Proyecto de aula y estrategias didácticas

Older adults were reported to slip further with greater velocity while the CoM horizontal anterior velocity reduces with age (Lockhart et al., 2003). The risk of balance loss due to anterior heel slipping is increased in older adults because when the lead-foot slips forward, the reduced CoM velocity is disadvantageous in preventing the CoM from catching up with the BoS that is moving away from the CoM (You et al., 2001). Safety adaptations to reduce the risk of slipping after heel contact include greater heel contact angle and flatter foot contact (i.e. less dorsiflexion) as discussed (Brady et al., 2000; McGorry et al., 2010). It can be reasonably assumed that older adults have flatter foot contact because they show reduced dorsiflexion prior to heel contact compared to young adults (Nagano et al., 2011). It is unknown whether heel contact angle increases or decreases with age. In one perspective, shorter step length seen in older adults can be due to limited maximum lead-limb forward stroking. If swing limb is placed down in the more conservative length, higher vertical-shank

floor contact angle can be expected in older adults. To achieve greater heel contact angle, the CoM should be located ideally straight above contact limb including foot, knee and hip joints.

2.8 Minimum Lateral Margin (MLM)

Toe-off and heel contact influence slipping risks when encountering a walking surface with an unexpectedly low CoF while at MFC, tripping related anterior balance loss is most likely. In contrast, the limited studies have investigated ML balance. In understanding ML balance minimum lateral margin (MLM) is critical, defined as the shortest ML distance between the CoM and the stance heel during the mid-swing phase (Figure 2.8.1). In definition, ML acceleration and velocity of the CoM should be zero at MLM because the CoM medial redirection takes place approximately at MLM. Same as MFC, MLMd (distance) and MLMt

(timing) describe MLM. Failure to provide successful medial redirection of the CoM at MLM is linked to the increased risk of lateral balance loss. In other words, unexpected ML balance perturbation at MLM is more likely to result in lateral balance loss than for any other part of swing phase. Although Åberg et al. (2010) measured MLMd between older adults with and

without fear of falling as described earlier in Figure 2.6.2, this concept has not been yet fully explored. One motivation for the current research in relation to ML balance is more advanced examination of MLM including MLMd and MLMt. Furthermore, as balance maintenance is

considered more difficult during single support, the period from toe-off to MLM (MLMt) can

be considered critical in terms of ML balance. Lateral CoP displacement and average medial acceleration on the CoM during this period have been thus captured for more comprehensive understanding underlying the mechanism of lateral balance perturbation at MLM.

Figure 2.8.1 Image of minimum lateral margin (MLM). MLMd = minimum medio-lateral

distance between stance heel and the centre of mass (CoM). Stance foot = shaded (black) foot, swing foot = non-shaded foot. AP = anterior-posterior, ML = medio-lateral. Time percentage (%) relative to the entire swing phase at MLM is MLMt.

Chapter 3

A Safety Zone Model of Balance When Walking

The previous section 2.6 has illustrated the traditional model of balance loss assessment method based on the centre of mass (CoM) and the base of support (BoS), but the limitation of the model in application of balance during single support has been also clarified for balance during single support. In this chapter, the Safety Zone Model is first defined and then, its application to assess balance at the swing phase events described in section 2.7 and 2.8 is explained. Furthermore, utility of safety zone model for balance loss simulations at these four events are described.

Previous biomechanical investigations of balance control when walking were limited because they did not capture the “dynamic” or time-varying nature of balance. Stability models are restricted to a theoretically defined event at double support which provides a “snapshot” of balance rather than balance in which the time-varying the base of support (BoS) changes in shape and extent as the gait cycle progresses. To address the research questions in this project a comprehensive dynamic Safety Zone Model was developed to more thoroughly investigate ageing effects on walking balance.

3.1Modelling the Safety Zone

Patla et al. (1991) first introduced the idea of ‘virtual base of support’ based on the transverse locations of the two feet during single support. This concept is necessary to further explore balance during single support. Regardless of whether or not the foot is in contact with the walking surface, the area between the two feet could be considered a safety zone, where the CoM can travel without hazardous balance loss (Figure 3.1.1). The Safety Zone during single support will represent the BoS in the case of swing foot placement vertically. Balance control during walking depends, therefore, critically, on controlling the CoM within the Safety Zone but in this model the swing foot’s trajectory continuously changes the Safety Zone boundaries, both along anterior-posterior (AP) and medio-lateral (ML) directions. Furthermore, centre of pressure (CoP) excursion controls the CoM.

Figure 3.1.1 Image of the Safety Zone Model. The Safety Zone = light blue shaded area. X+ = lateral(right), y+ = anterior-posterior, z = vertical.

First, in the Safety Zone Model, the spatial transverse area between the two toe-heel lines of both feet (Figure 3.1.2) represents conservative boundaries for lateral CoP movement. The line between two toes defines the anterior boundary of the Safety Zone while the two heels define the posterior boundary.

The typical CoP trajectory in normal gait begins approximately at the heel, passing through the plantar surface area by taking a slight lateral curve and disappearing at toe (Shanthikumar et al., 2010). Despite functional necessity in lateral CoP movement to redirect CoM medially, any excessive lateral CoP displacement can be a risk factor for lateral balance perturbation (Han et al., 1999; Rougier, 2009; Rugier, 2008; Winter et al., 1996). At heel contact, for example, as little as extra 2cm lateral deviation of CoP can cause lateral balance loss (Rietdyk et al., 1999).

Compared to the relatively stable double support phase, it can be more difficult to maintain balance during single support. ML CoP displacement therefore tends to be

minimised during single support (Hof et al., 2005) as shown in Figure 3.1.2 as the typical CoP path during normal gait. In an event of the swing foot’s unexpected placement, sudden onset of CoP in the area lateral to the typical CoP path (e.g. the very lateral edge of the foot) could raise a risk of inversion sprain and associated risk of ML balance perturbation. Typical CoP path instead of the entire contact area may be therefore more appropriate to define safety zone especially during single support. For this reason, the lateral boundaries of the Safety Zone Model are defined by toe-heel lines of both feet (Figure 3.1.2).

Figure 3.1.2 Toe-heel line and typical CoP path during stance: from heel contact to toe-off (Cook et al., 2008).