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Kathryn Mills, Aula Idris, Thu-An Pham, John Porte, Mark Wiggins and Manolya Kavakli

first be established. The objectives of this study were (1) to evaluate the validity of a VR game in evaluating the projected frontal plane knee angle when landing from a jump and (2) to assess the reliability of this evaluation over time. Methods A cross-sectional laboratory study was used to determine

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Niell G. Elvin, Alex A. Elvin, Steven P. Arnoczky and Michael R. Torry

Impact forces and shock deceleration during jumping and running have been associated with various knee injury etiologies. This study investigates the influence of jump height and knee contact angle on peak ground reaction force and segment axial accelerations. Ground reaction force, segment axial acceleration, and knee angles were measured for 6 male subjects during vertical jumping. A simple spring-mass model is used to predict the landing stiffness at impact as a function of (1) jump height, (2) peak impact force, (3) peak tibial axial acceleration, (4) peak thigh axial acceleration, and (5) peak trunk axial acceleration. Using a nonlinear least square fit, a strong (r = 0.86) and significant (p ≤ 0.05) correlation was found between knee contact angle and stiffness calculated using the peak impact force and jump height. The same model also showed that the correlation was strong (r = 0.81) and significant (p ≤ 0.05) between knee contact angle and stiffness calculated from the peak trunk axial accelerations. The correlation was weaker for the peak thigh (r = 0.71) and tibial (r = 0.45) axial accelerations. Using the peak force but neglecting jump height in the model, produces significantly worse correlation (r = 0.58). It was concluded that knee contact angle significantly influences both peak ground reaction forces and segment accelerations. However, owing to the nonlinear relationship, peak forces and segment accelerations change more rapidly at smaller knee flexion angles (i.e., close to full extension) than at greater knee flexion angles.

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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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Wolfgang Potthast, Gert-Peter Brüggemann, Arne Lundberg and Anton Arndt

The purpose of this study was to quantify relative contributions of impact interface, muscle activity, and knee angle to the magnitudes of tibial and femoral accelerations occurring after external impacts. Impacts were initiated with a pneumatically driven impacter under the heels of four volunteers. Impact forces were quantified with a force sensor. Segmental accelerations were measured with bone mounted accelerometers. Experimental interventions were hard and soft shock interfaces, different knee angles (0°, 20°, 40° knee flexion), and muscular preactivation (0%, 30%, 60% of maximal voluntary contraction) of gastrocnemii, hamstrings, and quadriceps. Greater knee flexion led to lower impact forces and higher tibial accelerations. Increased muscular activation led to higher forces and lower tibial accelerations. The softer of the two shock interfaces under study reduced both parameters. The effects on accelerations and forces through the activation and knee angle changes were greater than the effect of interface variations. The hardness of the two shock interfaces explained less than 10% of the variance of accelerations and impact forces, whereas knee angle changes explained 25–29%, and preactivation changes explained 35–48% of the variances. It can be concluded that muscle force and knee joint angle have greater effects in comparison with interface hardness on the severity of shocks on the lower leg.

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Ayoub Asadi, Alireza Farsi, Behrouz Abdoli, Esmaeel Saemi and Jared M. Porter

conducted to evaluate if any of the conditions differed in terms of jump distance or knee angle throughout the duration of the experiment. Participants Volunteers were 30 male undergraduate students with a mean age of 23 ± 4 years. Half of the participant pool (mean weight 74 ± 10.36 kg; mean height 183 ± 7

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Marie A. Johanson, Brian J. Cuda, Jonathan E. Koontz, Julia C. Stell and Thomas A. Abelew

Context:

Stretching exercises are commonly prescribed for patients and healthy individuals with limited extensibility of the gastrocnemius muscle.

Objective:

To determine effects of gastrocnemius stretching on ankle dorsiflexion, knee extension, and gastrocnemius muscle activity during gait.

Design:

Randomized-control trial.

Setting:

Biomechanical laboratory.

Participants:

Sixteen volunteers (9 men and 7 women, mean age = 27 y) with less than 5° of passive ankle-dorsiflexion range of motion randomly assigned to an experimental or control group.

Intervention:

The experimental group performed gastrocnemius stretching for 3 wk.

Main Outcome Measures:

Maximum ankle dorsiflexion, maximum knee extension, and EMG amplitude of the gastrocnemius muscles were measured between heel strike and heel-off before and after intervention.

Results:

No significant effect of group or time was found on maximum ankle dorsiflexion, maximum knee extension, or EMG activity of the medial or lateral gastrocnemius muscles between heel strike and heel-off. The experimental group had significantly greater passive ankle-dorsiflexion range of motion bilaterally at posttest than the control group.

Conclusions:

Stretching did not alter joint angles or gastrocnemius muscle activity in the early to midstance phase of gait.

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Dionne A. Noordhof, Carl Foster, Marco J.M. Hoozemans and Jos J. de Koning

A meaningful association between changes (Δ) in push-off angle or effectiveness (e) and changes in skating velocity (v) has been found during 5000-m races, although no significant association was found between changes in knee (θ0) and trunk angle (θ1) and Δv. It might be that speed skating event, sex, and performance level influence these associations.

Purpose:

To study the effect of skating event, sex, and performance level on the association between Δe and Δv and between Δθ0 and Δθ1 and Δv.

Methods:

Video recordings were made from frontal (e) and sagittal views (θ0 and θ1) during 1500- and 5000-m men’s and women’s World Cup races. Radio-frequency identification tags provided data of v.

Results:

Skating event influenced the association between Δe and Δv, which resulted in a significant association between Δe and Δv for the 5000-m (β = –0.069, 95% confidence interval [–0.11, –0.030]) but not for the 1500-m (β = –0.011 [–0.032, 0.010]). The association between Δθ0 and Δθ1 and Δv was not significantly influenced by skating event. Sex and performance level did not substantially affect the association between Δe and Δv and between Δθ0 and Δθ1 and Δv.

Conclusions:

Skating event significantly influenced the association between Δe and Δv; a 1° change in e results in a 0.011-m/s decrease in v during the 1500-m and a 0.069-m/s decrease in v during the 5000-m. Thus, it seems especially important to maintain a small e during the 5000-m.

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Danny Lum and Abdul Rashid Aziz

was performed at 2 knee flexion angle positions of 90° (ISqT90) and 120° (ISqT120; where full knee extension was 180°). The order of the test for each knee angle position was randomized. The athletes were asked to adopt the same feet placement as they would do for their back squat exercise. A bar was

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Jonathan S. Goodwin, Robert A. Creighton, Brian G. Pietrosimone, Jeffery T. Spang and J. Troy Blackburn

frontal plane knee angle at heel strike, peak knee varus angle, peak internal knee valgus moment (ie, the internal response to external varus loading), and frontal plane knee angular impulse (ie, the area under the moment vs time curve) via custom computer software (LabVIEW; National Instruments Corp

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Curtis Kindel and John Challis

°, −15°, and 0°(neutral), with 2 different knee angles (0° and 90°). The order of testing was randomized. The biarticular hamstring muscles (biceps femoris, semitendinosus, and semimembranosus) contribute to the hip extension moment, but also cross the knee. Therefore by using 2 knees angles, the lengths