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Jeffrey J. Chu and Graham E. Caldwell

Studies on shock attenuation during running have induced alterations in impact loading by imposing kinematic constraints, e.g., stride length changes. The role of shock attenuation mechanisms has been shown using mass-spring-damper (MSD) models, with spring stiffness related to impact shock dissipation. The present study altered the magnitude of impact loading by changing downhill surface grade, thus allowing runners to choose their own preferred kinematic patterns. We hypothesized that increasing downhill grade would cause concomitant increases in both impact shock and shock attenuation, and that MSD model stiffness values would reflect these increases. Ten experienced runners ran at 4.17 m/s on a treadmill at surface grades of 0% (level) to 12% downhill. Accelerometers were placed on the tibia and head, and reflective markers were used to register segmental kinematics. An MSD model was used in conjunction with head and tibial accelerations to determine head/arm/trunk center of mass (HATCOM) stiffness (K1), and lower extremity (LEGCOM) stiffness (K2) and damping (C). Participants responded to increases in downhill grade in one of two ways. Group LowSA had lower peak tibial accelerations but greater peak head accelerations than Group HighSA, and thus had lower shock attenuation. LowSA also showed greater joint extension at heelstrike, higher HATCOM heelstrike velocity, reduced K1 stiffness, and decreased damping than HighSA. The differences between groups were exaggerated at the steeper downhill grades. The separate responses may be due to conflicts between the requirements of controlling HATCOM kinematics and shock attenuation. LowSA needed greater joint extension to resist their higher HATCOM heelstrike velocities, but a consequence of this strategy was the reduced ability to attenuate shock with the lower extremity joints during early stance. With lower HATCOM impact velocities, the HighSA runners were able to adopt a strategy that gave more control of shock attenuation, especially at the steepest grades.

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Janet S. Dufek, John A. Mercer and Janet R. Griffin

The purpose of the study was to examine the effects of running speed and surface compliance on shock attenuation (SA) characteristics for male and female runners. We were also interested in identifying possible kinematic explanations, specifically, kinematics of the lower extremity at foot-ground contact, for anticipated gender differences in SA. Fourteen volunteer recreational runners (7 male, 7 female) ran at preferred and slow speeds on an adjustable bed treadmill, which simulated soft, medium, and hard surface conditions. Selected kinematic descriptors of lower extremity kinematics as well as leg and head peak impact acceleration values were obtained for 10 left leg contacts per subject-condition. Results identified significant SA values between genders across conditions and more specifically, across surfaces for females, with male runners demonstrating a similar trend. Regression modeling to predict SA by gender for surface conditions elicited unremarkable results, ranging from 30.9 to 59.9% explained variance. It appears that surface compliance does affect SA during running; however, the runner’s ability to dissipate the shock wave may not be expressly explained by our definition of lower extremity kinematics at contact.

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Joshua M. Thomas and Timothy R. Derrick

The purpose of this research was to determine the effects of step uncertainty on shock attenuation and knee/subtalar synchrony. Uncertainty was manipulated by decreasing the intensity of light and introducing bumps to the running surface. Twelve experienced distance runners ran at their chosen pace on a treadmill with two surfaces (smooth and irregular) and three light intensities (light, medium, dark). Knee angle, subtalar angle, leg impacts, and head impacts were recorded at 1,000 Hz. Heart rate was also monitored. Injury potential was assessed by evaluating the impacts and asynchronous activity between the knee and subtalar joint. Stride length was not influenced by either source of uncertainty. Heart rate increased with the intensity of light on the smooth running surface but decreased with the intensity of light on the irregular surface. The knee was more flexed at heel contact during the irregular surface conditions but was not affected by the intensity of light. This decreased the effective mass of the impact and allowed greater peak leg accelerations and greater impact attenuation during irregular surface running. There was a decrease in the rearfoot angle at contact on the irregular surface that approached significance (p = 0.056). Knee/subtalar asynchrony increased with the intensity of light on the smooth surface but decreased on the irregular surface. It appears that participants used the knee joint to adapt to the irregular surface and thus accommodate changes in the terrain. The subtalar joint may have become more stable during irregular surface running to minimize the chance of inversion sprains. The effects of intensity of light were small and generally mediated the irregular surface effects. Overall, these adaptations likely reduced the potential for injury during irregular surface running but may have been detrimental to performance.

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Kristian M. O'Connor and Joseph Hamill

Roads are generally designed with a camber to facilitate drainage. Running on a cambered road has been suggested as a potential cause of injury. Two possible mechanisms are mediolateral control and impact shock. The purpose of this study was to investigate the effect of a cambered surface on rearfoot motion and impact shock. Twelve runners ran at 3.83 m/s on both a flat and a cambered surface with the left side raised for all of them. Selected rearfoot kinematic and tibial acceleration measures were evaluated using a 2 × 2 repeated-measures ANOVA. The touchdown angle was less supinated on the left (high) side than on the right (low) side on the cambered surface. Maximum pronation was greater on the left (high) high side than on the right (low) side, as was total rearfoot motion. Maximum velocity of pronation was greater under the left (high) limb than under the right (low) limb while running on the cambered road. Time to maximum pronation did not differ, nor were there differences in peak acceleration or time to peak acceleration. The results of this study suggest that running on a cambered road caused changes in rearfoot motion kinematics that may predispose an individual to injury. Also, since the impact shock did not change with changes in rearfoot motion, perhaps the role of pronation on shock attenuation should be reexamined.

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Songning Zhang, Kurt Clowers, Charles Kohstall and Yeon-Joo Yu

The purpose of this study was to examine effects of shoe midsole densities and mechanical demands (landing heights) on impact shock attenuation and lower extremity biomechanics during a landing activity. Nine healthy male college athletes performed 5 trials of step-off landing in each of 9 test conditions, i.e., a combination of landings in shoes of 3 midsole densities (soft, normal, hard) from each of 3 landing potential energy (PE) levels (low, median, high). Ground reaction forces (GRF), accelerations (ACC) of the tibia and forehead, and sagittal kinematic data were sampled simultaneously. A 3 × 3 two-way (surface × height) repeated-measures analysis of variance (ANOVA) was performed on selected kinematic, ACC, and GRF variables; a 3 × 3 × 3 three-way (surface × height × joint) ANOVA was performed on variables related to eccentric muscular work. The GRF results showed that the forefoot peak GRF in the normal and hard midsoles was significantly greater than the soft midsole at the low and median PEs. Rearfoot peak GRF was significantly greater for the hard midsole than for the soft and normal midsoles at the median and high PEs, respectively. The peak head and tibia peak ACC were also attenuated in similar fashion. Kinematic variables did not vary significantly across different midsoles, nor did energy absorbed through lower extremity extensors in response to the increased shoe stiffness. Knee joint extensors were shown to be dominant in attenuating the forefoot impact force across the landing heights. The results showed limited evidence of impact-attenuating benefits of the soft midsole in the basketball shoes.

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Tim Blackmore, David Jessop, Stewart Bruce-Low and Joanna Scurr

Despite manufacturer claims that athletic socks attenuate force during exercise, no device exists to assess this. Therefore, this study outlines the development of a custom-built impact-testing device for assessing the cushioning properties of socks. The device used a gravity-driven impact striker (8.5 kg), released from 0.05 m, which impacted a no-sock, sock or a basic shoe/sock condition in the vertical axis. A load cell (10,000 Hz) assessed peak impact force, time to peak impact force and loading rate. Reliability was investigated between day, between trial and within trial. Excellent reliability (coefficient of variation < 5% adjusted for 95% confidence limits) was reported for peak impact force in all conditions, with no evidence of systematic bias. Good reliability (coefficient of variation < 10% adjusted for 68% confidence limits) was reported for time to peak impact force and loading rate with some evidence of systematic bias. It was concluded that the custom-built impact-testing device was reliable and sensitive for the measurement of peak impact force on socks.

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Hung-Ta Chiu and Tzyy-Yuang Shiang

The purpose of this study was to investigate the effects of insoles and additional shock absorption foam on the cushioning properties of various sport shoes with an impact testing method. Three commercial sport shoes were used in this study, and shock absorption foam (TPE5020; Vers Tech Science Co. Ltd., Taiwan) with 2-mm thickness was placed below the insole in the heel region for each shoe. Eight total impacts with potential energy ranged from 1.82 to 6.08 J were performed onto the heel region of the shoe. The order of testing conditions was first without insole, then with insole, and finally interposing the shock absorption foam for each shoe. Peak deceleration of the striker was measured with an accelerometer attached to the striker during impact. The results of this study seemed to show that the insole or additional shock absorption foam could perform its shock absorption effect well for the shoes with limited midsole cushioning. Further, our findings showed that insoles absorbed more, even up to 24–32% of impact energy under low impact energy. It seemed to indicate that insoles play a more important role in cushioning properties of sport shoes under a low impact energy condition.

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Oren Tirosh, Guy Orland, Alon Eliakim, Dan Nemet and Nili Steinberg

and attenuated throughout the skeletal system and thus affect all segments of the body. Shock attenuation is a measure used to understand the magnitude and characteristics of the collision between the foot and ground and the absorption of the impact energy between the foot and body segments as it

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Trampas M. TenBroek, Pedro A. Rodrigues, Edward C. Frederick and Joseph Hamill

The purpose of this study was to: (1) investigate how kinematic patterns are adjusted while running in footwear with THIN, MEDIUM, and THICK midsole thicknesses and (2) determine if these patterns are adjusted over time during a sustained run in footwear of different thicknesses. Ten male heel-toe runners performed treadmill runs in specially constructed footwear (THIN, MEDIUM, and THICK midsoles) on separate days. Standard lower extremity kinematics and acceleration at the tibia and head were captured. Time epochs were created using data from every 5 minutes of the run. Repeated-measures ANOVA was used (P < .05) to determine differences across footwear and time. At touchdown, kinematics were similar for the THIN and MEDIUM conditions distal to the knee, whereas only the THIN condition was isolated above the knee. No runners displayed midfoot or forefoot strike patterns in any condition. Peak accelerations were slightly increased with THIN and MEDIUM footwear as was eversion, as well as tibial and thigh internal rotation. It appears that participants may have been anticipating, very early in their run, a suitable kinematic pattern based on both the length of the run and the footwear condition.

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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.