Date of Award
Master of Science (Sports Science)
School of Medical and Health Sciences
Professor Anthony Blazevich
Field of Research Code
Motor unit double discharges (i.e. doublets), which are excitatory potentials that occur at shorter-than-normal intervals (e.g. 5-10 ms) during normal muscle activation, are known to cause muscle force to exceed that predicted from a standard, linear summation of twitch forces. However, although a marked increase in the occurrence of motor unit doublets at the onset of a contraction has been observed after explosive-type exercise training, and has been correlated with changes in RFD (Van Cutsem et al., 1998), little is known about the influence of strength training on the physiological and biomechanical benefits derived from the phenomenon. The present research examined the effects of 4 weeks of ‘explosive’ isometric knee extensor strength training on voluntary and electrically-evoked contractile RFD (calculated as the time derivative of the moment-time curve) in 8 untrained male participants. Electrical stimulation (NMES) trains were delivered to the muscle at 20 Hz and 40 Hz and incorporated short (5 and 10 ms) inter-pulse intervals (IPIs) at the onset of stimulation (i.e. variable-frequency trains; VFT). The influence of the short inter-pulse interval was assessed by comparison to a constant frequency train (i.e. the VFT:CFT ratio). Following the training, substantial improvements in maximum isometric knee extensor strength (MVC) (24.3 ± 13.3%, p = 0.002) and RFD measured to time intervals of 50 (55.5 ± 50.3%, p = 0.011), 100 (34.0 ± 47.2%, p = 0.01) and 150 ms (31.9 ± 38.2%, p = 0.02) were observed. RFD normalised to MVC (RFDnorm), measured to time intervals of 50 and 100 ms from the onset of contraction, improved by 44.9 ± 38.8% (p = 0.04) and 13.8 ± 12.2% (p = 0.01), respectively. There was a significant reduction in the VFT:CFT ratio after training when a 10-ms IPI preceded a 20-Hz train when measured to 30 (-13.7 ± 11.3%, p = 0.03), 50 (-13.9 ± 8.4%, p = 0.007), 100 (-8.6 ± 10.2%, p = 0.04), and 200 ms (-8.1 ± 5.3%, p = 0.009) as well as in the interval 100-200 ms (-7.4 ± 6.6%, p = 0.02). However, no significant changes were observed for other stimulation frequency-IPI combinations. Moderate-to-very strong positive correlations were observed between changes in RFDnorm and changes in VFT:CFT when measured within some time periods, particularly in the early phase of the contraction (r = 0.02 – 0.91). In conclusion, the effect of a high-frequency double discharge at stimulation onset remained unchanged or, under some conditions, was reduced after 4 weeks of explosive-type knee extensor training. Additionally, training-dependent improvements in the ability to rapidly reach a specified torque level relative to peak MVC torque (i.e. RFDnorm) were greater for those participants whose VFT:CFT ratio either did not decline or declined the least. These data provide evidence that explosive training may reduce the effect of a high frequency discharge at the onset of a contraction, and that greater increases in RFD may occur in those who most retain this ability.
Murray, D. V. (2018). The effect of maximal isometric training on doublet-induced force enhancement and its relationship with changes in voluntary rate of force development. Retrieved from https://ro.ecu.edu.au/theses/2074