(a)

(b) dy dy surf

Figure 3: Illustration of the spring-mass model for running, showing centre of mass displacement (y) and surface deformation (ysurf) (from Ferris et al., 1998)

displacement of the stiffer surface, resulting in a consistent leg–surface stiffness as surface stiffness changes. Reductions in leg stiffness for stiffer

surfaces have been demonstrated to be a result of changes in initial geometry of the lower limb and resulting reductions in joint stiffness. For example, Hardin and colleagues reported a straighter leg (less knee

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Centre of mass vertical displacement (cm)

5 0

-5 ground contact 0

Surface displacement (cm)

-5 0 0.1 12 0.2 0.3 0.4 Time (s) 0.5 0.6 0.7 0.8

flexion) at initial ground strike when running on stiffer surfaces (13). Since peak knee flexion was maintained at similar levels for different surface conditions, the straighter leg at ground strike resulted in a greater knee flexion excursion over the period from initial ground contact to peak knee flexion. This greater knee flexion excursion contributes to reduced lower limb stiffness on stiffer surfaces (13). However, although the greater knee flexion excursion results in a reduced lower limb stiffness during the entire flexion phase of ground contact, the straighter knee at ground impact results in a stiffer lower limb for the initial impact phase of running, in contrast to previous suggestions of a more flexed knee (3). This straighter knee is likely to result in an increase in impact shock (13) and thus have injury implications. In particular, higher impact shock has been associated with bone injuries such as stress fractures (14). In terms of running performance, in general more compliant (less stiff) surfaces appear to be preferred. For example, Ferris and colleagues described how the stiffer lower limb and lower knee excursion adopted for a less stiff surface requires lower muscle moments and forces to exert the same ground contact force, reducing the energy cost of running (12). In addition, a compliant surface may provide some rebound, again reducing the energy cost (15). However, a running surface may be too compliant, resulting in a feeling of running on sand, increasing the energy cost (13). Consideration of the entire contact phase rather than just

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Figure 4: Centre of mass overall displacement and surface displacement for a transition from a soft to a hard surface (from Ferris et al., 1999)

impact has highlighted evidence that runners adjust lower limb stiffness for different surfaces. This results in a consistent centre-of-gravity vertical oscillation, ground contact time and stride frequency (12). Thus, the observed adjustments in lower limb stiffness appear to be in order to allow consistent performance, possibly at the expense of injury concerns. Based on the evidence presented

so far concerning the impact phase and the entire contact phase, suggested responses to the first two questions posed at the start of this article are:

Q What changes occur in lower limb biomechanics with changes in running surface? A Adjustments in kinematics and joint moments to influence lower limb stiffness (at impact and throughout ground contact) Q What criteria govern the changes in biomechanics? A Maintenance of impact forces, and/or maintenance of consistent centre-of-gravity displacement

When we consider the question “What influence do these changes have on lower limb chronic injury and performance?” the evidence presented in the second section of this article suggests that there may be an

increase in impact shock as a result of a straighter leg at ground contact for stiffer surfaces, which may increase injury risk. There is also evidence that performance may be improved on more compliant (less stiff) surfaces but that this depends on the specific surface characteristics (may be too compliant/soft).

EVIDENCE FROM IN-SHOE PRESSURE MEASUREMENT Recent technological developments have allowed the measurement of pressure within the shoe during running, previously hampered by a lack of robust systems and limited sampling rates of pressure data. In contrast to studies using force plates, as described earlier, this approach has revealed greater heel loading for stiffer surfaces (Fig. 5) (16,17) and when running in stiffer shoes (18). It is suggested that pressure insoles are more sensitive than force plates to changes in running surface because the measurement takes place at the foot plantar surface,

sportEX medicine 2008:38(Oct):10-13