# Investigating the effects of age and conditioning stimulation intensity on SMA–M1 connectivity in younger, middle-aged, and older adults

## TL;DR

This study examines how age and stimulation intensity affect brain connectivity related to motor control in younger, middle-aged, and older adults.

## Contribution

The study provides new insights into age-related changes in SMA–M1 connectivity and bilateral motor performance.

## Key findings

- Older adults showed significantly poorer motor performance compared to younger and middle-aged adults.
- No conclusive age-related differences in SMA–M1 connectivity were found.
- Middle-aged adults exhibited motor performance similar to younger adults.

## Abstract

This study aimed to investigate bilateral motor control and connectivity between supplementary motor area (SMA) and primary motor cortex (M1) in younger, middle-aged, and older healthy adults.

32 younger (mean age 22.8 ± 5.3 years), 18 middle-aged (47.6 ± 6.5 years), and 23 older (75.8 ± 6.7 years) adults were tested. Bilateral motor control was assessed using the Purdue pegboard. Dual-site transcranial magnetic stimulation (TMS) was used to measure SMA–M1 connectivity at different conditioning stimulation intensities.

Older adults had significantly poorer motor performance than younger and middle-aged in all pegboard subtests. Notably, there were no conclusive differences in motor performance between younger and middle-aged adults. There was no conclusive evidence supporting age-related and intensity-related differences in SMA–M1 connectivity between younger, middle-aged, and older adults. There was also no conclusive evidence to support clear associations between SMA–M1 connectivity and bilateral motor control.

Age-related declines in bilateral motor functioning was found in older, but not middle-aged adults. The bilateral motor functioning of middle-aged adults is more young-like than old-like. The lack of conclusive age- and intensity-related differences in SMA–M1 connectivity, and lack of conclusive association with bilateral motor performance, might be due to high inter-individual variability in SMA–M1 connectivity. Potential factors contributing to this variability include SMA and M1 morphometry, the structural connectivity between these regions, and the localisation of SMA.

The online version contains supplementary material available at 10.1007/s00421-025-05904-0.