Sex and limb comparisons of neuromuscular function in the morning versus the evening

Abstract

The time-of-day influence on neuromuscular function is well-documented, but important details remain elusive. It is currently unknown whether males and females differ in their diurnal variation for optimal neuromuscular performance. The purpose of this study is to identify the time-of-day influence on neuromuscular function between sexes and determine whether these responses differ for the upper versus lower limbs. A group of males (n = 12) and females (n = 15) completed neuromuscular performance testing in the morning (07:00-09:00) and evening (17:00-19:00) on separate days in a randomized order. Maximal force, the normalized rate of force development, EMG, normalized EMG rise, and submaximal force steadiness were compared between morning and evening hours. The main findings show that maximal force was greater in the evening for the knee extensors (d = 0.570, p < 0.01) but not the elbow flexors (d = 0.212, p = 0.281), whereas maximal muscle excitation was greater in the evening for the biceps brachii (d = 0.348, p < 0.01) but not the vastus lateralis (d = 0.075, p = 0.526) with no influence of sex. However, force steadiness during knee extension was superior in the evening versus the morning for males (d = 0.734, p = 0.025) and compared to evening values for females (g = 1.19, p = 0.032). Overall, these findings show that time-of-day affects the knee extensors more than the elbow flexors and that diurnal variability between sexes appears to be task-dependent.

Keywords: EMG; RER; RFD; diurnal; force steadiness; sex differences; time of day.

FIGURE 1

Plots of maximal isometric force values during knee extension in the morning (07:00–09:00 h) and the evening (17:00–19:00 h) for each individual. The darker circles and lines reflect females (n = 15), and the lighter circles and lines reflect males (n = 12). Means are represented by the horizontal black bar and variability (SD) by the grey shading. *Significant (p < 0.01, d = 0.570) difference between morning and evening

FIGURE 2

Plots of maximal isometric force values during elbow flexion in the morning (07:00–09:00 h) and the evening (17:00–19:00 h) for each individual. The darker circles and lines reflect females (n = 15), and the lighter circles and lines reflect males (n = 12). Means are represented by the horizontal bar and variability (SD) by the grey shading

FIGURE 3

Plots of maximal EMG amplitude during elbow flexion in the morning (07:00–09:00 h) and the evening (17:00–19:00 h) for each individual. The darker circles and lines reflect females (n = 15), and the lighter circles and lines reflect males (n = 12). Means are represented by the horizontal bar and variability (SD) by the grey shading. *Significant (p < 0.01, d = 0.348) difference between morning and evening

FIGURE 4

Plots of the coefficients of variation during steady submaximal isometric contractions of the elbow flexors and knee extensors in the morning (07:00–09:00 h) and the evening (17:00–19:00 h) for males (n = 12) and females (n = 15). Means are represented by the horizontal bar and variability (SD) by the grey shading. *Significant (p = 0.025, d = 0.734) difference between morning and evening, ^#significant (p = 0.032, g = 1.19) sex difference in the evening