Revisiting the effect of manipulating lumbar stability with load magnitudes and positions: The effect of sex on trunk muscle activation.

Background: Lumbar spine stability is regularly studied by positioning different loads at different heights and distance and measuring trunk muscle activation changes. Some of these studies have reported sex differences, but this needs to be revisited while controlling for confounding factors.

Method: 20 males and 20 females sustained three static standing postures, with various loads (0, 5 and 10% of body weight), to evaluate the effect of height and distance. Activation of 12 trunk muscles was recorded with surface electromyography (EMG).

Results: Females activated their external obliques a little more than males, with increases ranging between 1.5 and 2.3% of maximal voluntary activation (MVA), which corresponds to strong effect sizes (Cohen's d ranging between 0.86 and 1.13). However, the significant Sex × Height, Sex × Distance and Sex × Load interactions observed for different trunk muscles led to small differential effects (≤1% MVA). Increasing load height slightly increased and decreased back and abdominal muscle activation, respectively, generally by less than 1% MVA.

Conclusion: The higher activation of the external obliques observed in females might be of clinical value, relative to the required overall trunk muscle activation (5%), to preserve lumbar stability. Other effects were negligible.