Body configuration as a predictor of centre of mass displacement in a forward reactive step

Application of Principal Component Analysis to Forward Reactive Stepping: Whole-body Movement Strategy Differs as a Function of Age and Sex

BACKGROUND

Differences in reactive stepping strategy to recover balance have been investigated as a function of age and sex, but to date have been measured using discrete step or joint specific measures. It is unknown how whole-body strategy or underlying motor control objectives differ between age and sex groups in forward reactive stepping.

RESEARCH QUESTION

Does whole-body movement and/or motor control strategy differ as a function of age or sex in a forward reactive step to maintain balance?

METHODS

Forty young and older adults (45 females, 35 males) participated in this study. All participants performed five reactive stepping trials in response to a forward balance perturbation while whole-body kinematics and ground reaction forces were collected. Features of whole-body movement strategy were determined using a principal component analysis model. Average principal component (PC) scores were compared between groups as a measure of whole-body movement strategy and within participant relative standard deviation of PC scores were compared to determine if motor control objectives differed across groups.

RESULTS

Significant differences in reactive stepping strategy were observed both as a function of age and sex. Older adults had a greater step length and width, greater anterior trunk and pelvis translation, greater knee flexion angles and anterior translation of the hip joint on the stepping leg compared to young participants. Males had lesser step length and width, as well as greater trunk flexion compared to females. No differences in relative standard deviation of PC scores were observed between age or sex-based groups suggesting that motor control objectives were similar between groups.

SIGNIFICANCE

This study demonstrates how whole-body movement strategy differs as a function of age and sex, which explains why previously reported discrete outcomes occur. Additionally, it does not seem that motor control strategy objectives differ between age or sex groups in forward reactive stepping.

Analysis of invoked slips while wearing flip-flops in wet and dry conditions: Does alternative footwear alter slip kinematics?

Minimal footwear has become more ubiquitous; however, it may increase slip severity. This study specifically examined the slipping kinematics of flip-flop sandals. Invoked slips from standing were evaluated in dry and wet tile, and a unique wet footbed + wet tile condition, with 40, 50, and 60% bodyweight (BW) committed to the slipping foot. Water did not alter peak slip velocity (PV) at 40% BW, but PV increased with greater slip-foot force on wet tile by ~1 m/s. Interestingly, when floor-contact was lost during the slip, the flip-flops could come off the heel. This decoupling occurred most often when both the tile and footbed were either dry or wet. Given that both decoupling and greater PV were observed on wet tile, slipping in flip-flops under wet conditions may have more serious consequences. The results highlight that slips may occur at both the foot-flip-flop, and flip-flop-tile interfaces.