Special issue: Bending stiffness: performance and injury effects

Forefoot bending stiffness is a footwear property capable of influencing both athletic performance and injury. Despite this, it has yet to attract the full attention of researchers within the realm of footwear science compared to other footwear properties, such as outsole traction and midsole cushioning.

For readers looking for an introduction to forefoot bending stiffness and the background on existing research, Stefanyshyn and Wannop (2016) provide a review article, highlighting relevant studies that have been conducted regarding athletic injury and performance. Observations indicate that increasing the forefoot bending stiffness of footwear may have a positive influence in terms of injury prevention, specifically by limiting excessive forefoot extension during athletic movements in order to reduce the risk of injuries such as turf-toe (Clanton & Ford, 1994; Hockenbury, 1999; McCormick & Anderson, 2009). While increasing forefoot bending stiffness may be a method of reducing the risk of metatarsophalangeal (MTP) joint injury, information on the forefoot bending stiffness of many existing shoe models or even a standardized methodology of measuring forefoot bending stiffness is not currently available. In an effort to address these shortcomings, Lessley, Crandall, Frederick, Kent, and Sherwood (2016) have presented, in this issue, forefoot stiffness data from a wealth of American football shoes, in addition to proposing a testing apparatus and methodology.

Although previous studies have reported the influence of forefoot bending stiffness on MTP joint motion during various movements (Riley et al., 2013), one important confounding factor in those studies is that the motion of the shoe was being recorded, not the actual motion of the foot within the shoe. Within this special issue, Ford et al. (2016) used an electromagnetic motion analysis system to measure and compare the motion of the shoe and foot as forefoot bending stiffness was increased. Interestingly, their results were different depending on whether the foot kinematics or shoe kinematics were investigated. This study provides evidence that the kinematic effects of altering forefoot bending stiffness may be different between the foot and shoe.

In terms of performance, research has shown that altering shoe forefoot bending stiffness can influence the performance of running (Roy & Stefanyshyn, 2006), sprinting (Stefanyshyn & Fusco, 2004), jumping (Stefanyshyn & Nigg, 1998) and multidirectional movements (Tinoco, Bourgit, & Morin, 2010). Various researchers have speculated as to the mechanisms behind the increases in performance, with alterations in minimizing energy loss and maximizing energy return (Stefanyshyn & Nigg, 2000), gearing ratio at the ankle joint (Carrier, Heglund, & Earls, 1994), or optimization of the musculoskeletal system (Nigg, Stefanyshyn, & Denoth, 2000) being suggested.

Within this special issue multiple studies have provided insight as to the mechanisms by which forefoot bending stiffness improves performance. The study by Vienneau, Nigg, Tomaras, Enders, and Nigg (2016) collected physiological data on soccer players showing that forefoot bending stiffness can influence soccer performance during game-like situations/testing. Their results also indicate that athletes may respond to a medium level of stiffness, with higher and lower stiffness shoes resulting in decreased performance. Madden, Sakaguchi, Wannop, and Stefanyshyn (2016) collected oxygen consumption and kinematic data on athletes performing overground running in different magnitudes of forefoot bending stiffness. The study identified kinematic differences of runners who responded positively (increased performance) in stiff footwear and those who responded negatively (decreased performance) in stiff footwear. The authors speculated that these kinematic differences may be due to differences in the musculoskeletal properties between athletes, influencing the reaction of each runner to the stiff footwear.

Willwacher, Kurz, Menne, Schrodter, and Bruggemann (2016) investigated the initial acceleration phase during a sprint start in three different stiffness conditions. A gearing function of bending stiffness was evident with increased MTP and ankle joint ground reaction force lever arms, with the authors speculating that this may offer a potential for athletes to improve the effectiveness of the horizontal force application. They speculated that some athletes may not have had the individual strength capacities in order to utilize this increased lever arm to optimize their performance. Similarly, a study by Smith, Lake, Sterzing, and Milani (2016) reached similar conclusions with athlete-specific performance results when sprinting in shoes differing in stiffness. The authors highlighted the fact that identification of key anatomical and biomechanical factors that influence foot function during sprinting are needed, speculating that the strength profiles of individual athletes may provide much needed insight.

Collectively, these performance studies indicate that forefoot bending stiffness can improve athletic performance, however, individual athletes have differing needs in terms of forefoot bending stiffness. The mechanism underlying performance improvements induced by forefoot bending stiffness remains unclear. Early studies highlighted optimized foot energetics, with the negative work of the MTP joint being reduced in stiff footwear. However, recent studies have suggested that MTP joint energetics may play a minimal role compared to the influence that altering the gearing ratio or movement of the centre of pressure under the foot has on performance. In addition, there appears to be a specific amount of forefoot bending stiffness to elicit performance increases, with most athletes performing optimally with footwear of medium stiffness and increases or decreases in this optimal stiffness resulting in performance detriments. Future research should focus on identifying the exact mechanisms of performance improvement with modified bending stiffness, with a focus on athlete-specific solutions that may be different for different movements. This information could then be used to develop methods to determine optimal bending stiffness for a particular movement or a particular athlete to maximize performance.

Altering forefoot bending stiffness in footwear also has great potential to reduce athletic injury risk. While studies have shown that increased forefoot bending stiffness could play a significant role in the prevention of turf-toe, there are surprisingly limited in-depth studies examining how altered forefoot bending stiffness will influence injury through an altered MTP joint range of motion or through changes to the magnitude and/or location of the loading on the foot. The effectiveness of forefoot stiffening at restricting MTP joint range of motion during movements and reducing the risk of athletes suffering an injury are currently not based on actual injury data. Further complicating the injury prevention results are the multitude of additional factors, including the magnitude of the bending stiffness and the influence that altering forefoot bending stiffness may have on the injury risk of other joints such as the ankle and knee.

This special issue on forefoot bending stiffness in footwear has emphasized some key advances that have been made within this research area, and also highlighted some areas that require additional research. Hopefully, this recent work will inspire continued research efforts towards the effects of forefoot bending stiffness on athlete performance and injury risk.