Muscle imbalance among elite Australian rules football players: a longitudinal study of changes in trunk muscle size.

CONTEXT: Trunk muscles, such as the transversus abdominis (TrA) and multifidus, play a key role in lumbopelvic stability, which is important in athletic performance. Asymmetry or imbalance in these and other trunk muscles could result from the specific requirements of the game of Australian rules football.

OBJECTIVE: To determine whether seasonal variations in the sizes of key trunk muscles associated with lumbopelvic stability occur in Australian Football League players.

DESIGN: Cross-sectional study.

SETTING: Hospital.

PATIENTS OR OTHER PARTICIPANTS: The number of players eligible to participate at each of the 4 time points was 36 at the start of preseason 1 (T1), 31 at end of season 1 (T2), 43 at the end of preseason 2 (T3), and 41 at the start of preseason 3 (T4). The group with data at all 4 time points (n = 20) was used in the analyses and was shown to be representative of the total sample.

INTERVENTION(S): Magnetic resonance imaging was used to determine the cross-sectional areas (CSAs) of the multifidus (vertebral levels L2 to L5) and lumbar erector spinae (LES) muscles (L3), as well as the thickness of the TrA and internal oblique (IO) muscles at L3.

MAIN OUTCOME MEASURE(S): Cross-sectional areas of the multifidus and LES muscles and thickness of the TrA and IO muscles.

RESULTS: By the end of the playing season, results showed 11.1% atrophy for multifidus CSA at L3 and 21% atrophy for TrA thickness at rest. In comparison, the CSA of the LES muscles increased by 3.6%, and the thickness of the IO muscle increased by 11.8% compared with the start of the preseason.

CONCLUSIONS: The results indicated an imbalance of the key muscles associated with lumbopelvic stability.