Whole-body angular momentum during step initiation in young and older adults

1

Keywords:

• Dynamic stability
• step initiation
• elderly
• angular momentum
• biomechanics

1. Introduction

Evidence suggests that humans regulate the angular momentum about the body’s centre of mass (CoM), commonly referred to as whole-body angular momentum (WBAM), for maintaining dynamic balance during daily living activities (Robert et al. 2009). Interestingly, it has been reported that poor regulation of WBAM is associated with a poor balance and a high risk of falling in people with mobility impairments such as amputees and post-stroke patients (Neptune and Vistamehr 2018). In healthy old adults, a previous study revealed that these people had difficulties to reduce the sagittal WBAM during the recovery phase of tripping (Pijnappels et al. 2005), which predisposed them to a fall after tripping. Nevertheless, to date it remains unclear whether and how the normal aging affects the WBAM regulation during volitional activities, such as walking or stepping. Yet, it has been shown that frequent falls in elderly people occur during these activities (Robinovitch et al. 2013).

The aim of the present study was to investigate the influence of aging on the regulation of three-dimensional WBAM during the initiation of volitional stepping. We hypothesize that older adults (OA) would exhibit higher ranges of WBAM than young adults (YA) during initiation of stepping, suggesting a greater balance control challenge in the elderly.

2. Methods

Nineteen healthy OA (age: 65 ± 3.3 years) and nineteen healthy YA (age: 22 ± 3.1 years) participated in this experiment. All participants were physically active and without fall experience within the last 12 months. All subjects gave their written consent after being fully informed of the test procedure.

Stepping was initiated from a first force-plate. A second force-plate was located immediately in front of this initial force-plate so that the subjects naturally step onto it. Thirteen retro-reflective markers were fixed on bony landmarks according to a simplified kinematic model (Tisserand et al. 2016) and three others were fixed bilaterally on the hallux, fifth metatarsal and calcaneus. A motion capture system equipped with 12 cameras was used to collect simultaneously the 3D whole-body kinematic data at 200 Hz and force-plate data at 1000 Hz. Initially, subjects were instructed to step forwards at their self-selected preferred pace after receiving a verbal GO-signal. They were asked to initiate step with their dominant leg and to follow through with the non-dominant leg to stop in a confortable upright posture. After two familiarization trials, five trials were collected.

The anteroposterior and mediolateral coordinates of the centre of pressure (CoP) were calculated from force-plate data. The three-dimensional coordinates of the body’s CoM position were computed as the weighted sum of each body segment’s CoM from a 9-segment biomechanical model (Tisserand et al. 2016). The net external moment about the body’s CoM (M_ext), which is equivalent to the time rate of change of WBAM, was calculated as the cross product of the moment-arm vector (i.e., the vector between CoM and CoP) and the GRF vector plus the free moment vector about the CoP (only the vertical component is not nil). WBAM was computed by numerical integration of M_ext using the trapezoidal rule (Pijnappels et al. 2005).

Step initiation was divided into two phases: the double-support phase (i.e., the time delay between the onset of movement to the toe-off of the swing leg) and the step execution phase (i.e., the time between the swing toe-off and the swing heel-contact). Spatial-temporal parameters such as the durations of the two phases, the progression velocity, the step length and width were calculated. Mean M_ext in both phases of stepping were calculated to quantify the regulation of WBAM (Neptune and Vistamehr 2018). In addition, the range of WBAM defined as the peak-to-peak value was calculated in each phase. M_ext were normalized by the participant’s weight and height. WBAM was normalized by the participant’s mass, height and$\sqrt{g.l},$ where g corresponds to the gravitational constant and l is the subject height. Student t-tests were conducted on each of these variables to test for differences between young and older groups.

3. Results and discussion

Compared to younger, OA initiated stepping with a slower progression velocity (OA: 0.70 ± 0.13 m.s⁻¹ vs. YA: 0.79 ± 0.14 m.s⁻¹; p < 0.05), shorter step (OA: 0.54 ± 0.09 m vs. YA: 0.61 ± 0.08 m; p < 0.05) and longer step duration (OA: 0.43 ± 0.08 s vs. YA: 0.39 ± 0.06 s; p < 0.05). Further, our results showed that WBAM differed between both age groups (Table 1). In the double support phase, older subjects exhibited smaller ranges of WBAM in the sagittal and transversal planes compared with their younger counterparts. These lower WBAM ranges were mainly explained by the smaller mean Mext generated in these two planes by the older adults (Table 1), which resulted in lower WBAM backwards and toward the stance leg in the sagittal and transversal planes, respectively.

Table 1. The mean range of normalized WBAM (x10⁻³) and the mean normalized external moment (Mext x10⁻³) for YA and OA.

	YA (19)	OA (19)	P-value
Double support phase
WBAMFrontal	2.2 ± 0.7	2.0 ± 0.8	N.S
WBAMSagittal	5.8 ± 1.3	4.5 ± 1.2	<0.01
WBAMTransversal	1.6 ± 0.5	1.2 ± 0.3	<0.01
MextFrontal	−1.0 ± 0.7	−0.8 ± 0.6	N.S
MextSagittal	3.1 ± 1.3	2.3 ± 1.0	<0.05
MextTransversal	0.9 ± 0.3	0.6 ± 0.2	<0.05
Step execution phase
WBAMFrontal	3.3 ± 2.0	4.8 ± 1.9	<0.05
WBAMSagittal	5.9 ± 1.5	8.4 ± 2.0	<0.001
WBAMTransversal	2.1 ± 0.6	2.3 ± 1.1	N.S
MextFrontal	3.0 ± 2.7	3.7 ± 2.8	N.S
MextSagittal	−4.9 ± 2.5	−6.6 ± 2.2	<0.05
MextTransversal	−2.2 ± 0.8	−2.0 ± 1.1	N.S

Possibly due to the decrease in ranges of WBAM in the double support phase, OA had higher ranges of WBAM in the frontal and sagittal planes than YA during the subsequent step execution phase, which may impose a greater balance control challenge during this latter phase in the elderly. These higher WBAM ranges during the step execution phase were mainly explained by the longer duration of this phase, i.e., the longer time of application of M_ext, resulting in larger WBAM towards the swing leg and forwards in the frontal and sagittal planes, respectively. Nevertheless, in the sagittal plane, a greater M_ext was also found in the OA (Table 1), which also contributed to the higher range of WBAM in these individuals. These results are consistent with the findings from Pijnappels et al. (2005), who showed that the range of WBAM in the sagittal plane was larger in OA than in YA during the recovery phase after tripping. This insufficient reduction in WBAM after tripping was associated with a higher rate of falls in OA. Future studies should therefore examine why older individuals are less able to reduce WBAM during the step execution phase.

4. Conclusions

The present study shows that aging affects the regulation of WBAM during initiation of stepping. In particular, older adults exhibit a higher range of WBAM during step execution than young adults, which may impose a greater balance control challenge in the elderly and thus could predispose them to a higher risk of falling during stepping.

Additional information

Funding

A Regional Research Grant from the Réunion Region and from the European Regional Development (FEDER) supported this work.