Propulsive Force Deterioration and the deceleration phase of sprinting

Understanding the mechanics of sprinting across the acceleration, constant velocity and deceleration phases is an important step towards developing strategies for enhancing performance. In sprinting research, a greater emphasis is usually put on acceleration and maximum velocity. The deceleration phase is sometimes overlooked, despite its crucial role in race outcomes.

However, in close finishes, the ability to maintain speed through to the final metres can be decisive, as seen in the men’s 400m final at the 2024 Paris Olympics and in several 100m Diamond League races, such as those in London and Rome.

A recent study published in the Journal of Science and Medicine in Sport presents an original perspective on a key variable influencing the deceleration phase, termed the “rate of propulsive force deterioration” (or PFD rate). The study explores its relationship with maximum velocity, deceleration rate, and finishing speed. You can read the full article here.

What is the PFD rate?

At the start of a sprint race, there is a theoretical maximum propulsive force that muscles generate to propel the sprinter forwards and upwards. In earlier studies on the acceleration phase, this force, let’s call it F0, was assumed to remain constant. Under this assumption, a sprinter accelerates to their top speed and holds it, meaning no deceleration occurs. This model has been widely adopted and validly used to describe the velocity-time relationship and to build force-velocity sprint profiles based on acceleration phase data.

However, in race situations, because all outdoor sprinting events (the 100m, 200m, and 400m) involve a deceleration phase this model cannot be used to model the velocity-time relationship in these events; the constant F0 model doesn’t fully capture what happens across these races. Taking a modelling approach, this new study builds on past research by exploring how a sprinter’s F0 might decline from the start to the finish (i.e., the PFD rate), and how that decline impacts velocity during the deceleration phase.

What were the main results?

1. Athletes with a greater PFD rate experienced greater deceleration across all events. However, the effect was greatest in the 200m highlighting the importance of a lower PFD rate in this event.
2. PFD rate and finishing velocity were negatively correlated across all events suggesting a trade-off. Athletes with higher PFD rates tended to finish with a slower velocity. This relationship was particularly prominent in the 200m and 400m events.
3. Sprinters with greater maximum velocities in the 200m and 400m events had higher PFD rates. That is, 200m and 400m sprinters who achieved higher maximum velocities were more likely to experience a greater rate of loss in propulsive force and velocity later in the race (leading to a slower finishing velocity). This relationship was not significant in the 100m. Higher top speeds were not necessarily associated with a higher PFD rate.

Early vs late-race velocity: managing the trade-off

These findings highlight that sprinting is not simply about reaching the highest possible top speed as quickly as possible. It is also about regulating the PFD rate to optimise finishing velocity.

The 200m and 400m results may reflect a functional trade-off to conserve limited energy from anaerobic metabolism. A higher PFD rate, representing a faster rate of energy from anaerobic metabolism, produces greater velocity early in the race and a higher maximum velocity, but often results in a slower finishing velocity. On the other hand, a lower PFD rate may be associated with a lower maximum velocity but help maintain a stronger finish. In essence, the relationship between a more aggressive maximum velocity and a slower finishing speed is mediated by the PFD rate.

Pacing

This trade-off has broader implications for pacing. The 200m sprinters in the study had lower F0 values compared to the 100m sprinters, suggesting they weren’t going “all out” from the start, possibly due to the biomechanical demands of curve running. In the 400m, where maximum effort can’t be sustained over the full race distance, athletes appear to adopt a deliberate strategy to accelerate with submaximal effort. Supporting this theory, the data showed a smaller F0, a lower acceleration constant, and an earlier time to top speed. These findings suggest that greater sprint performance is about generating greater forces for reaching a higher top speed and about how well propulsive force capacity can be maintained throughout the race.

Conclusion and application of the model

The algorithm used in this study can be used to model the velocity-time relationship in races. An example is here.

The Propulsive Force Deterioration rate is a theoretical metric derived from the velocity-time relationship generated from sprinting events. It may provide increased context for understanding sprinting performance in the outdoor events. Where margins are small, the ability to maintain a low PFD rate may be the difference between fading late or finishing fast.