5. Overview of the 4x4 body

It is well known that walking and running bipedally with a heel strike entails a temporary, rapid increase in vertical ground reaction force at the beginning of stance (see Glossary), called an impact force (Fig. 2A,C). These impact forces have been implicated in tissue damage leading to joint degenerative disease and other musculoskeletal injuries (Whittle, 1999), although the biomechanical factors determining these forces and how they are transmitted through the body is not fully understood. Recently, several studies have developed and tested a biomechanical model of foot strike in humans that demonstrates that impact forces occur when some portion of the lower extremity, the effective mass, decelerates abruptly,

requiring rapid dissipation of mechanical energy (Chi and Schmitt, 2005; Lieberman et al., 2010). According to this model, the impact peak is a function of foot velocity at impact, ankle and knee joint compliance, and the duration of deceleration, itself dependent on compression of a thick, fibrous fat pad below the calcaneal tuber (Gefen et al., 2001). This heel pad dissipates 17–45% of the energy returned from the ground at impact in humans (Gefen et al., 2001; Pain and Challis, 2001). Great apes often produce impact forces when walking bipedally (Fig. 2B) (Crompton et al., 2008; Pontzer et al., 2014), but also possess thick heel pads that presumably dissipate some energy at foot strike (Raven, 1936; Vereecke et al., 2003).

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