Energy absorption of impacts during running at various stride lengths.

PURPOSE: 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. METHODS: Running speed (3.83 m x 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. RESULTS: 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. CONCLUSION: 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.

Impact shock and attenuation during in-line skating.

Although impact and shock attenuation associated with foot contact during running has been studied extensively, much less is known about these phenomena during in-line skating (ILS). The purpose of this study was to describe these impact characteristics for ILS and to test the hypothesis that there is lower impact shock during ILS than in running at preferred velocities. Subjects ran and skated on a treadmill at preferred velocity, with low-mass accelerometers attached to both the distal tibia and head. Tibia and head acceleration data during stance were used to calculate peak acceleration (PA), peak frequency (PF), and median frequency (MedF). Impact attenuation (IA) between the tibia and head was quantified by comparison of PA values and by calculating a transfer function between the head and tibia frequency spectra. PA, PF, and MedF values were significantly lower in ILS than in running for both tibial and head data, whereas ILS was similar between the two movements. The ILS condition exhibited almost no power in the frequency range between 10 and 20 Hz, which has been associated with the foot impacting the ground. It is concluded that in-line skating results in less impact shock to the body with each foot contact, and may be a useful exercise modality for those wishing to reduce impact shock during aerobic training.

The relationship between preferred and optimal positioning during submaximal cycle ergometry.

This study was designed to determine how changes in oxygen uptake (VO2) and heart rate (HR) during submaximal cycle ergometry were determined by changes in cycle geometry and/or lower-limb kinematics. Fourteen trained cyclists [Mean (SD): age, 25.5 (6.4) years; body mass 74.4 (8.8) kg; peak VO2, 4.76 (0.79) 1 x min(-1) peak] were tested at three seat-tube angles (70 degrees, 80 degrees, 90 degrees) at each of three trunk angles (10 degrees, 20 degrees, 30 degrees) using a modified Monark cycle ergometer. All conditions were tested at a power output corresponding to 95% of the VO2 at each subject's ventilatory threshold while pedalling at 90 rpm and using aerodynamic handlebars. Sagittal-view kinematics for the hip, knee, and ankle joints were also recorded for all conditions and for the subjects' preferred positioning on their own bicycles. No combination of seat-tube and trunk angle could be considered optimal since many of the nine conditions elicited statistically similar mean VO2 and HR values. Mean hip angle (HA) was the only kinematic variable that changed consistently across conditions. A regression relationship was not observed between mean VO2 or HR and mean hip angle values (P > 0.45). Significant curvilinear relationships were observed, however, between deltaVO2 (VO2 - minimum VO2) and deltaHA (mean HA - preferred HA) using the data from all subjects (R = 0.45, SEE = 0.13 1 x min(-1)) and using group mean values (R = 0.93, SEE = 0.03 1 x min(-1)). In both cases deltaVO2 minimized at deltaHA = 0, which corresponded to the subjects' preferred HA from their own bicycles. Thus, subjects optimized their VO2 cost at cycle geometries that elicited similar lower-limb kinematics as the preferred geometries from their own bicycles.

Evaluation of time-series data sets using the Pearson product-moment correlation coefficient.

The Pearson product-moment correlation has been used by researchers to compare time series data sets to assess the temporal similarities. Computer generated data, vertical ground reaction force (VGRF) data and hybrid data (constructed by combining features of computer generated and VGRF data) were used to investigate the influence of timing and amplitude differences on the effectiveness of this technique. Under a specific set of conditions the correlation coefficient is a valid and reliable indicator of temporal similarity. Deviations from these conditions, however, result in interactive effects between timing and amplitude components with subsequent reductions in the value of the coefficient. Although GRF data were evaluated, the results apply equally to other types of curves as well. The correlation coefficient is easy to use and can be used to evaluate the entire curve as opposed to discrete data points. Its usefulness is jeopardized, however, since it can be influenced by timing and amplitude differences as well as the characteristics of the curves being analyzed. A high coefficient is always indicative of temporal similarity but a lesser value does not guarantee a lack of temporal similarity.