Effect of shoe bending stiffness on lower limb kinetics of female recreational runners

Running performance is strongly influenced by the stretch-shortening cycle mechanisms in which the force production is potentiated by the storage-restitution of elastic energy in the muscle-tendon structures (Komi 2000). Moreover, gender-specificity were found in the muscle-tendon structures, like the lower patellar or Achilles tendon stiffness observed in females compared to males (Onambélé et al. 2007; Burgess et al. 2009). Then, the shoe longitudinal bending stiffness was shown to be another parameter which greatly influences running biomechanics and therefore performance (Stefanyshyn and Wannop 2016). Nevertheless most of the previous studies were performed with male participants. Only one study compared male and female response to different shoe bending stiffnesses (Kleindiest et al. 2005). The authors reported gender specific reaction pattern at the metatarsophalangeal joint, but they did not investigate the global lower limb pattern. The aim of the present study was to investigate the role of the shoe longitudinal bending stiffness in the lower limb kinetics of female runners. It was hypothesized that increasing the shoe bending stiffness compared to a standard shoe would improve the stretch-shortening cycle mechanism, favouring the energy storage in the braking phase and the energy restitution in the push-off phase.


Introduction
Running performance is strongly influenced by the stretch-shortening cycle mechanisms in which the force production is potentiated by the storage-restitution of elastic energy in the muscle-tendon structures (Komi 2000). Moreover, gender-specificity were found in the muscle-tendon structures, like the lower patellar or Achilles tendon stiffness observed in females compared to males (Onambélé et al. 2007;Burgess et al. 2009).
Then, the shoe longitudinal bending stiffness was shown to be another parameter which greatly influences running biomechanics and therefore performance (Stefanyshyn and Wannop 2016). Nevertheless most of the previous studies were performed with male participants. Only one study compared male and female response to different shoe bending stiffnesses (Kleindiest et al. 2005). The authors reported gender specific reaction pattern at the metatarsophalangeal joint, but they did not investigate the global lower limb pattern.
The aim of the present study was to investigate the role of the shoe longitudinal bending stiffness in the lower limb kinetics of female runners. It was hypothesized that increasing the shoe bending stiffness compared to a standard shoe would improve the stretch-shortening cycle mechanism, favouring the energy storage in the braking phase and the energy restitution in the push-off phase.
After a warm-up of 5 min with each pair of shoes, the participants ran on a 15 m pathway at 10 ± 1 km.h -1 . Running speed was checked with photocells 2 m before and after the force plate. Kinematics of the right lower limb (Qualisys, Sweden) and ground reaction force (Kistler, Switzerland) were recorded for five trials for both shoe conditions. Two identical pairs of shoes were tested, which differed only in term of longitudinal bending stiffness. The bending stiffness was tested with the FlexerII device (Exeter Res., USA) at 2 Hz with 45 deg of flexion. The bending stiffness of the 'stiff ' shoes was 485 Nmm.deg -1 compared to 210 Nmm.deg -1 for the 'standard' shoes.
Ankle, knee and hip joint angles, net moments and net powers were calculated with the Visual3D software (C-Motion, USA) according to ISB recommendations (Wu et al. 2002). The adjusted Zatsiorsky-Seluyanov's anthropometric table (De Leva 1996) was preferred to match young female characteristics. The total eccentric (braking phase) and concentric (push-off phase) works were also calculated for each joints. Both kinematics and kinetics were time normalized to the stance phase.
In order to investigate the influence of bending stiffness on the whole kinetics time series, statistical parametric mapping (SPM) was preferred (Pataky et al. 2015). SPM paired t-tests were performed on the ground reaction forces and kinetics time series. Simple Student t-tests were performed on the net joint works. Statistics were computed in Matlab (The Mathworks, USA) with the α-level set at 0.05.

Results and discussion
With the stiff shoes, participants exerted lower push-off forces (79-88% of the stance phase) in the antero-posterior axis compared to the standard shoes (Figure 1). Vertical forces during the push-off phase (55-92% of the stance phase) were also lower with the stiff shoes compared to the standard shoes. Thus, the stiff shoes induced a reduction in propulsion forces in female recreational runners like it was previously observed in male runners (Flores et al. 2017).
The stiff shoes induced lower ankle and knee powers during the push-off phase (77-96% and 71-73% of the stance phase, respectively) compared to the standard shoes ( Figure 2). These changes in joint powers were associated with significant reductions of the concentric work at the knee joint (0.21 ± 0.09 vs. 0.23 ± 0.09 J.kg -1 ; p = 0.04) and the hip joint (0.45 ± 0.11 vs. 0.51 ± 0.16 J.kg -1 ; p = 0.02) during the push-off phase with the stiff shoes compared to the standard shoes.
As there was no change in the braking power and joint energy absorption, the reduced power in the pushoff phase and the lower energy generation might reflect an improvement of the stretch-shortening cycle during running with the stiff shoes (Komi 2000). The increase of bending stiffness might have been beneficial in the storage-restitution of elastic energy of the human-shoe system. This would have limited the need of energy production of the lower limb joints to run at the same speed with the stiff shoes compared to the standard shoes.

Conclusions
To conclude, the increase of longitudinal bending stiffness of the shoes could have a positive effect on the stretch-shortening cycle and therefore on running performance of female recreational runners. However, previous studies in male runners have determined an optimal bending stiffness to warrant an optimal running performance (Stefanyshyn and Wannop 2016). Thus, investigation about the optimal bending stiffness would be useful to ensure performance without increasing the injury risk in female runner population.