(4) ASSOCIATION OF VERTICAL JUMP DISPLACEMENT WITH MULTIPLE INDICES OF RATE OF FORCE DEVELOPMENT

Rate of force development (RFD) is often assumed to be positively associated with power-dependent performance. It may be calculated as the slope of the force-time curve (delta force for designated starting and ending points divided by the corresponding elapsed time). Paradoxically, RFD appears to be only modestly associated with vertical jump performance. Therefore, either our logic is faulty, or we have sub-optimally represented RFD during jumping. 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). We propose to also calculate five slopes from the force-time curve to represent the RFD 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: t) or 2) 50% of delta force to peak (kinetically based: k).

PURPOSE
To determine the association of vertical jump displacement with nine proposed iterations of RFD obtained during vertical jumping in biological men and women.

METHODS

Sixty young adults (31 men, 29 women), 18 to 35 years of age, performed three 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 on the right side of the body, respectively. The anchor point for RFD measurements was the first sign of a positive slope of the force time curve. Nine indices of RFD were calculated: (Zatsiorsky’s Indices)- S-gradient, A-gradient, IES, RC; (Slope-based Indices)- Full RFD (initial positive slope to peak force), Early-stage RFD_t (initial positive slope to 50% of the time to reach peak force), Early-stage RFD_k (initial positive slope to 50% of delta force to peak), Late-stage RFD_t (from 50% of the time to reach peak force to peak force), Late-stage RFD_k (from 50% of delta force to peak to peak force).

Data were extracted from the highest jump at each session. An average of the two sessions was used in calculating bivariate correlations (associations).

RESULTS

See Table 1.

CONCLUSIONS
  - Associations of VJ with nine indices of RFD were negligible-to-low regardless of biological sex.

PRACTICAL APPLICATIONS
  - RFD during CMVJs appears to have little association with jump performance.

  - Although RC is a normalized expression of IES, normalizing the remaining RFD indices for body mass might alter these findings.
  - Using unilateral force output herein was a study limitation.

Acknowledgements: None