(1) INTERCORRELATIONS OF MULTIPLE INDICES OF RATE OF FORCE DEVELOPMENT DURING VERTICAL JUMPING

Rate of force development (RFD) conceptually appears to reflect speed strength. It may be calculated as a slope in which delta force from a designated anchor point on the force-time curve to a designated point on the curve is divided by the corresponding elapsed time. However, alternative iterations of RFD have been suggested. In 1995, Vladimir Zatsiorsky proposed four indices of RFD including 1) starting gradient (S-gradient: 50% of peak force / elapsed time to reach it), 2) acceleration gradient (A-gradient: 50% peak force / elapsed time from that point to peak force), 3) index of explosive strength (IES: peak force / time to peak force, and 4) reactivity coefficient (RC: peak force / (time to peak force x body mass). Five slope-based iterations of the force-time curve might be considered as additional RFD indices including 1) full RFD to peak force, 2) early-stage RFD, and 3) late-stage RFD with the point of demarcation between early and late stages being either 1) 50% of the time to reach peak force (temporally based or t) or 2) 50% of delta force to peak (kinetically based or k). Indices not highly associated with each other might be considered as having unique qualities while those highly associated with each other might be considered as having redundant qualities.

PURPOSE
To determine the association between nine proposed iterations of RFD during vertical jumping in apparently healthy men and women.

METHODS
Sixty young adults (31 men, 29 women), 18 to 35 years of age, performed three countermovement vertical jumps (CMVJ) on two occasions using a self-selected countermovement depth and constrained arm swing. A nine-camera 3D motion capture system (240 Hz, Qualisys Inc., Sweden) and force platform (1200 Hz, AMTI, Watertown, MA, USA) were used to collect 3D marker position data and vertical ground reaction force (vGRF) data for the right side of the body, respectively. The designated anchor point for RFD measurements was the first sign of a positive slope (start of eccentric phase) of the force time curve. Each of the nine proposed indices of RFD were extracted from the highest CMVJ on each of two testing days and were subsequently averaged. Associations were based on bivariate correlations. Indices of RFD having at least 50% shared variance (r ≥ 0.71) were deemed redundant. Indices of RFD having less than 50% shared variance (r < 0.71) were deemed unique.

RESULTS
For significant (p ≤ 0.05) correlations (See Table 1), shared variances ranged from 11% to 98%.

CONCLUSIONS
Considering that redundant and unique RFD variables exist over a continuum rather than as a dichotomy, the nine proposed indices of RFD appear to be associated with each other over a wide spectrum.

PRACTICAL APPLICATIONS
  - Interchanging these nine RFD indices should be done cautiously.
  - The most redundant variables appear to be A-gradient with Late-Stage RFD_{t and k}, IES with RC, Early-Stage RFD_{t and k}, and Late-Stage RFD_t with A-gradient, RFD, and Late-Stage RFD_k.

Acknowledgements: None