Heel Contact

Figure 8.2.4 illustrates that older adults had a small anterior margin to balance loss at dominant heel contact in narrow walking. In contrast, at non-dominant heel contact, lead foot toeing-in extended the anterior margin, clearly seen in the narrow and wide walking conditions below (Figure 8.2.4).

Figure 8.2.4 Safety Zone at heel contact. HY = healthy young adults; HO = healthy older adults; CONTACT = the point of balance loss initition based on CoM velocity. Safety Zone, yellow = common to HY and HO; blue = only for younger adults; green = only for older adults. X axis (cm), positive (> 0) = direction to the dominant and negative (0>) to the non- dominant foot. Y axis (cm) = anterior-posterior. Locations relative to the CoM (0, 0).

 ‘Toeing-in’ of the Non-dominant foot :

Lead Foot: Toe position was most medial in narrow walking (4.9cm) followed by wide

(9.8cm) and preferred width walking (10.2cm); F (2, 53) = 40.1, p < .01, confirming toeing-in during width-controlled walking. Lead heel location was, in contrast, consistent with the step width manipulations, more lateral in wide and medial in narrow (F (2, 53) = 35.9, p < .01). The non-dominant lead heel was more laterally displaced from the CoM compared to the dominant lead heel (1.8cm); F (1, 54) = 7.0, p < .05, accounting for lead foot toeing-in only for the non-dominant lead contact foot. As clearly seen in Figure 8.4.1, non-dominant lead foot toeing-in during width controlled walking conditions was unique to the older group (age x width: F (2, 53) = 3.7, p < .05).

Trail Foot: The older group showed a 0.9cm more medial trail toe (F (1, 54) = 4.4, p < .05) despite no differences in trail heel. Trail foot was most lateral in wide and least in narrow walking (toe: F (2, 53) = 71.5, p < .01, heel: F (2, 53) = 13.5, p < .01). Toeing-in was characterised in the older group’s medial non-dominant trail toe, more in narrow and less in wide walking (age x width: F (2, 53) = 8.6, p < .01).

 Higher CoM Variability in Older Adults:

Lead Foot: Older adults showed higher variability in ML CoM of the lead foot (>0.3cm) at toe (F (1, 54) = 19.7, p < .01) and (>0.6cm) at the heel (F (1, 54) = 21.9, p < .01). The effect was more pronounced at non-dominant lead foot contact (age x limb: toe F (1, 54) = 9.4, p < .01; heel F (1, 54) = 10.7, p < .01).

Trail Foot: More variable ML CoM in the trail foot’s Safety Zone was detected in the older group by (>0.4cm); toe: F (1, 54) = 26.3, p < .01, heel: F (1, 54) = 34.1, p < .01.

 Effects of Width Controlled Walking on the ML CoM Variability

Lead Foot: During width controlled walking ML CoM control of the lead foot’s side became less variable at toe (F (2, 53) = 10.1, p < .01) but not significantly with lead heel. This effect was observed only in the older group (age x width: F (2, 53) = 15.0, p < .01).

Trail Foot: During width controlled walking reduced variability was seen in ML CoM

Only older adults showed this effect (age x width: toe: F (2, 53) = 17.0, p < .01, heel: F (2, 53) = 13.5, p < .01).

 Less AP Foot Separation in Older Adults

Lead Foot: The older group had reduced anterior lead foot relative to the CoM at toe (F (1, 54) = 6.2, p < .05) but not significantly different at heel (F (1, 54) = 3.6, p = .065).

Trail Foot: Posterior separation of the trail foot from the CoM was smaller (6cm); toe F (1, 54) = 45.6, p < .01; heel F (1, 54) = 20.9, p < .01.

8.3 Chapter Summary

The four most notable ageing effects on the Safety Zone are 1) non-dominant foot’s toeing-in during width controlled walking, 2) less consistent ML CoM control 3) more posterior swing foot location from the CoM at MFC and MLM and 4) less AP separation of both feet relative to the CoM.

Older adults’ non-dominant lead foot’s toeing-in extended the anterior boundary of the Safety Zone at toe-off and heel contact. When in trail foot, however, toeing-in reduced the AP Safety Zone. Non-dominant foot’s toeing-in appeared only when older adults walked with controlled width most clearly in narrow and less in wide walking.

Although step width variability was not differentiated between the age groups (Chapter 8), the more variable ML CoM position within the Safety Zone was characterised in the older group, possibly reflecting age-related loss of ML balance control.

Swing foot location at MFC and MLM were found to be more posterior in older adults. Consequently, swing foot at MLM was relatively more parallel to stance foot, causing the shorter anterior boundary of the Safety Zone. Consistent with the previous finding (Lugade et al., 2011), older adults showed smaller AP separation of both feet from the CoM at toe-off and heel contact possibly reflected in reduced step length in older adults.

Width controlled walking reduced ML CoM control especially in the older adults’ non-dominant stance foot. Walking on lines can be therefore considered to assist the more consistent ML Safety Zone control.

CHAPTER 9

Gait Data Computation for Balance Simulation

9.1 Overview

One application of the Safety Zone Model is in devising methods for balance loss simulation due to any perturbations to either CoM or foot, as outlined in section 3.3. This chapter presents balance loss simulation using the experimental data obtained from both young and older adults during preferred step width walking. All the balance loss simulations at the examined swing phase events required segment mass of CoM and for this reason, the transverse pelvis CoM was used for simulation as proposed by Kingma et al. (1995). Except for lateral balance loss simulation where perturbation is modelled directly into CoM, it was necessary to estimate how perturbation of the foot would affect CoM movement. Following the inverted pendulum model, this concept can be described by expressing perturbation as kinetic inputs on foot that generates net force transferred to CoM.

9.2 Results

The leg length and segment mass of the foot and pelvis for the young and older groups were obtained by Visual 3D modelling (Table 9.2.1).

Table 9.2.1 Average segment mass of foot and pelvis and lower limb length of the two age groups. Lower limb length = from lateral malleolus to greater trochanter

Young Older

1 Foot Segment Mass (kg) 1.10 1.11

2 Pelvis Segment Mass (kg) 10.7 10.9

3 Leg Length (m) 0.81 0.77