Enter your vertical jump height (cm), drop-jump ground contact time (ms), body weight and back squat 1RM to compute your Reactive Strength Index (RSI = jump height in metres / contact time in seconds), Sayers et al. (1999) peak anaerobic power in watts, and whether your training should prioritise heavy force or fast elastic plyometrics.
Core Equation & Guard-Rail
RSI = Jump Height (m) ÷ Contact Time (s) · Peak Watts = 60.7 × Jump (cm) + 45.3 × Body Mass (kg) − 2055
Flanagan and Comyns (2008) discuss fast stretch-shortening cycle (SSC) plyometrics in terms of ground contact times of about 250 milliseconds (0.25s) or less. If your contact time drifts above that, lower the drop-box height or focus on stiff-ankle pogo hops. Sayers’ equation was built from squat-jump height and has a typical error of several hundred watts, so treat the watts as a ballpark. The force versus elastic-speed read-out is a rule of thumb, not a test.
Methodology, Guides & Coaching
2.18
Fast SSC (<= 250ms)
4301 W
56.6 W/kg (Sayers)
Balanced Force-Velocity Profile
1.64x BW Squat
Balanced Force-Velocity Profile: Combine 1 heavy lower-body strength day (80-88% 1RM) with 1 fast-SSC plyometric session (< 220ms contacts).
General fitness and training estimates only. Not medical advice.
The link carries the numbers you entered in the part of the address after the #, which is never sent to a server. Share it only with people you are happy to show those numbers to.
RSI = Jump Height (m) ÷ Contact Time (s) · Peak Watts = 60.7 × Jump (cm) + 45.3 × Body Mass (kg) − 2055
Flanagan and Comyns (2008) discuss fast stretch-shortening cycle (SSC) plyometrics in terms of ground contact times of about 250 milliseconds (0.25s) or less. If your contact time drifts above that, lower the drop-box height or focus on stiff-ankle pogo hops. Sayers’ equation was built from squat-jump height and has a typical error of several hundred watts, so treat the watts as a ballpark. The force versus elastic-speed read-out is a rule of thumb, not a test.
General fitness and training estimates only, labelled EST. Not medical advice or a clinical diagnosis.
Verified Literature
Contact time and the Reactive Strength Index (jump height divided by ground contact time) are used to prescribe and track fast stretch-shortening cycle plyometrics, which are usually described as ground contacts of about 250 milliseconds or less.
Flanagan, E.P. & Comyns, T.M. "The Use of Contact Time and the Reactive Strength Index to Optimize Fast Stretch-Shortening Cycle Training." Strength and Conditioning Journal, 2008;30(5):32-38. doi:10.1519/SSC.0b013e318187e25b
View the sourcePeak lower-body anaerobic power in watts can be estimated from vertical jump height and body mass using the cross-validated Sayers regression equation.
Sayers, S.P., Harackiewicz, D.V., Harman, E.A., Frykman, P.N. & Rosenstein, M.T. "Cross-validation of three jump power equations." Medicine & Science in Sports & Exercise, 1999;31(4):572-577. doi:10.1097/00005768-199904000-00013
View the sourceThis clinical commentary describes counting ground contacts per plyometric session, building them up gradually with experience, and leaving about 48 to 72 hours between plyometric sessions. It is a practical guide, and the authors note the best dose is not settled.
Davies, G., Riemann, B.L. & Manske, R. "Current concepts of plyometric exercise." International Journal of Sports Physical Therapy, 2015;10(6):760-786.
View the sourceDeep-Dive Training Guides
Automatic Tracking & Adaptation
Inside Pelaris, you never need to punch numbers into a spreadsheet. Every logged set, run and check-in updates your estimates automatically and adapts next week’s sessions.
Strength & Power
1RM & RIR Load Calculator
Calculate your Epley one-rep max (1RM), adjust working weights for Reps in Reserve (RIR), and generate a 1 to 15 rep load table. Free, no sign-up.
Strength & Power
Deload & Readiness Calculator
Score four fatigue signals to see whether to keep progressing or deload, with suggested deload sets (-40% volume) and bar weights (85-90% load) at 3 to 4 RIR.
Strength & Power
Plyometric Contacts Calculator
Plan plyometric ground contacts per session (about 65 to 125) by athlete tier, sport focus and season phase, with 48 to 72 hours between plyometric days.
Using jump height in metres divided by ground contact time in seconds, an RSI of 1.5 to 2.0 is solid for club athletes, 2.0 to 2.5 represents strong reactive elasticity for netball, basketball and AFL, and above 2.5 is elite.
Sports science distinguishes the fast stretch-shortening cycle (< 250 milliseconds, such as sprint ground contacts, pogo hops and depth jumps) from the slow stretch-shortening cycle (> 250 milliseconds, such as countermovement jumps).
As a rule of thumb: if you jump well with low contact times but your back squat is below 1.5x body weight, building squat and trap-bar strength is likely your biggest opportunity. If you squat over 1.75x body weight but have slow contact times (over 250ms) and a modest jump, fast elastic plyometrics are the likelier win.