BY DR SHARON DIXON PhD

of surfaces. We can experience a transition from concrete to soft turf and continue in our running stride with very little adjustment. Although we are aware that the surface has changed, it is difficult to identify specifically what we do differently in order to cope with differences in surface stiffness. Researchers have commented

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that an observer watching the upper body of an individual running from a hard surface on to a soft surface (or visa versa) would not detect that the performer has run on different surfaces, because of the similar movement patterns observed (1,2). This suggests that upper body and centre-of-gravity movement are the same, regardless of surface. Does this mean that we run the same on any surface, regardless of the surface’s properties?

STUDIES OF IMPACT FORCES Initial studies addressing the question of whether we run the same on any surface utilised a force platform to investigate whether the force occurring during ground contact as a result of human interaction (ground reaction force) differed for different running surfaces. In particular, the peak impact force and its rate of loading (slope of initial section of graph) have been compared (Fig. 1). If a runner were to perform with an identical running style regardless of surface, then the peak impact force would be expected to vary according to the stiffness of the surface, with a stiffer surface resulting in a greater impact force. In contrast, several studies have demonstrated similar impact force values regardless of running surface (3,4). This maintenance of consistent ground reaction force values regardless of surface suggests that lower limb biomechanics are adjusted in order to accommodate different surfaces.

uring running, humans are able to adapt to an incredible range

Important questions arising from this suggestion include: n What changes occur in lower limb biomechanics with changes in running surface? n What criteria govern the changes in biomechanics? n What influence do these changes have on lower limb chronic injury and performance? If we consider first the evidence for

changes in lower limb biomechanics, we see that possible strategies for biomechanical adjustment include changes in lower limb movement (kinematics) and changes in joint moments (5). Movement changes observed in the literature as being influential on impact forces include knee joint flexion and heel impact velocity. For example, some authors have observed a greater knee flexion at ground strike when the shoe–surface interface is relatively stiff (3,6). Similarly, reductions in heel impact velocity have been detected on stiffer surfaces (3,7). A greater knee flexion when impacting

Peak impact force

Vertical GRF (N)

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stiffer shoe–surface interface, resulting in similar impact forces regardless of surface conditions.

Loading rate 50 100 time (ms)

Figure 1: Typical ground reaction force (GRF) for heel–toe running, illustrating peak impact force and peak rate of loading

the ground is suggested to increase the compliance of the lower limb at this time, utilising intrinsic cushioning mechanisms of the human to a greater extent. Similarly, a reduced heel impact velocity indicates a more controlled placement of the foot on the ground, contributing to a reduced impact loading. These kinematic strategies are suggested to compensate for the reduced cushioning provided by the

HIGHLIGHTED EVIDENCE THAT RUNNERS ADJUST LOWER LIMB STIFFNESS FOR DIFFERENT SURFACES.

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CONSIDERATION OF THE ENTIRE CONTACT PHASE, RATHER THAN JUST IMPACT, HAS

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Changes in joint moments have also been suggested as a possible strategy for the control of impact forces in running (5). The joint moment represents the resultant torque applied by all muscles acting about a joint and will influence the stiffness of the joint. Few studies have focused on lower limb joint moments during the impact phase of running for different surface conditions. A recent study has, however, highlighted that ankle joint moments immediately following ground contact may differ for different surface conditions (8). This study monitored the peak ankle dorsiflexor moment for surfaces of distinct stiffness, including concrete and foam. The peak ankle dorsiflexor moment represents the overall muscle action attempting to control the initial rapid plantarflexion of the foot during the impact phase of heel–toe running (Fig. 2). It was found that the peak dorsiflexor moment was significantly higher for stiffer surface conditions, suggesting that a greater demand is placed on the dorsiflexor muscles (eg. tibialis anterior) to control the impact when running on a stiff surface.

Using the evidence presented sportEX medicine 2008:38(Oct):10-13