Musculoskeletal attenuation of impact shock in response to knee angle manipulation. 
Shock waves resulting from the foot-ground impact are attenuated by biological tissues within the body.  It has been suggested that the primary site for shock attenuation is the knee joint.  The purpose of this study was to determine if knee flexion affects the filtering characteristics of the musculoskeletal system in response to impacts.  Impacts were delivered to 10 participants during inline skating on a treadmill at 2.0 m/s. Four knee angle conditions (0, 10, 20, and 30 degrees) were investigated using real-time visual feedback of motion capture data.  Shock attenuation between the leg and head was determined using accelerometry.  The cutoff frequency of the body was determined by progressive filtering of the leg acceleration until differences between head acceleration and filtered leg acceleration were minimized.  A nonlinear increase in shock attenuation (p < .001) and a nonlinear decrease in the cutoff frequency of the body (p < .001) were observed as the knee became more flexed.  These results suggest that the knee joint acts as a low-pass filter allowing greater shock attenuation with increased knee flexion.  Flexing the knee may shift the shock-attenuating responsibilities away from passive biological tissue toward active muscular contraction.