Foot strike patterns

A preliminary investigation into the distribution of foot strike patterns was carried out for all 28 participants. A scatter plot of the strike index (Figure 33) indicates a clear bimodal distribution in the foot strike index, which is consistent across speeds. For this reason, any participant exhibiting a foot strike index up to and including 33 % was defined as a

rearfoot striker, while anyone exhibiting a foot strike index equal to or greater than 50 % was defined as a forefoot striker. Those participants, at a particular speed, exhibiting a foot strike index between these values (34 to 49 %) were excluded from further analysis. Figure 33 shows that only three participant’s fall into this “middle” group of foot strike indexes, and all these are at speed 3. Furthermore, one of these participants is very much on the border of being classified as a forefoot striker.

An interesting relationship between foot strike pattern and speed also becomes apparent from Figure 33. It appears that some athletes transitioned from rearfoot patterns at the

slower speeds to forefoot strike patterns at the faster speeds (Figure 33 and Table 4). In general, participants had only a small variability in their foot strike index as speed increased; this is evident from the clusters in Figure 33. However, eight participant’s transitions from a rearfoot pattern at speed 1, 2 or 3 to a forefoot pattern at speed 4; these are indicated by the dotted vertical lines in Figure 33. Furthermore, three of these eight were those participants who had a strike index between these 34 to 49 % at speed 3.

Figure 33 – Scatter plot showing the distribution of the foot strike index among (n=28) participants. The black, red, green, and blue markers represent speeds 1 – 4, respectively. The dotted vertical lines show the participants who transitioned from a rearfoot strike pattern at the slower speeds to a forefoot strike pattern at faster speeds.

Table 4 - Proportion of forefoot and rearfoot strikers at each speed.

Speed 1 Speed 2 Speed 3 Speed 4

Forefoot 11 12 13 19

4.2.3. Force-length relationships

In this investigation, the lower limb length was defined as the distance from the

instantaneous hip joint centre (used as a surrogate for the CoM) to average CoP of the foot (over the stance phase) (Figure 34). In the literature there are a variety of ways in which the lower limb length is defined, including estimating the distance from the CoM to CoP throughout stance (Lipfert et al. 2012) and using the height of the greater trochanter during standing as the resting length and estimating the change in length using the velocity, contact time and the change in vertical CoM position (McMahon et al. 1990, Farley et al. 1996, Morin et al. 2007).There appears to be no clear rational for choosing one over the other. However, using the CoP as the distal end of the spring violates the assumption of a fixed contact point and essentially changes the spring throughout stance. On the other hand, estimating the length change with the vertical CoM displacement assumes

appropriate integration of the vertical acceleration and that maximum spring compression occurs at exactly the point of the maximum vertical GRF, which is not necessarily the case. In contrast, the definition chosen in this investigation satisfies the assumption of the spring mass model, a fixed ground contact point, and provides an in-plane estimate of the CoM. Furthermore, since the Visual3D model used here does not include the arms and head, the estimated CoM will be offset from the anatomical CoM in both the AP and vertical directions (Gill et al. 2017), suggesting using the hip joint centre as an

approximation of the CoM movement is appropriate. If and how the definition of lower limb length affects the force-length relationship will be discussed in more detail later (chapter 5.2.2 and 5.3.1).

Figure 34 - Definition of the lower limb length.

To address the fundamental assumption of the spring mass model (that the spring exhibits linear elastic properties) force-length relationships for individuals were determined. The lower limb force was calculated as the component of the GRF acting along the line of the lower limb (defined above) from the GRFs, the approach angle of the lower limb and the angle of the resultant GRF (Figure 35, Eq. 11). In this case, 𝑥 and 𝑧 are the horizontal and vertical distances between the proximal and distal endpoints of the lower limb,

respectively. Therefore, the lower limb force, Flimb, was calculated by first determining the resultant GRF, and projecting this vector onto the lower limb axis. It is worth noting that the force perpendicular (Fperp in Figure 35) to the lower limb force is neglected in this scenario; a simplification inherent to the spring mass model. Although this perpendicular component would have an influence on the moment acting at the contact point and the rotational velocity of the spring, the contact point is assumed to be a fixed frictionless pin joint and only the relationship between the spring compression and the force acting along the lower limb are considered to be important here.

𝐹𝑙𝑖𝑚𝑏 = GRF ∗ cos 𝜃𝑑

𝜃𝑑 = 𝜃𝐺𝑅𝐹− 𝜃0

Figure 35 – Lower limb force, Flimb, determined by projecting the resultant GRF onto the lower limb axis, where θd represents the difference between the resultant angle,

θGRF, and the approach angle, θ0.