Upper body accelerations during planned gait termination in young and older women
Introduction
Stabilization of the head with respect to the environment is fundamental to preserve whole body balance during locomotion (Cromwell et al., 2001, Mulavara and Bloomberg, 2003, Pozzo et al., 1990). As an inertial guide platform, head stability helps to maintain gaze by optimizing input from visual, vestibular and somatosensory systems (Cromwell et al., 2004, Pozzo et al., 1990). This mechanism is threatened over each gait cycle by the heel strike that provokes a sudden impact acceleration, which is transmitted from the lower body structures to the head (Cappozzo, 1991, Mulavara and Bloomberg, 2003). During level walking, young adults adopt a ‘‘head stabilization in space’’ strategy performed at the upper body level and aimed at attenuating the acceleration transmitted from pelvis to head, following a strain-reducing criterion (Cappozzo, 1991).
Head stability is influenced by gait speed (Menz et al., 2003a, Moe-Nilssen, 1998), with increasing upper body accelerations at fast speed (Hirasaki et al., 1999, Kavanagh et al., 2006, Menz et al., 2003a). In fact, older individuals typically adopt a slowness strategy by reducing walking speed and head accelerations in the antero-posterior (AP) and vertical (VT) directions, but not in the medio-lateral (ML) direction, compared to young individuals during level walking (Mazzà et al., 2008, Menz et al., 2003b). This reduced attenuation of ML acceleration may further deteriorate in more challenging circumstances than walking, leading to augmented head lateral acceleration and, thus, to an unstable reference platform for vision and vestibular systems (Lord et al., 1996).
Transitory locomotor tasks, in particular, involve complex interactions between neural and mechanical factors which may challenge upper body stability (Laudani et al., 2006) and whole-body balance to a greater extent than unconstrained walking (Nagano et al., 2013). This challenge may help to explain why the number of falls in older individuals are frequent during locomotor transitions such as gait termination (Sparrow and Tirosh, 2005, Winter, 1995). From a biomechanical perspective, gait termination is defined as a transition between steady-state walking and upright standing, which requires a minimal level of extensor torque in lower limb joints and an adjustment of foot placement: both factors are aimed at keeping the Center of Mass (CoM) behind the feet center of pressure (CoP). This regulation of CoM-CoP vector generates decelerating forces that are therefore able to reduce forward speed and terminate gait (Jian et al., 1993). The literature reports that older adults have a diminished activation of lower limb extensor muscles when terminating gait in response to a visual cue (Tirosh and Sparrow, 2005, Tirosh and Sparrow, 2004), thus outlining a reduced involvement of lower body without taking into account the role of the upper body in performing the gait termination task. In fact, as the upper body represents 2/3 of the whole body mass, its movement could effectively help in completing the task by controlling the CoM position and, therefore, peculiar acceleration patterns could be expected for this transitory task. To the authors’ knowledge, there is no study in the literature looking at the pattern of upper body accelerations during gait termination and how these accelerations are transmitted through the body along the three directions of space. These quantities are directly related to dynamic balance and have the potential to unveil peculiar motor strategies adopted by either young or older individuals (Mazzà et al., 2008).
The aim of this study was to compare upper body patterns of acceleration during planned gait termination between young and older women. It was hypothesized that older women might present lower attenuation of acceleration from pelvis to head than young, especially in the ML direction.
Section snippets
Participants
Ten healthy young women (age: 23.1 ± 1.1 years; height: 1.66 ± 0.06 m; body mass: 56.9 ± 6.6 kg) and ten healthy and independent community-dwelling older women (age: 73.8 ± 2.4 years; height: 1.60 ± 0.06 m; body mass: 62.1 ± 13.6 kg) volunteered to participate in this study. The homogeneity of the two groups with respect to the body mass parameter was verified through an independent sample t-test (p > .05). The age groups were divided according to the following inclusion criterion: 20–30 years
Results
Both mean AP CoM speed and duration in each phase are reported in Table 1. Older women had lower AP CoM speed than young women in the Approaching and Breaking Phases at each of the three speeds (F >23.384; p < .001) and in the Stabilization Phase only at fast speed (F = 10.266; p < .005). While a progressive increase of AP CoM speed was reported between speed conditions at each of the three phases in young women, such increase was observed only at the Approaching Phase in older women. The
Discussion
The main finding of this study was that older women showed lower acceleration RMS and attenuation of acceleration from pelvis to head than young, with specific patterns in each gait termination phase. In the Approaching Phase, older women showed lower ML accelerations at pelvis than young women and, associated to this, lower attenuation from pelvis to trunk on the same axis. In the Braking Phase, older women exhibited lower pelvis-to-trunk attenuation even in the AP direction. Finally, in the
Acknowledgments
The authors wish to thank Dr. Amy Maslivec for her support in data acquisition.
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