Towing heavy resisted sprint sleds produces loss of kinematic specificity but increased kinetic overload beneficial to acceleration

This study compared the biomechanical effect of towing a resisted sprint-sled loaded with two different loads. Five amateur athletes (age 21 ± 4.1 years, stature 175.48 ± 7.0 cm, body mass 67.96 ± 10.54 kg) equipped with 39 reflective markers performed 5 m maximal acceleration sprints. Three-dimensional motion analysis system consisting 9 digital cameras (250 Hz) reconstructed a full-body pipeline model of the fastest trial for three different conditions; 1) non-resisted, 2) harnessed to a sled equivalent to 30% of the subject’s body mass (30%BM) and 3) a sled equivalent to 80% body mass (80%BM). Kinetic data was obtained from integrated force platforms (2000 Hz) measuring second step ground contact forces. Results demonstrated increasing sled load slowed 5 m sprint time by reducing take-off velocity from 4.93 ± 0.51 m/s by -14.70% (ES = 1.55; p = .008) with 30%BM and -36.21% (ES = 3.94; p = .006) with 80%BM sleds. Contact time increased by 45.79%, step length shortened by 38.88% and step rate slowed by 13.82% using 80%BM sleds (p<.05) but no significant loss of specificity with 30%BM. Hip, knee and ankle joint angles at touchdown and take-off were not significantly altered. Significantly greater ground reaction forces were experienced for total impulse (+32.92%; ES = 1.45; p=.012), vertical impulse (+30.56%; ES = 1.13; p=.012) and net propulsive impulse (+57.24%; ES =2.36; p=.034) for 80%BM sleds but not 30%BM. In conclusion towing heavy sleds produces selected reduced kinematic specificity yet may provide kinetic overload useful to acceleration phase of sprinting.