DERRICK, T. R., D. DEREU, and S. P. MCLEAN. Impacts and kinematic adjustments during an exhaustive run. Med. Sci. Sports Exerc., Vol. 34, No. 6, pp. 998–1002, 2002. 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. 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. The knee became significantly more flexed at heel impact (start: 164.9 ± 2.3°; end: 160.5 ± 2.9°;P < 0.05). The rearfoot angle became more inverted at impact (start: 12.2 ± 1.6°; end: 13.6 ± 1.9°;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. 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.

The foot-ground impact experienced during running produces a shock wave that is transmitted through the human skeletal system. This shock wave is attenuated by deformation of the ground/shoe as well as deformation of biological tissues in the body. The goal of this study was to investigate the locus of energy absorption during the impact phase of the running cycle. Running speed (3.83 m·s-1) was kept constant across five stride length conditions: preferred stride length (PSL), +10% of PSL, -10% of PSL, +20% of PSL, and -20% of PSL. Transfer functions were generated from accelerometers attached to the leg and head of ten male runners. A rigid body model was used to estimate the net energy absorbed at the hip, knee, and ankle joints. There was an increasing degree of shock attenuation as stride length increased. The energy absorbed during the impact portion of the running cycle also increased with stride length. Muscles that cross the knee joint showed the greatest adjustment in response to increased shock. It was postulated that the increased perpendicular distance from the line of action of the resultant ground reaction force to the knee joint center played a role in this increased energy absorption.

Human locomotion has been modeled as a force-driven harmonic oscillator (FDHO). The minimum forcing function in locomotion has been shown to occur at the resonant frequency of the FDHO and results in the suggestion that oxygen cost may be considered an optimality criterion for locomotion. The purposes of this study were twofold: first, to determine the relationship between stride frequency and shock attenuation, and second, to determine whether shock attenuation may also be considered an optimality criterion. Ten healthy young adult males served as subjects in this study. Each subject's preferred running speed and preferred stride frequency (PSF) were determined. In addition to the PSF, they ran at stride frequencies corresponding to −20%, −10%, +10%, and +20% of the PSF at the preferred running speed. Metabolic data as well as leg and head acceleration data were collected during a steady state run at each of the stride frequency conditions. The metabolic data produced a U-shaped curve hypothesized by the FDHO model. Spectral analysis on the leg and head acceleration data were used to develop transfer functions for each of the stride frequency conditions. Analysis of the transfer function indicated that there was a gain at the low frequencies and an attenuation at the higher frequencies. The transfer function at the higher frequencies indicated that the impact shock signal was attenuated as it passed through the body. However, the transfer functions appeared to vary according to the amount of shock input to the system with the result that the head accelerations remained constant. It would appear that impact (high frequency) shock attenuation increases with stride frequency and thus does not fit the FDHO model as an optimization criterion. At all stride frequencies, regardless of the impact shock, head accelerations were maintained at a constant level.

DERRICK, T. R. The Effects of Knee Contact Angle on Impact Forces and Accelerations. Med. Sci. Sports Exerc., Vol. 36, No. 5, pp. 832–837, 2004. This article will summarize findings from several studies that together allow: 1) the examination of the effect that knee contact angle has on the

The fatigue life of bone is inversely related to strain magnitude. Decreasing stride length is a potential mechanism of strain reduction during running. If stride length is decreased, the number of loading cycles will increase for a given mileage. It is unclear if increased loading cycles are detrimental to skeletal health despite reductions in strain. To determine the effects of stride length and running mileage on the probability of tibial stress fracture. Ten male subjects ran overground at their preferred running velocity during two conditions: preferred stride length and 10% reduction in preferred stride length. Force platform and kinematic data were collected concurrently. A combination of experimental and musculoskeletal modeling techniques was used to determine joint contact forces acting on the distal tibia. Peak instantaneous joint contact forces served as inputs to a finite element model to estimate tibial strains during stance. Stress fracture probability for stride length conditions and three running mileages (3, 5, and 7 miles·d−1) were determined using a probabilistic model of bone damage, repair, and adaptation. Differences in stress fracture probability were compared between conditions using a 2 × 3 repeated-measures ANOVA. The main effects of stride length (P = 0.017) and running mileage (P = 0.001) were significant. Reducing stride length decreased the probability of stress fracture by 3% to 6%. Increasing running mileage increased the probability of stress fracture by 4% to 10%. Results suggest that strain magnitude plays a more important role in stress fracture development than the total number of loading cycles. Runners wishing to decrease their probability for tibial stress fracture may benefit from a 10% reduction in stride length.