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Timothy R. Derrick, Graham E. Caldwell and Joseph Hamill

A modified mass-spring-damper model was used to simulate the vertical ground reaction forces of a human runner as stride length was altered. Spring stiffness values were selected by an optimizing routine that altered model parameters to match the model ground reaction force curve to a runner’s actual ground reaction force curve. A mass in series with a spring was used to simulate the behavior of body structures that produce the active portion of the ground reaction force. A second mass in series with a spring-damper system was used to simulate the behavior of those components that cause the impact portion of the ground reaction force. The stiffness of the active spring showed a 51% decrease as subjects increased their stride length. The stiffness value of the impact spring showed a trend opposite that of the active spring, increasing by 20% as strides lengthened. It appears that the impact stiffness plays a role in preventing the support leg from collapsing in response to the increased contact velocities seen in the longer strides.

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Joseph Hamill, Jeffrey M. Haddad and William J. McDermott

Variability is a critical aspect of a dynamical systems analysis. Because there are a number of numerical techniques that can be used in such an analysis, the calculation of variability has several issues that must be addressed. The purpose of this paper is to present a variety of quantitative methods for investigating variability from a dynamical systems perspective. The paper is divided into two major sections covering discrete and continuous methods. Each of these sections is subdivided into two sections. Within discrete methods, we discuss, first, the calculation of the discrete relative phase from a time-series history of two parameters and, second, the use of return maps. Using continuous methods, we present procedures for using angle-angle plots in the evaluation of relative phase. We then discuss the use of phase plots in the calculation of the continuous relative phase. Each of these methods presents unique problems for the researcher and the method to be used is determined by the nature of the question asked.

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Bryan C. Heiderscheit, Joseph Hamill and Richard E.A. van Emmerik

The purpose of this investigation was to determine whether individuals with patellofemoral pain (PFP) display a reduction in intralimb joint coordination variability compared to nonimpaired persons. In addition, it was hypothesized that the variability of the stride characteristics would be similar between groups. Eight individuals with unilateral PFP and 8 nonimpaired participants ran on a treadmill at a fixed (2.68 m·s–1) and preferred speed while stride characteristics and 3-D kinematics of the bilateral lower extremities were recorded. Intralimb coordination variability was measured using a vector coding technique applied to relative motion plots of various joint couplings. The PFP group displayed greater stride length variability during running at the preferred speed. However, this was not the case during running at the fixed speed. When averaging across the entire stride cycle, coordination variability for all joint couplings was consistent between the two groups. However, further analysis about heel-strike revealed reduced joint coordination variability for the thigh rotation/leg rotation coupling of the PFP group’s injured limb compared to that of the nonimpaired group. With the exception of the transverse plane rotations at heel-strike, it would appear that the level of pain experienced by the PFP participants may not be great enough to produce a change in the intralimb coordination patterns during running.

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Nancy L. Greer, Joseph Hamill and Kevin R. Campbell

Ground reaction force patterns during walking were observed in 18 children 3 and 4 years of age. The children walked barefoot at a self-chosen walking pace. Selected variables representing the vertical, anteroposterior, and mediolateral force components were evaluated. The results indicated that children in this age range contact the ground with greater vertical force measures relative to body mass than do adults. In addition, the minimum vertical force was lower, the transition from braking to propulsion occurred earlier, and the mediolateral force excursions were higher than typically found in adults. When the children were divided into groups on the basis of sex, differences were observed between those groups. The boys exhibited a greater difference in the vertical peak forces, a lower minimum force, a greater braking force, and a higher mediolateral force excursion value. The results indicated that children display a different ground reaction force pattern than do adults and that differences between boys and girls may be observed as early as ages 3 and 4 years.

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Christopher L. MacLean, Irene S. Davis and Joseph Hamill

The purpose of this study was to analyze the influence of varying running shoe midsole composition on lower extremity dynamics with and without a custom foot orthotic intervention. Three-dimensional dynamics were collected on 12 female runners who had completed 6 weeks of custom foot orthotic therapy. Participants completed running trials in 3 running shoe midsole conditions—with and without a custom foot orthotic intervention. Results from the current study revealed that only maximum rearfoot eversion velocity was influenced by the midsole durometer of the shoe. Maximum rearfoot eversion velocity was significantly decreased for the hard shoe compared with the soft shoe. However, the orthotic intervention in the footwear led to significant decreases in several dynamic variables. The results suggest that the major component influencing the rearfoot dynamics was the orthotic device and not the shoe composition. In addition, data suggest that the foot orthoses appear to compensate for the lesser shoe stability enabling it to function in a way similar to that of a shoe of greater stability.

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Katherine A. Boyer, Julia Freedman Silvernail and Joseph Hamill

Injury rates among runners are high, with the knee injured most frequently. The interaction of running experience and running mechanics is not well understood but may be important for understanding relative injury risk in low vs higher mileage runners. The study aim was to apply a principal component analysis (PCA) to test the hypothesis that differences exist in kinematic waveforms and coordination between higher and low mileage groups. Gait data were collected for 50 subjects running at 3.5 m/s assigned to either a low (< 15 miles/wk) or higher (> 20 miles/wk, 1 year experience) mileage group. A PCA was performed on a matrix of trial vectors of all force, joint kinematic, and center of pressure data. The projection of the subjects’ trial vectors onto the linear combination of PC7, PC10, PC13, and PC19 was significantly different between the higher and lower mileage groups (d = 0.63, P = .012). This resultant PC represented variation in transverse plane pelvic rotation, hip internal rotation, and hip and knee abduction and adduction angles. These results suggest the coordination of lower extremity segment kinematics is different for lower and higher mileage runners. The adopted patterns of coordinated motion may explain the lower incidence of overuse knee injuries for higher mileage runners.

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Christopher L. MacLean, Richard van Emmerik and Joseph Hamill

The purpose of this study was to analyze the influence of a custom foot orthotic (CFO) intervention on lower extremity intralimb coupling during a 30-min run in a group of injured runners and to compare the results to a control group of healthy runners. Three-dimensional kinematic data were collected during a 30-min run on healthy female runners (Shoe-only) and a group of female runners who had a recent history of overuse injury (Shoe-only and Shoe with custom foot orthoses). Results from the study revealed that the coordination variability and pattern for the some couplings were influenced by history of injury, foot orthotic intervention and the duration of the run. These data suggest that custom foot orthoses worn by injured runners may play a role in the maintenance of coordination variability of the tibia (transverse plane) and calcaneus (frontal plane) coupling during the Early Stance phase. In addition, it appears that the coupling angle between the knee (transverse plane) and rearfoot (frontal plane) joints becomes more symmetrical in the late stance phase as a run progresses.

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W. Brent Edwards, Timothy R. Derrick and Joseph Hamill

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.

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Saunders N. Whittlesey, Richard E.A. van Emmerik and Joseph Hamill

Many studies have assumed that the swing phase of human walking at preferred velocity is largely passive and thus highly analogous to the swing of an unforced pendulum. In other words, while swing-phase joint moments are generally nonzero during swing, it was assumed that they were either zero or at least negligibly small compared to gravity. While neglect of joint moments does not invalidate a study by default, it remains that the limitations of such an assumption have not been explored thoroughly. This paper makes five arguments that the swing phase cannot be passive, using both original data and the literature: (1) Computer simulations of the swing phase require muscular control to be accurate. (2) Swing-phase joint moments, while smaller than those during stance, are still greater than those due to gravity. (3) Gravity accounts for a minority of the total kinetics of a swing phase. (4) The kinetics due to gravity do not have the pattern needed to develop a normal swing phase. (5) There is no correlation between pendular swing times and human walking periods in overground walking. The conclusion of this paper is that the swing phase must be an actively controlled process, and should be assumed to be passive only when a study does not require a quantitative result. This conclusion has significant implications for many areas of gait research, including clinical study, control theory, and mechanical modeling.

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Kristian M. O’Connor and Joseph Hamill

The ankle joint has typically been treated as a universal joint with moments calculated about orthogonal axes and the frontal plane moment generally used to represent the net muscle action about the subtalar joint. However, this joint acts about an oblique axis. The purpose of this study was to examine the differences between joint moments calculated about the orthogonal frontal plane axis and an estimated subtalar joint axis. Three-dimensional data were colected on 10 participants running at 3.6 m/s. Joint moments, power, and work were calculated about the orthogonal frontal plane axis of the foot and about an oblique axis representing the subtalar joint. Selected parameters were compared with a paired t-test (α = 0.05). The results indicated that the joint moments calculated about the two axes were characteristically different. A moment calculated about an orthogonal frontal plane axis of the foot resulted in a joint moment that was invertor in nature during the first half of stance, but evertor during the second half of stance. The subtalar joint axis moment, however, was invertor during most of the stance. These two patterns may result in qualitatively different interpretations of the muscular contributions at the ankle during the stance phase of running.