Flight-to-Contact Ratio: Diagnosing Whether a Jump Problem Is Slow Contact or Low Flight

A physio pulls up the weekly jump sheet and sees it again: an athlete's RSI has slid from 2.4 to 1.9 over three weeks. The usual response on most staffs is to add more plyometric volume and hope the number climbs back. Sometimes it works. Sometimes the athlete gets slower off the mark on the field and the RSI barely moves, because the program guessed wrong about what was actually breaking down underneath that single number.

RSI is a ratio, and a ratio can fall for two opposite reasons that call for opposite training fixes. Either the athlete is not getting off the ground as fast or as high (flight time drops while ground contact stays about the same), or the ground contact itself has gotten slower (contact time rises while flight time barely changes). Averaged into one trend line, both look identical: reactive strength going down. They are not the same problem. This guide walks through splitting the ratio back into its two raw time components before deciding what goes in the next training block.

What RSI Is Actually Built From

Reactive strength index is jump height divided by ground contact time, and in most field systems jump height itself is calculated from flight time using projectile motion. That means a practical, field-usable version of the same ratio can be written directly from the two raw clocks a force plate or jump sensor already records: Flight-to-Contact Ratio (FT:CT) = Flight Time (ms) ÷ Ground Contact Time (ms). It moves in the same direction as RSI and tracks the same underlying quality, but because it skips the height conversion step, it stays closer to what the sensor actually measured.

Case One: The Low-Flight Pattern

An athlete lands and leaves the ground in roughly the same amount of time as always, but each jump sends them noticeably lower into the air. Flight time drops 8 to 12%, contact time holds flat. The limiting factor here is force production during the push-off — the athlete simply is not generating enough vertical impulse in the window available. This is a propulsion problem.

Case Two: The Slow-Contact Pattern

The athlete still gets nearly the same height, but takes noticeably longer on the ground to do it. Contact time climbs 20 to 30%, flight time barely changes. The athlete is compensating for a braking or amortization deficit by spending extra time on the ground to build up the same force output. This is a stretch-shortening cycle (SSC) efficiency problem, not a strength problem.

Both cases can produce the identical RSI drop from 2.4 to 1.9. Program the wrong one — heavy squats for a slow-contact athlete, or ankle hops for a genuine force deficit — and six weeks pass without the number moving.

The Composite Score Blind Spot

Ebben and Petushek (2010) tested the reliability of RSImod across five plyometric variants in 49 college athletes (26 men, 23 women) — countermovement jump, tuck jump, single-leg jump, squat jump, and dumbbell countermovement jump. The composite score itself proved highly reliable within each exercise, but the study also found a significant main effect for exercise type (p ≤ 0.001): RSImod values are not interchangeable across jump variants, because the movement-time and height components contribute differently depending on the task. The limitation worth sitting with is that reliability was established for the composite number, not for its two time components measured independently. A stable RSI trend line can still be produced by a flight-time component and a contact-time component moving in opposite directions and roughly canceling out — exactly the scenario this diagnostic is built to catch.

The Height Conversion Adds Its Own Noise

Gonçalves, Baptista, Tufano, Blazevich, and Vieira (2024) modeled how much error creeps into flight-time-derived jump height when ankle position differs between takeoff and landing — a common and largely unconscious technique drift over a training block. Their simulation found the flight-time method can overestimate jump height by roughly 18% on average for a typical 30 cm jump, and by up to 60% for taller athletes producing a lower jump. It is a simulation rather than a field measurement of real athletes, so the true error band will vary with each athlete's landing consistency, but the direction of the finding matters: converting flight time into a height number before calculating RSI adds a source of noise that raw flight time, used on its own, does not carry.

Contact Time Has Its Own Physiological Zones

Flanagan and Comyns (2008) argued that ground contact time should be tracked as a variable in its own right, not folded silently into a single ratio. Building on Schmidtbleicher's earlier classification, they split stretch-shortening cycle actions into fast SSC (contact time under roughly 250 ms — sprinting, hopping, most reactive jumps) and slow SSC (contact time above roughly 250 ms — countermovement jumps, back squats). Their point was that an athlete drifting from a fast-SSC contact time into slow-SSC territory is undergoing a qualitatively different movement, not just a worse one — and that shift is invisible if you only look at the combined RSI number. Because the 250 ms line comes from a synthesis of earlier classification work rather than a large athlete-specific dataset, treat it as a coaching heuristic to confirm against your own baseline, not a fixed cutoff for every sport.

Equipment

You need a device that reports flight time and contact time as raw numbers, not only a derived RSI or height figure. A force plate remains the reference standard; a validated wearable sensor or contact mat works for field use as long as it exports both raw clocks — many consumer apps only surface the composite score.

Protocol

1. Standardize a drop height (30 cm box is the most common reference point in published norms) and use the same box for every session.
2. Athlete steps off the box, lands on both feet, and immediately rebounds as high as possible with minimal ground contact — the standard drop jump cue.
3. Discard the first rep as familiarization. Record 5 scored trials with 60 to 90 seconds rest between attempts.
4. Log flight time and ground contact time individually for every trial, not just the resulting RSI. Take the average of the best 3 trials by RSI for each metric.
5. Repeat this exact protocol weekly, at the same time of day and training-week position, for at least 3 sessions before drawing any conclusion about a trend.

Setting a Meaningful-Change Threshold

Jump testing carries session-to-session noise even in reliable protocols, so a single week-to-week wobble is not automatically a real change. Use the athlete's first 3 sessions to establish a personal baseline and typical variation, then treat any single-metric shift smaller than roughly 5 to 8% as within normal noise. A shift larger than that in one clock while the other stays flat is the signal this protocol is built to surface.

Contact Time Reference Zones

Once both clocks are logged separately across at least 3 sessions, compare the direction and size of each change against the interpretation matrix below rather than reading the composite ratio alone.

The two middle rows are the ones staffs most often misread, because RSI drops by roughly the same margin in both — the fix, though, points in opposite directions.

Athlete A — Low Flight

Baseline: flight time 580 ms, contact time 180 ms, RSI-equivalent ratio 3.22. Week 6: flight time 525 ms (down 9.5%), contact time 184 ms (up 2.2%, effectively flat). Ratio falls to 2.85. Contact time never left the fast-SSC zone, so the drop is not a stretch-shortening cycle issue — the athlete simply is not producing the same push-off force. The reactive strength index alone would have flagged a generic reactive-strength decline and likely triggered more plyometric volume, which would not have addressed a force-production gap.

Athlete B — Slow Contact

Baseline: flight time 560 ms, contact time 175 ms, ratio 3.20. Week 6: flight time 552 ms (down 1.4%, effectively flat), contact time 226 ms (up 29.1%), crossing from fast SSC into slow SSC territory. Ratio falls to 2.44 — a bigger apparent drop than Athlete A despite a nearly identical jump height. This athlete is compensating for a braking deficit with extra ground time. More heavy strength work would not shorten that contact window; targeted fast-SSC drills would.

Same monitoring alert, same downward RSI trend line, and two athletes who needed completely different six-week blocks.

For the Low-Flight Pattern

Address force production directly. Heavy back squats or trap bar deadlifts (3 to 5 sets of 3 to 5 reps, 80 to 90% 1RM) twice weekly rebuild the strength base, paired with loaded jump squats (20 to 30% body mass, 3 sets of 5) to keep it expressed at speed. Retest at week 4, not week 1 — force adaptations lag the stimulus by roughly 3 to 4 weeks.

For the Slow-Contact Pattern

Address stretch-shortening cycle efficiency, not raw strength. Ankle hops with minimal knee bend (3 sets of 20 to 30 contacts, near-daily) and drop jumps from a lower box than baseline (20 cm, cueing touch-and-go) retrain the fast-SSC pattern. Cut heavy, slow eccentric lower-body work back to maintenance volume during this block — it can reinforce the slower pattern you are removing. Recheck weekly; this pattern typically resolves faster than a genuine strength deficit, often within 2 to 3 weeks.

For the Combined-Decline Pattern

Treat this as a fatigue flag before a training-quality flag. Drop total jump and sprint volume by 30 to 40% for a week, prioritize sleep and recovery modalities, and only reassess against baseline after the deload. Programming more of anything into a fatigued system at this point compounds the problem regardless of which drill you choose. See also: choosing the right drop height for RSI testing and RSImod for CMJ-based monitoring when a box drop jump is not practical for a given athlete.