Torso muscle onset in response to an unexpected lower body perturbation in young adults, older adults and trained participants

In older adults, risk of fall has been associated with poor muscle quality, dysfunction and reduced muscle thickness. Research has typically focussed on muscle quality through capacity tests and the identification of sarcopenia. However, little is understood about the quality of the torso muscle in older populations and particularly torso muscle onset when recoveringfrom an unexpected perturbation. Recovery from unexpected perturbations like a slip, require fast and efficient motor control. However research that has focussed on torso muscle onset, have used tasks that are self-initiated which, if repeated, can lead to modulated motor control responses. Therefore, the aim of this study was to develop a test that would reveal torso muscle thickness and onset responses to an unexpected lower body perturbation event that mimicked a unilateral slip, in younger, trained and older participants. Developing a test that could identify deeper and superficial torso muscle onset simultaneously required the synchronisation of two methods of onset detection; a perturbation device and motion capture to orchestrate the synchronised timing. Reliability of Rehabilitative Ultrasound Imaging (RUSI) B-mode to capture muscle thickness revealed very good to excellent agreement for sonographer reliability (ICC, 0.796 to 0.995) and measurement method (0.995 to 0.9997). The perturbation device was monitored throughout testing to assess the factors that affect the force and velocity of the perturbation. RUSI M-mode was used simultaneously with sEMG to assess deeper and superficial torso muscles onset. Reliability of onset detection methods were assessed using erector spinae sEMG signals. The addition of a TKEO application within signal processing improved accuracyand reliability of computerised algorithm detection methods on the sEMG signals (ICC 0.8152 95% CI 0.723 to 0.875). In order to use RUSI to measure TrA and IO muscle onset, the agreement between EO tissue deformation via RUSI with EO onset via sEMG was determined using Bland Altman Limits of agreement (LOA). The LOA for RUSI and sEMG were calculated separately for older (4.45 ms, 95% CI; -7.25 to 16.15), younger (-9.65 ms 95% CI; -5.91 to - 13.38) and trained (– 7.87 ms, 95% CI; -23.26 to 7.52) participants and applied to muscle onset values for final comparison. Older participants revealed significantly later onset times than younger participants (p