Journal of Strength and Conditioning Research: June 2022 - Volume 36 - Issue 6 - p 1524-1531 doi: 10.1519/JSC.0000000000003656

PURPOSE:

The effects of 5 loading conditions (0, 20, 40, 60, and 80% of body-mass [BM]) on resisted sprint performance and kinematics in male rugby players over different distances.

PROCEDURES:

Ten players from the Brazilian National Team performed 20-m sprints under the 5 loading conditions. Sprint times in 5, 10, and 20 m were recorded. Stride length (SL), and hip, knee, and ankle angles were measured using an eight-sensor motion analysis system. The kinematic parameters were calculated over the different distances.

CONCLUSIONS:

Heavier loads led to significantly greater velocity loss.

Significant reductions in SL were also observed when comparing 0% BM and all resisted sprints in all assessed distances.

Very heavy (80% BM) sled load provoked significantly greater decreases in SL than the rest of loading conditions.

Important kinematic alterations were observed for all loading conditions and sprint distances when compared with:

 - 0% BM.

-  Hip, knee, and ankle angles.

The 80% BM induced significantly higher hip flexion, lower knee flexion, and higher ankle dorsiflexion as compared to the 20% BM at distances of 5–10 and 10–20 meters.

Sled loads less than 40% BM may be better suited improve speed without making significant alterations in the unloaded sprinting technique, but heavier loads may be more suitable for optimizing horizontal force production and the ability to accelerate better.

IN PLAIN ENGLISH:

Pulling a sled to improve sprint performance is all relative to the amount of resistance of the sled:

The lighter it is, the faster it can be pulled, and the more it resembles body mechanics exhibited while running unabated by resistance.

The heavier it is, the slower it can be pulled, and the less it resembles body mechanics exhibited while running unabated by resistance.

In accordance with the Principle of Specificity, to improve proper sprinting mechanics it’s best to do so unabated by resistance. Using any weighted sled may offer a “strength” benefit provided it is heavy enough to create a strength-gain effect.