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BIOMECHANICS INJURY RISK

THE INFLUENCE OF CHANGES IN SPORTS SURFACE CUSHIONING ON THE LOADING OF THE BODY AND RUNNING BIOMECHANICS

There is evidence that surfaces of high stiffness may increase injury risk through increased impact loading. On the other hand, surfaces with suitable compliance appear to have the potential to reduce injury risk and to improve performance. This article considers whether we change our running biomechanics for surfaces of different stiffness, and the implications of any adjustments made, in terms of both injury risk and performance.

Peak ankle dorsiflexor Moment

40 20 0

-20 -40 -60 -80 -100 -120 -140 -160

0.05 0.1 0.05 0.2 Time (s)

Figure 2: Typical ankle dorsiplantar flexor moment time– history for heel–toe running

regarding the impact phase of running, gives us answers to the first two questions posed at the start of this article:

Q What changes occur in lower limb biomechanics with changes in running surface? A Adjustments in initial kinematics and joint moments at impact Q What criteria govern the changes in biomechanics? A Maintenance of impact forces

However, when we consider the

question “What influence do these changes have on lower limb chronic injury and performance?” the evidence presented so far might suggest that, if impact forces are maintained, then there will be similar injury profiles for

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different surfaces. We know from injury studies and from anecdotal evidence that this is not the case (9,10). In general, there is a greater incidence of injury reported on stiffer surfaces. Two suggestions are provided for the discrepancy in findings regarding impact forces and the incidence of injury for surfaces of different stiffness. It may be that the externally measured ground reaction force is not the most important factor when considering injury. For example, internal loading may change as a result of the observed kinematic adjustments, despite similar ground reaction forces. It is these internal forces that are likely to be important when considering injury mechanisms. However, we do not have evidence that internal impact loads vary with changes in cushioning of the shoe–surface interface. In contrast, the few studies to have estimated internal loads on structures such as the ankle joint (11) for different cushioning provision have detected no change in internal loading. An alternative explanation for the discrepancy between impact force results and injury findings is that the observed kinematic adjustments are not an attempt to reduce injury, but that there are alternative criteria governing the changes in biomechanics.

LOWER LIMB STIFFNESS An alternative approach to the study of human response to changes in surface stiffness has been through the monitoring of lower limb stiffness throughout ground contact, rather than focusing on the impact phase. In particular, Farley and co-workers have represented the human as a spring– mass model (Fig. 3), allowing monitoring of the vertical deformation of the human and the surface (1,2,12). Using this approach, Farley and colleagues have demonstrated that the combined deformation of the surface and the human centre of gravity relative to the surface are maintained at similar levels, regardless of the surface stiffness, resulting in a consistent vertical oscillation of the human centre of mass. This is illustrated most clearly in a study where subjects were monitored as they ran from a soft surface on to a hard surface (Fig. 4) (2). Within the first step on the hard surface, subjects increased the vertical displacement of their centre of mass relative to the surface, resulting in the same overall displacement of the centre of mass and surface as occurred during the prior step on the soft surface. The increased vertical displacement of the centre of mass relative to the surface therefore compensates for the reduced

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