Resisted Sprint Training: Force-Velocity Profiling With Actual Data
Summary
The weight on a friction sled is not the force your athlete pulls against. What the research says about resisted sprint loading, and why horizontal force has to be measured rather than assumed.
Resisted sprint training with a friction sled is one of the most effective tools available for developing horizontal force capacity and improving sprint acceleration. The research on this is consistent: resisted sprinting increases horizontal ground reaction force, improves 5–30 metre split times, and — when loading is prescribed correctly — produces meaningfully better acceleration outcomes than unresisted training alone.
The problem is the last part of that sentence: when loading is prescribed correctly.
You have been loading your athletes at what feels like the right weight — maybe 20% of body mass because a textbook suggested it, or 30 kg because the previous coach used it. Your athletes are working hard. The sessions look good.
But here is the question: do you actually know what horizontal force was delivered to each athlete on each rep? Not the weight on the sled. The actual force.
If you are using a standard friction sled, the honest answer is no. And the research backs this up.
Why Horizontal Force Is the Number That Matters in Sprint Training
Sprint acceleration is a horizontal problem. When an athlete accelerates, the primary mechanical requirement is producing force in the direction of travel — horizontal force, directed backward into the ground so the athlete moves forward.
Brughelli and colleagues at Auckland University of Technology (SPRINZ) demonstrated in 2011 that as sprint velocity increases, performance becomes progressively more dependent on horizontal ground reaction force than vertical. Vertical force keeps you in contact with the ground. Horizontal force is what drives you forward.
The connection goes further. Research by Morin, Brughelli and colleagues in 2015 identified the hamstrings as the primary muscle group driving horizontal ground reaction force during sprint acceleration. And a prospective study tracking 284 footballers across a full season found that athletes with low horizontal force production capacity at sprint initiation had significantly elevated hamstring strain injury risk (Edouard, Lahti, Brughelli et al., 2021).
Train horizontal force production and you improve acceleration. Measure it, and you have a direct injury risk signal. The case for building resisted sprint training into your programme is well established. The question is whether the tools you are using to implement it are doing what you think.
The Measurement Problem: Why Standard Friction Sleds Fall Short
A friction sled is a simple machine. You load it with weight, your athlete attaches a harness, and they sprint. The sled drags across the ground, creating resistance that the athlete must overcome with horizontal force.
In 2017, Tinwala and colleagues at Auckland University of Technology published a paper in the Journal of Sports Sciences that exposed a fundamental problem with this setup. The paper — Determining friction and effective loading for sled sprinting — quantified how the coefficient of friction of a sprint sled changes with varying mass and running velocity. The finding was unambiguous: the effective load delivered to an athlete cannot be calculated from the weight on the sled alone.
Friction is not constant. It varies with how heavy the sled is, how fast the athlete is moving, and the surface beneath the runners. Two athletes pulling the same weight at different velocities are experiencing different effective loads. An athlete in early acceleration experiences a different effective load than the same athlete at peak speed in the same session.
What this means: when you load a standard sled at 30 kg, you do not know what horizontal force your athlete is actually producing. You know what you put on the sled. Those are different numbers, and the difference varies in ways you cannot calculate without direct measurement.
A practitioner using a standard sled for resisted sprint training is flying blind.
Force-Velocity Profiling With a Sled: Why Optimal Load Is Individual
Force-velocity profiling — mapping the relationship between force production and movement velocity for an individual athlete — is one of the most powerful tools available for individualising sprint training. When applied to resisted sprinting, it tells you not just how fast an athlete is, but whether they are limited by force production capacity, velocity capacity, or both.
Research by Cross, Brughelli, Samozino, Brown and Morin (IJSPP, 2017) investigated optimal sled loading for maximising horizontal power output across a group of athletes. Using a battery of sled sprints at different loads, they built individual force-velocity-power profiles for each participant.
The finding: optimal load for maximising horizontal power ranged from 69% to 96% of body mass across the group. Not a tight cluster — a range spanning nearly 30 percentage points of body mass. What is optimal for one athlete is substantially wrong for another.
A subsequent training study (Cross et al., PLOS ONE, 2018) confirmed that athletes training at their individually identified maximal power load produced better acceleration improvements than those using a fixed, lower load. Individual force-velocity profiling works — but only if you know the actual force being delivered at each sled loading condition.
If your sled cannot measure force, it cannot support accurate force-velocity profiling. You are guessing at the most important variable in the prescription.
DynaSled: Resisted Sprint Training With Real Force Data
DynaSled is a friction-resistance training sled with a biaxial load cell integrated into the tow point. Where a standard sled tells you nothing about force produced, DynaSled measures actual horizontal and vertical force vectors in real time — for every rep, every session.
DynaSled captures a composite force trace from which left and right leg contributions can be identified based on the athlete's starting position, logged in the Swift SYNCRO app. Automatic bilateral detection via AI and machine learning is in active development. An athlete compensating for a recovering hamstring injury may show symmetric sprint times while producing meaningfully different force contributions from each leg — an asymmetry signal that timing gate data cannot provide.
Data is captured via Bluetooth to the Swift SYNCRO app, displayed in real time, and stored in Swift Labs for longitudinal tracking. The Force and Rate of Force Development (RFD) output shows not just peak force but how quickly an athlete develops force across each rep — a signal that becomes particularly relevant in fatigue monitoring and return-to-sport contexts.
In practical terms, accurate force measurement in resisted sprint training means:
You know the actual effective load, not an estimate based on sled weight and assumed frictionYou can build a genuine force-velocity profile from a sled sprint battery — the kind Cross and colleagues used to identify optimal loading in 2017You can track bilateral asymmetries session to session as a direct injury risk signalYou can confirm whether training is producing the intended adaptation, not just assume it is
The AUT Research Partnership
DynaSled was developed in a research partnership between Swift Performance and Auckland University of Technology. The original technology developers are Associate Professor Matt Brughelli, Dr Matt Cross, and Dr Farhan Tinwala — all from AUT's Sports Performance Research Institute New Zealand (SPRINZ).
The connection between their published research and the product is direct. Tinwala's friction paper identified the measurement gap. Cross and Brughelli's force-velocity profiling work established the framework for meaningful horizontal force metrics in the field. Brughelli's 2019 IJSPP paper developed the computational methodology DynaSled automates in real time.
Mark Fisher, CEO of Swift Performance: "It is great working with industry-leading sports science researchers, and we hope to have a long-term partnership with AUT going forward to help commercialise the sled technology."
The measurement approach in DynaSled is grounded in peer-reviewed methodology from researchers who were working on this specific problem before the product existed.
What This Looks Like in a Real Training Environment
Profiling. A sled sprint battery at three to five loads across the individual's force-velocity range builds a power profile. You identify the load at which each athlete produces maximal horizontal power. That becomes their training load. Force-deficient athletes are prescribed differently to velocity-deficient athletes. The protocol is the same across the group; the loading is individualised.
Session monitoring. Force and RFD data shows fatigue developing in real time. Declining RFD across reps is a different signal to declining peak force. Distinguishing between them changes volume and recovery decisions within a block.
Asymmetry tracking. For return-to-sport, bilateral force data across sessions gives a trend line for limb symmetry in the context of resisted sprinting. An athlete passing all standard RTS criteria but showing persistent 15% left-right force asymmetry is a different clinical picture to one whose asymmetry has resolved.
Longitudinal adaptation. Every session is logged in Swift Labs. Pre-season force-velocity profiles can be compared directly to mid-season profiles — objective measurement of whether training is moving each athlete's profile in the intended direction.
Conclusion
Resisted sprint training is well supported by the research as an effective method for developing horizontal force capacity. The gap has been in measurement: standard friction sleds cannot tell you what they are actually delivering to your athletes.
DynaSled is newly released. It does not have a decade of adoption behind it, and we will not claim otherwise. What it has is a clear research lineage from AUT, a measurement approach validated in peer-reviewed literature, and a direct proposition: if you are using resisted sprint training to develop horizontal force capacity and build force-velocity profiles, you should know how much horizontal force your athletes are actually producing.
Standard sleds cannot tell you that. DynaSled can.
Explore DynaSled →
References
Brughelli M, Cronin J, Chaouachi A. Effects of Running Velocity on Running Kinetics and Kinematics. Journal of Strength and Conditioning Research. 2011.Morin JB, Gimenez P, Edouard P, et al. Sprint Acceleration Mechanics: The Major Role of Hamstrings in Horizontal Force Production. Frontiers in Physiology. 2015;6:404.Cross MR, Brughelli M, Samozino P, Brown SR, Morin JB. Optimal Loading for Maximizing Power During Sled-Resisted Sprinting. IJSPP. 2017;12(8):1069–1077.Tinwala F, et al. Determining Friction and Effective Loading for Sled Sprinting. Journal of Sports Sciences. 2017;35(22):2198–2203.Cross MR, et al. Training at Maximal Power in Resisted Sprinting. PLOS ONE. 2018;13(4).Edouard P, Lahti J, Brughelli M, et al. Low Horizontal Force Production Capacity during Sprinting as a Potential Risk Factor of Hamstring Injury in Football. IJERPH. 2021;18(15):7827.Brughelli M, et al. Assessing Horizontal Force Production in Resisted Sprinting. IJSPP. 2019;14(5).
Disclosure: Swift Performance designs and manufactures the DynaSled friction-resistance training sled. This article is written by the Swift Performance team.
Mark Fisher
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