# Plantar Loads of Habitual Forefoot Strikers during Running on Different Overground Surfaces
## 4 Discussion  
This study examined the influence of surfaces on plantar loads among runners with habitual forefoot strike. The hypothesis in the current study was that lower plantar loads would be found on rubber and grass surfaces when compared with the loads on concrete surface. Our hypothesis was partially supported because running on synthetic rubber showed significantly lower plantar pressure in lateral forefoot and lower pressure-time integral in central forefoot and lateral forefoot when compared to running on concrete. However, no differences were found in plantar pressure and pressure-time integral for grass surface compared to synthetic rubber or concrete. No differences were found for maximum force, contact area, and force-time integral integral while runners run on the three surfaces. In the current study, there were significant different plantar loads on the lateral and central forefoot when participants were running at the target velocity of 3.3 \(\pm\) 0.2 m/s on synthetic rubber and concrete surfaces. Direct comparison with other studies was unreasonable, as there were few studies on plantar loads with forefoot strike pattern. Although the participants performed running on a forefoot strike pattern and significant difference of PTI in forefoot area was also found in a previous study [31], the participants ran with regular rearfoot strike pattern before participating in the trail. In a previous study, significantly different plantar loads were presented on the lateral and central forefoot [23]. However, the surfaces (natural grass, treadmill and concrete) and running speed (3.8 m/s) used in the study [23] were not exactly the same as these in the current study. Although similar plantar loads were showed in an investigation into different strike patterns at the same running speed (3.3 m/s) in another study [21], the data were only collected in synthetic rubber surface. Although the plantar loads of nonrearfoot strikes during running were investigated in a previous study [32], the study aimed to compare the plantar loading of rearfoot strikers and nonrearfoot strikers after receiving a 4-week running program with minimalist footwear instead of overground surfaces. In addition, the current results are in line with a previous study that showed different impact absorbing ability comparing overground surfaces [14; 23], although different strike patterns (rearfoot and forefoot) were tested in these previous studies. However, a study found that four different surfaces (grass, concrete, synthetic track and treadmill) had no significant effect on PP and PTI under the same running speed (12 km/h) [13]. Differences between the current work and the previous study [13] could be attributed to different effects of foot striking pattern (forefoot strike vs. rearfoot strike) on plantar loads distribution [21], different cushioning of foot wear (Shanghai Shoes Co. Ltd., Shanghai, China vs. ASICS, Osaka, OSA, Japan) [33], and different insole measuring devices (Version 4.4; T&T Medilagic Medizintechnik GmbH, Schonefeld, Germany vs. Version 25.3.6e, Novel, Munich, Germany, 2009) for plantar regional division [28]. It has been shown previously that running on different overground surfaces produces significant differences in plantar loads [14; 23; 31]. However, the participants in these previous studies were rearfoot strikers, which has larger plantar loads during the stance phase when compared with forefoot strike pattern. In the current study, the plantar loads were tested in habitual forefoot strikers and no significant differences were found on total foot. These findings were consistent with a previous study [3], confirming that the lower limbs can show similar loading rate with a forefoot strike pattern between barefoot and shod conditions. This study therefore indicates that the lower limbs can show more compliance with a forefoot strike pattern during running on different overground surfaces. Higher and repetitive plantar loading might play a risk factor leading to forefoot injuries [34].