hemolysis and muscle damage resulted. Hard midsoles increased shock and may prolong hemolysis and increase muscle damage. Part II. This study investigated impact shock attenuation, joint kinematics, muscle activation and oxygen consumption during a prolonged run. Ten males ran downhill (−12% grade, 3.4 m·s−1 , 30 minutes). Accelerometers sampled shock data. Joint kinematics and oxygen cost (O2) were collected. Electromyography data (EMG) were collected from six muscles. Shock magnitude, high frequency power and attenuation remained constant. Joint geometry was modified while peak joint velocities increased (p < 0.05). EMG timing was altered (p < 0.05; gluteus maximus, tibialis anterior and gastrocnemius). EMG activation increased (p < 0.05; rectus femoris, vastus lateralis) and O2 increased (p < 0.05). Shock may have remained constant by modifying joint geometry, increasing peak joint velocities, varying muscle timing, and increasing muscle activation and energy cost. Part III. This study investigated midsole hardness and surface stiffness influences on impact shock, joint kinematics, muscle activation and oxygen cost. Twelve males ran in six conditions, combinations of midsole hardness (40 and 70 Shore A) and surface stiffness (100 kN·m −1, 200 kN·m−1, 350 kN·m −1). Accelerometers sampled shock data. Joint kinematics and O2 were collected. EMG was collected from six muscles. Shock magnitude, the power of high frequencies, and attenuation increased (p < 0.05) with surface stiffness regardless of midsole. Peak joint velocities increased (p < 0.05) with increasing surface stiffness while O2 decreased (p < 0.05). Muscle activation levels decreased (p < 0.05; gluteus maximus, biceps femoris and gastrocnemius). Shock was attenuated by increasing peak joint velocities without a concomitant increase in energy cost or muscle activation.

Human performance will be altered by changes in movement speed. The inertial properties of human limbs may play an important role in these alterations. The effects of these changes may be observed with the measurement of joint and segmental mechanics, as well as muscular kinematics, kinetics and muscular activity patterns. In this study, the cycling motion was used to investigate these inertial effects, following the development of a new mechanical model that provided the theoretical basis of coordination changes with movement speed. The alteration of cycling performance with different pedaling speed was examined using: (1) Surface EMG as an indication of the changes in muscular coordination as a function of cadence; (2) Inverse dynamics and decomposition of mechanical parameters to identify the influence of gravitational, inertial, and external factors; and (3) Simulations via a musculoskeletal modeling approach to assess the contributions of individual muscles. As predicted by the theoretical model, an increase in pedaling speed produced greater changes at the hip joint compared to knee and ankle joint in both muscular activities and mechanical measures. The changes in muscular activity were evident in both the activity of the single joint hip extensor and the coordination among the synergistic muscles. The altered muscular activities with increased cadence were accompanied by changes in joint moments, in the order of hip, knee and ankle joint from greatest to smallest. Further, the responses in movement organization were not linearly related to the increased inertial influence as the pedaling speed increased. Finally, the simulation analysis demonstrated a compensatory relation between gastrocnemius and soleus muscular activities with different pedaling speeds, although the combined patterns of the two were consistent.