Footwear is an effective biomechanical solution to lower extremity joint problems. Variable stiffness shoe (VSS) is designed. It has been proved to reduce knee internal abduction (external adduction) moment [1]. This helps to slow down progression of medial knee osteoarthritis (OA) [2]. However, there is no study on the effects of VSS on lower extremity during dynamic activities besides walking. Influence of VSS in sports performance is also yet to be examined. This study aims to investigate the biomechanical influence of VSS on lower extremity during dynamic activities and to assess the potential of VSS in improving sports performance.
Materials and methods
15 female and 15 male subjects walked in 2 conditions: VSS and Control. VSS had a lateral sole 1.6 times stiffer than medial (Figure 1). The optimized ratio was obtained from finite element analysis of a simplified 2D knee model. Control had uniform stiffness outsole.3D kinematic and kinetic analysis was conducted during walking, running, stop jumping and lateral hopping. Rating on footwear comfort was also performed.
Figure 1
Images of shoe bottom and shoe cross section showing lateral (L) and media (M) soles.
Results
Increased posterior force during running and stop jumping (Table 1) ensured controlled gait termination and reduced the risk of fall. Increased anterior force during walking and running (Table 1) increased forward propulsion and acceleration. Knee internal abduction moment was generally reduced (Table 1). This showed potential of VSS as sportswear that helped to relieve medial knee loading in more vigorous activities such as running, stopping and jumping. Kinetic data and comfort data showed that VSS did not change gait kinematics much. Rating differences were all insignificant (p<0.05). The stiffness variation in VSS was hardly noticeable. Shoe comfort was not compensated in VSS.
Table 1
Table compares only the averages of kinematics (angles) and kinetics (moments and forces) data that are statistically significant (p<0.05) during the dynamic activities.
Activity
Joint
Variable Name
Control
VSS
%difference
Walking
Knee
Max adduction moment (%BWxHt)
0.37
0.35
-5.994
Walking
Max anterior force at push off (%BW)
19.73
20.97
6.302
Running
Knee
Max adduction moment (%BWxHt)
1.04
0.88
-14.716
Running
Max posterior force (%BW)
-24.50
-28.93
18.072
Running
Max anterior force (%BW)
29.02
30.78
6.078
Stop Jumping
Knee
Max adduction moment (%BWxHt)
0.83
0.67
-18.661
Stop Jumping
Max posterior force (%BW)
-67.67
-73.33
8.373
Conclusion
The study demonstrated great potential of VSS in improving sports achievement and protecting knee. Outsole configuration can be further modified by varying outsole stiffness along anteroposterior axis for better performance and protection.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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