Separate and combined effects of time of day and verbal instruction on knee extensor neuromuscular adjustments

We examined the effects of time of day and verbal instruction, separately and combined, on knee extensor neuromuscular adjustments, with special reference to rapid muscle force production capacity. Ten healthy male participants performed 4 experimental trials in counterbalanced order: morning "hard-and-fast" instruction, evening hard-and-fast instruction, morning "fast" instruction, and evening fast instruction. During each experimental trial, neuromuscular performance was assessed from the completion of 6 maximal isometric voluntary contractions (rest = 2 min) of the knee extensors with concomitant quadriceps surface electromyography recordings. For each contraction, we determined maximal voluntary force (F_max), maximal rate of force development (RFD_max) and associated maximal electromechanical delay (EMD_max), and maximal rate of muscle activation (RMA_max). Globally, oral temperature (+2.2%), F_max (+4.9%) and accompanying median frequency (+6.6%)/mean power frequency (+6.0%) as well as RFD_max (+13.5%) and RMA_max (+15.5%) were significantly higher in the evening than morning (p < 0.05). Conversely, evening in reference to morning values were lower for EMD_max (-4.3%, p < 0.05). Compared with a hard-and-fast instruction, RFD_max (+30.6%) and corresponding root mean square activity (+18.6%) were globally higher using a fast instruction (p < 0.05), irrespectively of the time of day. There was no significant interaction effect of time of day and verbal instruction on any parameter, except for EMD_max (p = 0.028). Despite diurnal variation in maximal or explosive force production of knee extensors and associated neuromuscular parameters, these adjustments occurred essentially independently of the verbal instruction provided.