Sled training: slow them down to speed them up?
Depending on who you talk to, follow, or have history with, you’ll get a wide range of opinions of the usefulness of weighted sleds in improving speed. If you talk to the old-school track coach, you may believe that it doesn’t have a place in training, because it “destroys an athletes sprint mechanics”, and “teaches them to be slow”. With a brief look at the research, you could argue, that yes it does. Since, with external load as little as 10 kg, a towing sled can reduce an athletes stride length by 13.5%, their frequency by 20%, as well as increase their forward lean by 20% (1,2). This leads them to them being over 10% slower when loads exceed 10% of bodyweight (3). Listening to this coach, you would be convinced that resisted sled training will mess your athletes up. However, I would argue that is an over simplification of the acute results of sled work, and an under appreciation for the process of training. Moreover, it shows a lack of understanding of underpinning factors of the overload principle, that improves an athlete’s ability to accelerate and sprint. We need to understand where resisted sprint training fits, with which populations, and when. I will take you through the research, and provide you what the best research methods for each situation, and show you that resisted work does have a place.
Speed or acceleration?
First, we need to appreciate that speed (maximal velocity) and acceleration (rate of change of velocity) are two separate qualities an athlete should possess. When an athlete is at top speed (or maximal velocity), their success is related to their ability to produce high peak forces to overcome gravity, high rate of force development, and the technical ability to reposition their limbs while in flight (4). Athletes with greater maximal velocity generally have shorter ground contact times then those who are slower (5). Though, during acceleration, an athlete has longer ground contact times, has to direct more force horizontally, and the precision of their horizontal force seems to differentiate between better and worse acceleration (6). This is important to know, since field sport athletes rarely accelerate for longer than 2 seconds at a time, and may repeat that 20-30 times per game (7). For court sport athletes, the dimensions of the surface limit the ability to reach maximal velocity. Track and field athletes are generally the only athletes guaranteed to hit maximal velocity, and where maximal velocity is a key performance indicator. Therefore acceleration is more important for field sport athletes, and maximal velocity for track athletes.
How much load?
I led off this article with the often used 10% of bodyweight loading rule. It may have usefulness in track, while not always being the right tool for team sport athlets. When it comes to producing the highest horizontal power, recent research found that horizontal power is realized with loads between 69-96% of body weight (8). Although this group did not study changes in sprint times, J.B. Morin’s group recently found that 80% bodyweight has a greater effect on 5m and 20m sprint times in soccer players, along with improving maximal force and force after 0.3 seconds (9). One novel finding of the study was that they found evidence that heavy loaded sprint training also improved sprint mechanics.
These previous findings were measured against control groups, essentially telling us that heavy load is superior than no load. We need to ask what the difference between various loads. Kawamori’s group (10) found that a load that reduced velocity by 30% (approximately 40-50% bodyweight), improved both 5 and 10 m sprint times compared to a light load, which only impacted 10 m sprint performance. Meanwhile, a more comprehensive investigation looked at three different loads (5, 12.5, 20%), determined that heavier favoured improvement in shorter sprint distances, and lighter was more effective for longer distances (11). A nice review by Petrakos (12) summarises the remaining literature to conclude that resisted sled training is effective in improving acceleration, more so than maximal velocity. He does caution that the research is mixed, and there is evidence that performing non-resisted sprint training can improve acceleration, and at the same time be more effective for top end speed.
Overall, more load might be better when it comes to improving acceleration (0-10m), and transition period (10-30m) sprint times. The shorter the sprint, the more the weight has been found to be effective. When it comes to long sprints, there doesn’t appear to be much use for resisted sprint work, since forces that need to be overcome are vertical in nature, and slowing an athlete down would limit vertical ground reaction forces. As a coach using a short to long approach to speed development, heavier sleds should be used earlier, and lighter implements as distances and maximal velocity increase. To individualise training load, Kawamori’s unique method of prescribing individual loads based on velocity decrement is a novel, and useful strategy for coaches. Since team sports generally have a variety of body shapes, sizes, weights, and force producing capabilities this could be superior to providing % of bodyweight loads (10).
Currently, most of the research talks about the benefits of sled towing, while other forms of resisted sprinting have been researched, and found to be useful in improving performance. Overload appears to be the influential factor in improving acceleration with resisted sprint training. A medium (1.2m x 1.2m) parachute, a resisted weight belt equal to 9% of body mass, appears to provide an athlete similar overload to a sled loaded at 16% of body mass (13). A vest load of 18.5% of body mass improved a flying sprint (18-53m) equally to that of a 10% sled sprint (14), even though it only represented a 1% improvement. Resisted push sleds (prowlers) are currently popular in the field, but have very little research behind them. Jay Hoffman published his Phd thesis (15) on the topic and found that 75% of bodyweight resulted in peak velocity dropping 40-51%, which is similar with appropriate reductions in sprint velocity previously found.
Sets and reps?
When it comes to implementing this in a program, we need to be able to periodise the training of the program. Looking at a review of the literature (12), it’s suggested that 6 weeks is necessary for seeing results. A typical session should have 5-35 sprints, and total between 60 and 340 m per session. Below is a sample that you can implement right away with the information outlined above.
Distance Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 5m 2x3 2x3 2x3 2x3 1x4 1x4 1x4 1x3 10m 1x3 1x4 1x5 1x3 1x4 1x5 1x5 1x3 20m 1x2 1x3 1x4 1x3
Resisted sprint training used to be limited to 10% of body weight. Current research and application is suggesting otherwise, that more can be better in improving acceleration and transition. If possible, try and determine load based on the drop off in speed that comes from adding resistance. To get proper overload without significant form decrease, upwards of 50% reduction can be appropriate. Don’t worry about having fancy apps, or high speed cameras to measure speed. A stopwatch, and good coaching eye can provide you the feedback necessary to make good decisions. Follow a short to long, heavy to light program, and you should see your athletes get faster!
References 1. Lockie, R. G., Murphy, A. J., & Spinks, C. D. (2003). Effects of resisted sled towing on sprint kinematics in field-sport athletes. Journal of Strength and Conditioning Research, 17(4), 760–767. 2. Letzelter, M., Sauerwein, G., & Burger, G. (1995). Resistance runs in speed development. Modern Athlete Coach, 33, 7–12. 3. Alcaraz, P. E., Elvira, J. L. L., & Palao, J. M. (2014). Kinematic, strength, and stiffness adaptations after a short-term sled towing training in athletes. Scandinavian Journal of Medicine & Science In Sports, 24, 279–290. 4. Weyand, P., Sandell, R., Prime, D., & Bundle, M. (2010). The biological limits to running speed are imposed from the ground up. Journal of Applied Physiology, 108(4), 950–961. 5. Weyand, P., Sternlight, D., Bellizzi, M., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89, 1991–1999. 6. Morin, J. B., Edouard, P., & Samozino, P. (2011). Technical ability of force application as a determinant factor of sprint performance. Medicine and Science in Sports & Exercise, 43(9), 1680–1688. 7. Lockie, R. G., Murphy, A. J., Knight, T. J., & De Jonge, X. A. K. (2011). Factors that differentiate acceleration ability in field sport athletes. Journal of Strength and Conditioning Research, 25(10), 2704–2714. 8. Cross, M., Brughelli, M., Samozino, P., Brown, S., & Morin, J.-B. (2017). Optimal loading for maximising power during sled-resisted sprinting. International Journal of Sports Physiology and Performance, 0(0), 1–25. 9. Morin, J.-B., Jimenez-Reyes, P., Brown, S., & Samozino, P. (2016). Very-heavy sled training for improving horizontal force output in soccer players. International Journal of Sports Physiology and Performance, 0(0), 1–13. 10. Kawamori, N., Newton, R., Hori, N., & Nosaka, K. (2014). Effects of weighted sled towing with heavy versus light load on sprint acceleration ability. Journal of Strength and Conditioning Research, 28(10), 2738–2745. 11. Bachero-Mena, B., & Gonzalez-Badillo, J. J. (2014). Effects of resisted sprint training training on acceleration with three different loads accounting 5, 12.5 and 20% of body mass. Journal of Strength and Conditioning Research, 28(10), 2954–60. 12. Petrakos, G., Morin, J.-B., & Egan, B. (2016). Resisted sled sprint training to improve sprint performance: a systematic review. Sports Medicine, 46, 381–400. 13. Alcaraz, P., Palao, J., Elvira, J., & Linthorne, N. (2008). Effects of three types of resisted sprint training devices on the kinetics of sprinting at maximal velocity. Journal of Strength and Conditioning Research, 22(3), 890–897. 14. Clark, K., Stearne, D., Walts, C., & Miller, A. (2010). The longitudinal effects of resisted sprint training using weighted sleds vs. weighted sleds. Journal of Strength and Conditioning Research, 24(12), 3287–3295. 15. Hoffman, J. R. (2014). An investigation of the sled push exercise: quantification of work, kinematics, and related physical characteristics. Electronic Theses and Dissertations, Paper 2422, 1–114.