Horizontal Force-Velocity Sprint Testing With Motorized Resistance

Two athletes on the same roster ran an identical 1.68s for their 10m sprint last testing block. One of them needs six weeks of heavy trap-bar work and hill sprints. The other needs almost the opposite: lighter, faster ballistic work and assisted overspeed sprints. A stopwatch or a single timing gate cannot tell you which is which, because a 10m split collapses two separate physical qualities, the ability to apply large horizontal force and the ability to keep applying force as velocity climbs, into one number.

Sled work, elastic bands, or a motorized resistance unit built into a device like a 1080 Sprint can pull that number apart, but only with enough loaded trials to build an actual force-velocity relationship rather than eyeballing one heavy sled push. The protocol below covers setting up a spread of resisted sprints, the regression that turns those trials into theoretical maximal force (F0), theoretical maximal velocity (V0), and maximal power (Pmax), what the sprint mechanics research says those numbers predict, and how to read a profile well enough to know whether an athlete's acceleration problem is a force problem or a velocity problem.

Why the Same 10m Time Can Hide Two Different Problems

During any single maximal sprint, the horizontal force an athlete can apply to the ground falls roughly linearly as running velocity rises, since muscles produce less force the faster they shorten. Extrapolate that line to zero velocity and you get a theoretical maximal force, F0. Extrapolate it to zero force and you get a theoretical maximal velocity, V0. The slope tells you whether an athlete leans toward force or velocity, and the area under it, Pmax, is their ceiling for horizontal power.

The catch is that one sprint only gives one point on that line, not the line itself. A 10m split from an unloaded sprint sits somewhere in the middle of the spectrum, and two athletes can land on the same split from very different starting profiles: one by producing a lot of force but topping out early on velocity, the other with modest force but a high velocity ceiling. Their Pmax values, and their short-sprint times, come out nearly identical. Their training needs do not.

This gap shows up constantly in staffs that test only unloaded splits, or add one arbitrary sled load and treat that as a profile. One data point cannot produce a slope; F0, V0, and Pmax require sampling several force-velocity pairs across a real spread of resistance, which is what a motorized resistance unit is built to standardize.

Equipment and Setup for Resisted F-V Testing

A profile can technically be built from sled towing and independent timing, but the setup below compares that manual route against a motorized resistance unit, since the two produce meaningfully different data quality.

Whichever route you use, the harness attachment matters more than most coaches budget time for. A waist belt that rides up during the drive phase changes the pulling angle and adds a vertical force component the horizontal model doesn't account for. Fit it low, above the hips, and recheck after the familiarization trial, not just before the first one.

Step-by-Step Testing Protocol

1. Warm-up (10-12 minutes): Jog, dynamic mobility, then 2-3 build-up strides to roughly 85% effort with a full stop.
2. Device setup: Attach the low waist harness to the tether, zero-calibrate the unit per its own procedure, and set the mode to constant resistance (isotonic), not constant speed. Constant-speed modes assist or match velocity rather than resisting it, and will not produce a force-velocity profile.
3. Familiarization: One submaximal trial each at the lightest and heaviest planned loads, mainly to check harness fit and confirm the athlete understands the start command.
4. Test sequence: One maximal unloaded sprint, followed by five maximal resisted sprints at increasing loads, for example roughly 5%, 10%, 15%, 20%, and 25% of body mass if the unit sets load as a percentage, or a comparable spread in kilograms otherwise. Run loads light to heavy and allow 3-5 minutes of full recovery between every maximal trial.
5. Analysis window: Use the same window for every trial, either the first 10m or the first 2.5-3 seconds from the start command, whichever the device reports consistently, and take the mean horizontal force and mean velocity within that window for each of the six trials.
6. Validity check per trial: Discard and rerun after full recovery if the start posture differed from the rest of the session, the harness slipped, or the athlete stumbled. A single bad trial in the middle of the load spread will bend the regression more than most coaches expect.

A full six-trial session, warm-up and recovery included, runs about 30-35 minutes per athlete, the main practical tradeoff against a single unloaded sprint test for a usable diagnosis rather than just a time.

Deriving F0, V0, and Pmax From the Loaded Sprints

Once you have six mean force-velocity pairs, one per load condition, the relationship between them is fit as a simple linear regression: F = F0 - (F0 / V0) x v, where F is mean horizontal force relative to body mass (N/kg) and v is mean velocity (m/s) for a given trial. The y-intercept of that fitted line is F0, and the x-intercept is V0. Check the fit before trusting the numbers: an R-squared below about 0.95 usually means a trial was compromised or the load spread was too narrow, not that the athlete has an unusual profile.

Maximal power comes directly from the two intercepts, since a linear force-velocity relationship produces a parabolic power-velocity curve with its peak at the midpoint: Pmax = (F0 x V0) / 4, expressed in watts per kilogram. The slope of the fitted line itself, sometimes written as Sfv, describes the athlete's force-velocity orientation independent of Pmax.

Worked example: Athlete A posts F0 of 8.4 N/kg and V0 of 8.6 m/s, giving Pmax of roughly 18.1 W/kg and a steep slope of about -0.98. Athlete B posts F0 of 6.3 N/kg and V0 of 11.6 m/s, giving Pmax of roughly 18.3 W/kg, essentially matching Athlete A, but on a much flatter slope near -0.54. Same power ceiling, same short-sprint time in practice, and opposite profiles. A is force-dominant and comparatively thin on velocity. B is velocity-dominant and comparatively thin on force. Loading them with the same off-season program would very likely widen the gap each already has rather than close it.

What the Research Says

Samozino, Rabita, Dorel, Slawinski, Peyrot, Saez de Villarreal, and Morin (2016), in the Scandinavian Journal of Medicine & Science in Sports, validated a simplified field method for computing F0, V0, and Pmax against a force-plate-and-radar reference system. Agreement was very large across all three parameters, with correlations generally above 0.90, the foundation that lets a motorized unit's directly measured force stand in for a full lab setup. Their stated limitation: the model assumes a fixed effective frontal area for air resistance, which breaks down somewhat for very tall or very short athletes and for testing into a meaningful headwind or tailwind.

Cross, Brughelli, Samozino, Brown, and Morin (2017), in the International Journal of Sports Physiology and Performance, mapped power output across a spectrum of resisted-sprint loads and found peak power at loads producing roughly a 50% reduction in unloaded maximal velocity, considerably heavier than the light sled loads common in practice at the time. That's a large part of why the spread above runs up to about 25% of body mass rather than stopping at a token light load. Their limitation was scope: the loading response was established in field-sport athletes and has not been confirmed to generalize identically to specialist sprinters.

Cross, Brughelli, Samozino, and Morin (2017), in Sports Medicine, reviewed methods for building a sprint F-V profile, including timing gates, radar, GPS, and motorized resistance devices, and flagged a structural difference: kinematic methods estimate force indirectly through an inverse-dynamics model built on assumed drag and mass, while a motorized load cell measures applied tension directly. Their stated caveat applies to every method equally: none fully separate a resistance device's own mechanical drag and inertia from the athlete's true output unless that friction is measured and subtracted first.

Normative Ranges and Reading the Profile

The bands below reflect the ranges typically reported across the sprint F-V profiling literature. Treat them as a coarse reference point for where an athlete sits, not a target divorced from their own sport, mass, and training age.

The band an athlete falls into matters less than which side of their own profile is thin relative to the other. Use both numbers together as a simple diagnosis:

Mistakes That Skew the Profile

Turning F0 and V0 Into a Training Fix

A force-deficit profile, flat slope with F0 below its band, responds best to heavy resisted sprints loaded above roughly 20% of body mass, paired with heavy strength work aimed at hip and knee extension. A velocity-deficit profile, steep slope with V0 below its band, responds better to the opposite: light or unloaded maximal sprints, assisted overspeed runs, and plyometrics that train rate of force development rather than peak force. Loading a velocity-dominant athlete with more heavy sled work is a common mistake, and it tends to make the slope steeper, moving the athlete further from balanced rather than closer.

Retest on a 4-6 week cycle, matched to the end of whatever block is currently running, since a test mid-block mostly reflects fatigue rather than adaptation. Watch the slope across retests more closely than either single number: a slope moving in the intended direction is a better sign of real change than a modest bump in Pmax alone, which shifts with day-to-day readiness almost as much as with six weeks of training.