January 21, 2019

M.I. B. Debenham 1, G.A. Power 2

1 University of British Columbia Okanagan, British Columbia, Canada
2 University of Guelph, Ontario, Canada

The optimization of explosive performance is important for enhancing athletic performance and for everyday life, such as recovering from a slip, fall or trip. Explosive performance can be quantified by rate of force development, which is how fast force is produced over a given period of time. Additionally, there is evidence that explosive performance may be optimized by reducing the time delay between the onset of electrical activity measured at the muscle and the onset of force production, known as electromechanical delay. As many movements during everyday life do not begin from rest (e.g. changing stride during walking) and occur at different muscle lengths, it is important to understand how a contracting muscle prior to an explosive contraction (preceding activity), and muscle length, influences explosive performance. In this study, we aimed to determine the influence of muscle length and preceding muscle activity on torque and activation characteristics during explosive isometric knee extensions (i.e., ‘kicks’). Healthy young adult males performed voluntary and electrically evoked (octet stimulation; 8 pulses at 300 Hz) isometric knee extensions at long (i.e., knee flexed) and short muscle lengths (i.e., knee extended) with various amounts of preceding quadriceps muscle activity.

We found that during both voluntary and electrically stimulated contractions, preceding activity limited rate of force development at both a long and short muscle length. However, greater amounts of preceding quadriceps activity were required to limit rate of force development at a long muscle length when compared to a short muscle length. These findings indicate that long muscle lengths offer a “protective effect” for explosive performance against preceding activity. As we saw similar reductions in rate of force development during both voluntary and electrically stimulated contractions, it is suggested that muscular mechanisms may be driving the effects of preceding activity on explosive performance. Additionally, preceding activity did not influence electromechanical delay or rate of activation which indicate that explosive performance may be limited by muscular mechanisms in addition to previously reported neural mechanisms. These findings may be relevant to an individual recovering from a trip or fall during normal gait, a movement involving low amounts of preceding activity. For example, if an individual attempts to recover from a trip at a long muscle length, they may have a greater success of recovery than if the attempt was at a short muscle length.

Summary of Key Points:

  • Changes in muscle length and preceding activity individually influence explosive performance, but there is a lack of understanding of how both preceding activity and muscle length influence explosive performance together.
  • Long muscle lengths offer a protective effect against preceding activity during explosive performance.
  • Deficits in explosive performance by preceding activity may be a result of muscular mechanisms in addition to previously reported neural mechanisms.

Original Article:

M.I. B. Debenham and G.A. Power The influence of preceding activity and muscle length on voluntary and electrically evoked contractions. Appl Physiol Nutr Metab. 2018 September 6. doi/10.1139/apnm-2018-0104#.XEYnP89Ki9S

This article is a summary of an article published in Applied Physiology, Nutrition & Metabolism. If you intend to cite any information in this article, please consult the original article and cite that source. This summary was written for the Canadian Society for Exercise Physiology and it has been reviewed by the CSEP Knowledge Translation Committee.