Impacts and kinematic adjustments during an exhaustive run.

PURPOSE To examine the kinematic adjustments that runners make during an exhaustive run and to look at the effects these adjustments have on shock and shock attenuation. METHODS Ten recreational runners ran to volitional exhaustion on a treadmill at a velocity equal to their average 3200-m running velocity at maximal effort (average time: 15.7 +/- 1.7 min). Head and leg accelerometers, a knee electrogoniometer, and a rearfoot electrogoniometer were attached to each subject. The data were sampled at 1000 Hz at the start, middle, and end of the run. RESULTS The knee became significantly more flexed at heel impact (start: 164.9 +/- 2.3 degrees; end: 160.5 +/- 2.9 degrees; P < 0.05). The rearfoot angle became more inverted at impact (start: 12.2 +/- 1.6 degrees; end: 13.6 +/- 1.9 degrees; P < 0.05). These kinematic changes resulted in a lower extremity that that had a lower effective mass during the impact. This decreased effective mass allowed the leg to accelerate more easily; thus, peak leg impact accelerations (start: 6.11 +/- 0.96 g; end: 7.38 +/- 1.05 g; P < 0.05) and impact attenuation (start: 74.5 +/- 5.4%; end: 77.5 +/- 4.1%; P < 0.05) increased during the progression of the run. CONCLUSIONS The increase in peak impact accelerations at the leg was not considered an increased injury risk because of the decreased effective mass. The altered kinematics may have resulted in increased metabolic costs during the latter stages of the exhaustive run.