# Plantar Loads of Habitual Forefoot Strikers during Running on Different Overground Surfaces
## 1 Introduction  
Running is recognized for its health benefits and low cost as a popular exercise [1]. However, running is also related to a high risk of injuries rate varying from 19.4% to 79.3% at the lower limbs [2]. Many factors contributed to running injuries, such as shod conditions [3], training volume per week [4] and fatigue [5]. The overground surface is also an essential factor that has been studied in recent years [6; 7]. Many investigators have identified the overground surface as an important variable that can affect load absorption mechanisms as revealed in the kinematics and kinetics of the lower limbs during running [7; 8; 9; 10]. The accumulation of plantar loads could lead to chronic injuries when running for more than 64 km per week [2] and excessive plantar loads are considered to be a major cause of overuse running injuries [11; 12]. Therefore, the overground surface plays an important part to prevent running injuries [4].  
Different surfaces may be associated with different plantar loads, which may contribute to running injuries [13; 14; 15]. Compared to an asphalt surface, running on a natural grass surface has been shown to lower peak pressures at the lateral forefoot, lateral and central rearfoot [16]. Lower maximum pressure was observed on natural grass at the lateral forefoot, central forefoot, lateral midfoot and total foot compared to concrete surface [14]. Therefore, it might lower the risk of musculoskeletal injuries when running on grass surface rather than other surfaces like asphalt, concrete and synthetic rubber [14; 15; 16]. However, these different surfaces (synthetic, grass, concrete, and treadmill surfaces) did not change the pressure-time integral, the 1 st and 2 nd maximal plantar pressures and peak pressure distribution for habitual rearfoot strike pattern runners [13]. Overall, results comparing different overground surfacesmay show lower plantar loads when running on a softer surface. However, inconsistent results have been discovered and the influence of different surfaces on plantar loads during running remains a contentious issue.  
There are three strike patterns in the long-distance runners, namely rearfoot, midfoot and forefoot strikes [17]. Although the proportion of the forefoot strike pattern is small among runners [17; 18], great attention has been paid to forefoot strike pattern in recent years because of the lower repetitive stress injury rate compared to rearfoot strikes [19; 20]. The force-time integral, maximum force and contact area at the total foot were lower in forefoot strike pattern than rearfoot strike pattern [21]. In addition, the modification from rearfoot strike pattern to forefoot strike pattern can potentially prevent running injuries such as patellofemoral pain syndrome, exertional compartment syndrome [3; 19; 22] and reduce pain and disability [3; 19; 22]. However, many studies on plantar loads characteristics of runners focus on rearfoot strike [23; 24]. Although the contentious influence of different surfaces on plantar loads appears in the rearfoot strike, few studies exist regarding the plantar loads while running on different surfaces for runners with habitual forefoot strike [21; 25].  
Therefore, this current study aimed to examine plantar loads characteristics of habitual forefoot strike runners while running on grass (G), concrete (C) and synthetic rubber (R) surfaces. Given that previous researchers identified a higher peak ground reaction force on a concrete surface than on an elastic surface [26], the hypothesis was that plantar loads during running on grass and synthetic rubber surface were lower than those during running on concrete surface.