Vol.:(0123456789) 1 3 European Journal of Applied Physiology https://doi.org/10.1007/s00421-017-3796-5 ORIGINAL ARTICLE A comparison between the force–velocity relationships of unloaded and sled-resisted sprinting: single vs. multiple trial methods Matt R. Cross 1,2  · Pierre Samozino 1  · Scott R. Brown 2  · Jean‑Benoît Morin 2,3 Received: 17 October 2017 / Accepted: 22 December 2017 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Purpose We sought to compare force–velocity relationships developed from unloaded sprinting acceleration to that compiled from multiple sled-resisted sprints. Methods Twenty-seven mixed-code athletes performed six to seven maximal sprints, unloaded and towing a sled (20–120% of body-mass), while measured using a sports radar. Two methods were used to draw force–velocity relationships for each athlete: A multiple trial method compiling kinetic data using pre-determined friction coefcients and aerodynamic drag at maximum velocity from each sprint; and a validated single trial method plotting external force due to acceleration and aero- dynamic drag and velocity throughout an acceleration phase of an unloaded sprint (only). Maximal theoretical force, velocity and power were determined from each force–velocity relationship and compared using regression analysis and absolute bias (± 90% confdence intervals), Pearson correlations and typical error of the estimate (TEE). Results The average bias between the methods was between − 6.4 and − 0.4%. Power and maximal force showed strong cor- relations (r = 0.71 to 0.86), but large error (TEE = 0.53 to 0.71). Theoretical maximal velocity was nearly identical between the methods (r = 0.99), with little bias (− 0.04 to 0.00 m s −1 ) and error (TEE = 0.12). Conclusions When horizontal force or power output is considered for a given speed, resisted sprinting is similar to its asso- ciated phase during an unloaded sprint acceleration [e.g. frst steps (~ 3 m s −1 ) = heavy resistance]. Error associated with increasing loading could be resultant of error, fatigue, or technique, and more research is needed. This research provides a basis for simplifed assessment of optimal loading from a single unloaded sprint. Keywords Explosive performance · Sprint · Power development · Resisted sprinting Abbreviations α Acceleration BM Body mass ES Cohen’s efect size F Horizontal force F aero Aerodynamic friction force F f Friction force F n Normal force F opt Optimal horizontal force F peak Horizontal force at maximum velocity Fv Horizontal force–velocity relationship F 0 Maximum theoretical horizontal force h t Attachment height of tether to athlete L opt Optimal external normal loading m System mass P Horizontal power P max Maximum horizontal power Pv Horizontal power–velocity relationship S Fv Slope of the linear Fv relationship TEE Typical error of estimate v Horizontal velocity v max Maximum horizontal velocity v opt Optimal horizontal velocity v 0 Maximum theoretical horizontal velocity θ Angle of pull μ k Coefcient of friction Communicated by Jean-René Lacour. * Matt R. Cross matthew.cross@univ-savoie.net 1 Université Savoie Mont Blanc, Laboratoire Interuniversitaire de Biologie de la Motricité, EA 7424, F-73000 Chambéry, France 2 Sports Performance Research Institute New Zealand, Auckland University of Technology, Auckland, New Zealand 3 Laboratoire Motricité Humaine, Education, Sport, Santé, Université Côte d’Azur, Nice, France