# Different trunk muscle responses to unexpected balance perturbations between older recurrent fallers and older non-fallers: a combined wearable ultrasound imaging and electromyography (EMG) study

## TL;DR

This study shows that older people who fall frequently have slower and weaker muscle responses in their trunk when balance is unexpectedly disturbed, compared to those who don't fall.

## Contribution

The study introduces a new method combining wearable ultrasound and EMG to explore muscle responses during reactive balance in older adults.

## Key findings

- Recurrent fallers showed 51% smaller internal oblique muscle thickness change after anterior perturbations.
- Fallers had slower and smaller EMG responses in rectus abdominis and internal oblique muscles during various perturbations.
- Wearable ultrasound imaging proved feasible for studying muscle morphological changes during reactive balance.

## Abstract

Falls are important public health issues among older people. Previous studies reported the fall-related alterations in electromyographic (EMG) activation of specific trunk muscles following a single direction of perturbation, while the activation patterns following multiple directions of balance perturbations remained unclear. Additionally, although the static structural/morphological characteristics of trunk muscles have been associated with balance performance, the fall-related trunk muscle contraction patterns for maintaining reactive balance remained unknown. This pilot observational study therefore aimed to comprehensively explore how trunk muscles’ activation and contraction patterns during reactive balance control differed between older recurrent fallers and older non-fallers.

Six community-dwelling older recurrent fallers (70.0 ± 5.1 years; 4 females; ≥2 falls in past one year), and six older non-fallers (70.8 ± 3.9 years; 5 females) were recruited. Eligibility criteria were aged 65 years or older, with independent mobility, without cognitive impairment or balance-affecting conditions, without recent injuries/surgery, and without recent structured exercise. Participants received unpredictable translational moving-platform balance perturbations during natural standing. The dominant-side trunk muscle thickness changes (measured by wearable ultrasound imaging) and electrical activities (measured by EMG) for maintaining reactive balance were focused, supplemented by analyzing pelvic motions and postural sways.

Compared to older non-fallers, older recurrent fallers had significantly: (1) 51% smaller rate (following anterior perturbations; d = 1.47; p = 0.029) and 50% smaller peak (following medial perturbations; d = 1.32; p = 0.045) of the internal oblique (IO) muscle thickness change; and (2) smaller peak of IO muscle EMG signal following medial perturbations, and slower/smaller EMG responses of the rectus abdominis (RA) muscle following posterior/medial/lateral perturbations (p < 0.05).

This study demonstrated the feasibility of using wearable dynamic ultrasound imaging to characterize fall-related morphological changes during reactive balance control in older adults. The participated recurrent fallers exhibited the perturbation-direction-specific slower/smaller activation and contraction of IO muscle and slower/smaller activation of RA muscle, as compared to non-fallers. Given the pilot study, future longitudinal research with adequately powered samples is warranted to confirm this preliminary finding, which may help complement the current fall-risk assessments and imply the targeted fall-prevention exercises in older adults.

The online version contains supplementary material available at 10.1186/s12877-026-07158-7.