When resisted-sprint training started to become more popular in strength and conditioning circles, the rule of thumb was not to use sled loads in excess of 10% of the athlete’s body weight. The original thinking behind this “rule” was that loads greater than 10% are likely to alter the athletes sprinting mechanics. Though sprint mechanics are transiently altered when towing heavy loads, there was not much evidence to suggest that this would lead to chronic adaptations in running form. More recently, many coaches have abandoned the 10% rule and are prescribing sled-towing with very have loads in effort to enhance sprint-specific force production to improve acceleration. Other possible uses for heavy sprint-resistance training is for post-activation potentiation purposes. However, research evaluating optimal resistive loads and duration of rest-periods is quite limited.
A new study published in the Journal of Strength and Conditioning research examined the acute potentiating effects of heavy sled-towing (of various loads) on sprinting speed. The sample was comprised of 22 trained male rugby players. In a randomized, crossover and counterbalanced design, the subjects performed 2 sled-towing protocols on separate days. Each protocol involved two 15 m sprints to establish baseline speed. Following this, the subjects performed sled pulls with either 75% of body weight (for 15 m) or 150% of body weight (for 7.5 m). Athletes were then tested again in the 15 m sprint at 4, 8 and 12 minutes following the sled-towing intervention. Sprint times were compared between interventions and recovery time points.
The results showed that there were no significant improvements in 15 m sprint time following sled-pulls with 150% of body mass, regardless of rest period. In contrast, meaningful but small improvements (Effect Size >0.20) in 15 m sprint time were observed at 8 and 12 minutes following sled-towing with 75% of body mass. The authors concluded that resisted sled pulling with a load equal to 75% of body mass is an adequate load to potentiate sprinting speed so long as sufficient rest (i.e., 8-12 minutes) is provided. Heavier loads (~150% of body mass) do not appear to be useful for transiently improving sprinting speed.
Winwood, P. W., Posthumus, L. R., Cronin, J. B., & Keogh, J. W. (2016). The acute potentiating effects of heavy sled pulls on sprint performance. The Journal of Strength & Conditioning Research, 30(5), 1248-1254.