Does jump peak force reflect an athlete’s RFD capabilities?
To the authors’ knowledge, and somewhat surprisingly, there is a scarcity of research exploring the relationship between peak force and RFD during jumping. McLellan et al45 reported a strong correlation at r = 0.63, suggesting RFD may account for ~40% of the variance (r2) in peak force. Although not exactly the same, Kawamori et al[40] reported associations between CMJ peak force and peak RFD in mid-thigh clean pulls at a range of intensities (r = 0.52 at 120% intensity, r = 0.35 at 90% intensity, r =0.22 at 60% intensity, and r = 0.51 at 30% intensity). The lack of consistency in these findings is likely a consequence of the erratic and unreliable nature of RFD as a metric,[18,46] together with the methodological differences in how RFD is calculated. For example, McLellan et al[45] reports peak RFD calculated from the maximum force that occurred over the first derivative of the force-time curve and Kawamori et al[40] reports peak RFD using a 0.002 mins moving time-window. Additionally, neither study clarified whether this was taken from the braking or propulsive phase, which are not comparable as peak force would occur earlier in the latter.[65] Consequently, the different ways by which RFD is calculated makes it challenging to gain any consistent insight into the relationship between vertical jump peak force and RFD.

Given this complexity, it seems logical to revert to first principles. The first limitation we highlight has been discussed already in the context of dynamic strength – ie, the metric is confounded by displacement or jump strategy. A practical way to infer dynamic RFD improvements (ie, during a jump), would be to concurrently monitor the metric of ‘time to take-off’, in addition to the force or impulse from a jump. A reduction in time-to-take-off with no increase in force or impulse would then indicate the athlete is achieving the same outcome, in less time; therefore, RFD has likely improved. Clearly, without the metric of time to take-off, this inference is impossible. As was the case for dynamic strength, the jump type will have a large bearing on the relationship between jump peak force and RFD. As previously discussed, the pre-existing high levels of force at the start of the propulsive phase in a CMJ reflect the fact the muscles have built up a high stimulation during the downward phase to create pre-tension in the musculotendinous unit. This affords greater joint moments over the early joint extension, and in turn, the ability to perform more work in the first part of the CMJ push-off phase.[13,14] As a consequence of this, the need for a rapid rise in force at the start of the push-off phase is negated. Therefore, peak force from a CMJ is likely a poor reflection of an athlete’s RFD capabilities. In fact, it is viable that the peak force may occur at zero velocity, so the RFD during the propulsive phase of the jump may even be negative.

On the other hand, peak force achieved in a SJ will somewhat reflect RFD capabilities, as it will directly influence the area under the force-time curve during the propulsive phase. However, it is worth noting that peak force typically occurs earlier (approximately 125-150 ms) in explosive concentric contractions because the high initial neuromuscular activation persists for longer than in other muscle activity types.[65] This therefore reduces the ‘impulse advantage’ of a greater RFD, as maximal strength (peak force) and specifically, the force that can be maintained at the specific aspect of the force-velocity curve, will have the greatest influence on the area under the force-time curve. Therefore, if using the SJ as the dynamic component of the DSI, one could theoretically infer some change in RFD within an athlete over time (assuming the IMTP peak force value remained stable), but this should be interpreted with caution as any improvements may be explained to a greater proportion by changes in the athlete’s force-velocity orientation (improved ability to produce greater force at the velocity-end).

UKSCA-IQ

The application of weightlifting to sprinting

This content is available free to registered users

Additional resources