Timothy R. Derrick

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.

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.

The effects of knee contact angle on impact forces and accelerations

This article will summarize findings from several studies that together allow: 1) the examination of the effect that knee contact angle has on the severity of the resulting impact, 2) examination of the relationship between vertical ground reaction impact forces and leg impact accelerations, and 3) exploration of the adaptations that occur in response to running during changing environmental conditions. Changing the knee flexion angle at contact can alter the effective mass during activities in which the foot impacts the ground. It has been shown that increasing the knee flexion angle at ground contact can reduce the peak vertical ground reaction impact force, but it can also increase the peak impact acceleration at the leg. Attenuation can be calculated from accelerometers on the leg and the head and combined with the leg acceleration values to give a more accurate impression of the severity of the impact. Lower-extremity joint contact angles can be used to examine the kinematic adaptations that take place in response to changing environmental conditions. One common adaptation that can occur when the internal or external environment is not ideal is an increase in the knee flexion angle at contact. More extended knee contact angles can increase the forces experienced by the body and therefore increase injury potential. Increased knee flexion may give the runner a larger margin for dealing with kinematic errors but this benefit likely has an associated metabolic cost that will reduce performance.

Effects of stride length and running mileage on a probabilistic stress fracture model.

UNLABELLED 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. PURPOSE To determine the effects of stride length and running mileage on the probability of tibial stress fracture. METHODS 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 x 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 x 3 repeated-measures ANOVA. RESULTS 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%. CONCLUSIONS 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.

Lower extremity joint stiffness characteristics during running with different footfall patterns

Abstract The purpose of this study was to examine the knee and ankle joint stiffness and negative joint work during running when participants utilised their preferred and non-preferred footfall pattern. A total of 40 healthy, young runners (20 habitual forefoot (FF) and 20 habitual rearfoot (RF) runners) served as participants in this study. Three-dimensional data were obtained using a motion capture system and a force platform. The participants completed over-ground trials in each of two conditions: 1. their natural footfall pattern; and 2. their non-preferred footfall pattern. Joint stiffness was calculated by the ratio of the change in joint moment and the change in joint angle during the energy absorption phase of support. Negative joint work was calculated as the integral of the joint power-time curve during the same time interval. It was observed that joint stiffness was different between the footfall patterns but similar for both groups within a footfall pattern. A stiffer knee and a more compliant ankle were found in the FF pattern and the opposite in the RF pattern. Negative work was greater in the ankle and less in the knee in the FF pattern and the reverse in the RF pattern. We conclude that runners, in the short term, can alter their footfall pattern. However, there is a re-organisation of the control strategy of the joint when changing from a FF to a RF pattern. This re-organisation suggests that there is a possible difference in the types of injuries that may be sustained between the FF and the RF footfall patterns.

Joint contact loading in forefoot and rearfoot strike patterns during running

Research concerning forefoot strike pattern (FFS) versus rearfoot strike pattern (RFS) running has focused on the ground reaction force even though internal joint contact forces are a more direct measure of the loads responsible for injury. The main purpose of this study was to determine the internal loading of the joints for each strike pattern. A secondary purpose was to determine if converted FFS and RFS runners can adequately represent habitual runners with regards to the internal joint loading. Using inverse dynamics to calculate the net joint moments and reaction forces and optimization techniques to estimate muscle forces, we determined the axial compressive loading at the ankle, knee, and hip. Subjects consisted of 15 habitual FFS and 15 habitual RFS competitive runners. Each subject ran at a preferred running velocity with their habitual strike pattern and then converted to the opposite strike pattern. Plantar flexor muscle forces and net ankle joint moments were greater in the FFS running compared to the RFS running during the first half of the stance phase. The average contact forces during this period increased by 41.7% at the ankle and 14.4% at the knee joint during FFS running. Peak ankle joint contact force was 1.5 body weights greater during FFS running (p<0.05). There was no evidence to support a difference between habitual and converted running for joint contact forces. The increased loading at the ankle joint for FFS is an area of concern for individuals considering altering their foot strike pattern.

Internal femoral forces and moments during running: Implications for stress fracture development

BACKGROUND Femoral stress fractures tend to occur at the neck, medial proximal-shaft, and distal-shaft. The purpose of this study was to determine the internal femoral forces and moments during running. It was expected that larger loads would occur at these common sites of femoral stress fracture. METHODS Ten subjects ran at their preferred running speed over a force platform while motion capture data were collected. Static optimization in conjunction with a SIMM musculoskeletal model was used to determine individual muscle forces of the lower extremity. Joint contact forces were determined, and a quasi-static approach was used to calculate internal forces and moments along a centroid path through the femur. FINDINGS The largest mean peak loads were observed at the following regions: anterior-posterior shear, 7.47 bodyweights (BW) at the distal-shaft (posteriorly directed); axial force, 11.40BW at the distal-shaft (compression); medial-lateral shear, 3.75BW at the neck (medially directed); anterior-posterior moment, 0.42BWm at the proximal-shaft (medial surface compression); torsional moment, 0.20BWm at the distal-shaft (external rotation); medial-lateral moment, 0.44BWm at the distal-shaft (anterior surface compression). INTERPRETATION The mechanical loading environment of the femur during running appears to explain well the redundancy in femoral stress fracture location. We observed the largest internal loads at the three femoral sites prone to stress fracture.

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.

Running injury and stride time variability over a prolonged run

Locomotor variability is inherent to movement and, in healthy systems, contains a predictable structure. In this study, detrended fluctuation analysis (DFA) was used to quantify the structure of variability in locomotion. Using DFA, long-range correlations (α) are calculated in over ground running and the influence of injury and fatigue on α is examined. An accelerometer was mounted to the tibia of 18 runners (9 with a history of injury) to quantify stride time. Participants ran at their preferred 5k pace±5% on an indoor track to fatigue. The complete time series data were divided into three consecutive intervals (beginning, middle, and end). Mean, standard deviation (SD), coefficient of variation (CV) and α of stride times were calculated for each interval. Averages for all variables were calculated per group for statistical analysis. No significant interval, group or interval×group effects were found for mean, SD or CV of stride time. A significant linear trend in α for interval occurred with a reduction in α over the course of the run (p=0.01) indicating that over the run, stride times of runners became more unpredictable. This was likely due to movement errors associated with fatigue necessitating frequent corrections. The injured group exhibited lower α (M=0.79, CI(95)=0.70, 0.88) than the non-injured group (p=0.01) (M=0.96, CI(95)=0.88, 1.05); a reduction hypothesized to be associated with altered complexity. Overall, these findings suggest injury and fatigue influence neuromuscular output during running.

Effects of running speed on a probabilistic stress fracture model.

BACKGROUND Stress fractures are dependent on both loading magnitude and loading exposure. Decreasing speed is a potential mechanism of strain reduction during running. However, if running speed is decreased the number of loading cycles will increase for a given mileage. It is unclear if these increased loading cycles are detrimental despite reductions in bone strain. The purpose of this study was to determine the effects of running speed on the probability of tibial stress fracture during a new running regimen. METHODS Ten male subjects ran overground at 2.5, 3.5, and 4.5m/s. Force platform and kinematic data were collected synchronously. Inverse dynamics and musculoskeletal modeling were used to determine joint contact forces acting on the distal tibia. Peak tibial contact force served as input to a finite element model to estimate tibial strains. Stress fracture probability for each running speed was determined using a probabilistic model based on published relationships of bone damage, repair, and adaptation. The effects of speed on stress fracture probability was compared using a repeated measures ANOVA. FINDINGS Decreasing running speed from 4.5 to 3.5m/s reduced the estimated likelihood for stress fracture by 7% (P=0.017). Decreasing running speed from 3.5 to 2.5m/s further reduced the likelihood for stress fracture by 10% (P<0.001). INTERPRETATION Runners wanting to reduce their risk for tibial stress fracture may benefit from a decrease in running speed. For the speeds and mileage relative to the current study, stress fracture development was more dependent on loading magnitude rather than loading exposure.