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Isabel S. Moore and Sharon J. Dixon

Interest in barefoot running and research on barefoot running are growing. However a methodological issue surrounding investigations is how familiar the participants are with running barefoot. The aim of the study was to assess the amount of time required for habitually shod runners to become familiar with barefoot treadmill running. Twelve female recreational runners, who were experienced treadmill users, ran barefoot on a treadmill for three bouts, each bout consisting of 10 minutes at a self-selected speed with 5 minute rest periods. Sagittal plane kinematics of the hip, knee, ankle, and foot during stance were recorded during the first and last minute of each 10-minute bout. Strong reliability (ICC > .8) was shown in most variables after 20 minutes of running. In addition, there was a general trend for the smallest standard error of mean to occur during the same period. Furthermore, there were no significant differences in any of the biomechanical variables after 20 minutes of running. Together, this suggests that familiarization was achieved between 11 and 20 minutes of running barefoot on a treadmill. Familiarization was characterized by less plantar flexion and greater knee flexion at touchdown. These results indicate that adequate familiarization should be given in future studies before gait assessment of barefoot treadmill running.

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Sharon J. Dixon and David G. Kerwin

In this study sagittal plane joint angles of the lower extremity were used to obtain an indication of the influence of heel lift manipulation on Achilles tendon strain in running. The influence of increased heel lift on lower extremity kinematics was investigated for 8 heel striking subjects. With increased heel lift, all subjects demonstrated reductions in peak ankle dorsi-flexion and consistent values of peak knee flexion, indicating that there were reductions in peak Achilles tendon strain. Group analysis demonstrated that the reductions in peak ankle angle were statistically significant (p < .01). Typically, subjects also demonstrated adjustments in initial ankle angle, whereby the amount of ankle dorsi-flexion at initial ground contact was reduced with increased heel lift. Group mean data indicated that a 15-mm heel lift resulted in a mean decrease in initial ankle dorsi-flexion of 3.9°, while peak ankle dorsi-flexion was also reduced by 3.9°. It is suggested that the initial ankle angle adjustment acted to maintain a similar range of ankle joint movement in the period from initial ground contact to peak ankle dorsi-flexion across heel lift conditions. The distinct behavior of one subject, who demonstrated an increased ankle dorsi-flexion at ground impact, has highlighted the importance of considering single subject results in studies of footwear variation in running.

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Sharon J. Dixon and David G. Kerwin

In this study, a modeling method was developed to estimate Achilles tendon forces in running. Owing to the common use of heel lift devices in the treatment of Achilles tendon injury, we investigated the influence of increased heel lift on Achilles tendon loading. The hypothesis was that heel lift manipulation can influence maximum Achilles tendon force. Responses to heel lift variation were found to differ among 3 elite runners demonstrating distinct running styles. A rearfoot and a midfoot striker demonstrated significant increases in maximum Achilles tendon force with increased heel lift, whereas a forefoot striker demonstrated no changes in maximum Achilles tendon force values with heel lift manipulation (p < .05). Analysis of the factors contributing to the observed changes in maximum Achilles tendon force highlighted the influence of the moment arm of ground reaction force and the moment arm of the Achilles tendon about the ankle joint center. The finding that increased heel lift may increase maximum Achilles tendon force suggests that caution is advised in the routine use of this intervention. The different responses to heel lift increase between subjects highlight the importance of classifying subjects based on running style.

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Sharon J. Dixon and David G. Kerwin

This study investigated the influence of heel lift interventions on the loading of the Achilles tendon for heel-toe runners. It was hypothesized that the peak Achilles tendon force and peak rate of loading would be reduced by the increase in heel lift, and that the peak Achilles tendon force would occur significantly later in stance. Achilles tendon forces were determined by calculating sagittal-plane ankle joint moments using inverse-dynamics techniques and dividing these moments by Achilles tendon moment arm lengths. Methods for estimating Achilles tendon moment arm length using skin markers were justified via MRI data for one participant. Seven participants underwent running trials under three heel lift conditions: zero, 7.5-mm, and 15-mm heel lift. Average magnitude and occurrence time of peak Achilles tendon force and peak rate of loading were determined for each condition over the 7 participants. Despite group reductions in peak Achilles tendon force and peak rate of loading for the increased heel lift conditions, statistical analysis (ANOVA) revealed no significant differences for these variables, p > 0.05. Individual participant observations highlighted varied responses to heel lift; both increases and decreases in peak Achilles tendon force were observed. For the group data, the time of peak impact force occurred significantly later in the 15-mm heel lift condition than in the zero heel lift, p < 0.05. It is suggested that the success of increased heel lift in treating Achilles tendon injury may be due to a later occurrence of peak Achilles tendon force in response to this intervention, reducing Achilles tendon average rate of loading. In addition, the individuality of Achilles tendon peak force changes with heel lift intervention highlights the need for individual participant analysis.

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Victoria H. Stiles and Sharon J. Dixon

Research suggests that heightened impacts, altered joint movement patterns, and changes in friction coefficient from the use of artificial surfaces in sport increase the prevalence of overuse injuries. The purposes of this study were to (a) develop procedures to assess a tennis-specific movement, (b) characterize the ground reaction force (GRF) impact phases of the movement, and (c) assess human response during impact with changes in common playing surfaces. In relation to the third purpose it was hypothesized that surfaces with greatest mechanical cushioning would yield lower impact forces (PkFz) and rates of loading. Six shod volunteers performed 8 running forehand trials on each surface condition: baseline, carpet, acrylic, and artificial turf. Force plate (960 Hz) and kinematic data (120 Hz) were collected simultaneously for each trial. Running forehand foot plants are typically characterized by 3 peaks in vertical GRF prior to a foot-off peak. Group mean PkFz was significantly lower and peak braking force was significantly higher on the baseline surface compared with the other three test surfaces (p < 0.05). No significant changes in initial kinematics were found to explain unexpected PkFz results. The baseline surface yielded a significantly higher coefficient of friction compared with the other three test surfaces (p < 0.05). While the hypothesis is rejected, biomechanical analysis has revealed changes in surface type with regard to GRF variables.

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Victoria H. Stiles, Igor N. Guisasola, Iain T. James and Sharon J. Dixon

Integrated biomechanical and engineering assessments were used to determine how humans responded to variations in turf during running and turning. Ground reaction force (AMTI, 960 Hz) and kinematic data (Vicon Peak Motus, 120 Hz) were collected from eight participants during running (3.83 m/s) and turning (10 trials per condition) on three natural turf surfaces in the laboratory. Surface hardness (Clegg hammer) and shear strength (cruciform shear vane) were measured before and after participant testing. Peak loading rate during running was significantly higher (p < .05) on the least hard surface (sandy; 101.48 BW/s ± 23.3) compared with clay (84.67 BW/s ± 22.9). There were no significant differences in running kinematics. Compared with the “medium” condition, fifth MTP impact velocities during turning were significantly (RM-ANOVA, p < .05) lower on clay (resultant: 2.30 m/s [± 0.68] compared with 2.64 m/s [± 0.70]), which was significantly (p < .05) harder “after” and had the greatest shear strength both “before” and “after” participant testing. This unique finding suggests that further study of foot impact velocities are important to increase understanding of overuse injury mechanisms.