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RSI Measurement Guide: Measure, Read, and Apply Reactive Strength

  • Jul 3
  • 8 min read

Picture the moment a foot lands. The first step of a sprint, a cut to change direction, catching a rebound and springing right back up. The foot stays on the ground for less than 0.2 seconds, and how fast an athlete pours force into that brief moment often decides performance. RSI is the metric that puts this "ability to spring back up quickly" into a number.

This article lays out, as a field workflow, how to measure RSI, how to read the numbers you get, and how to connect them to training decisions. We'll go in order — from what the metric means, to measurement types, to the steps for testing with PoinT GO, to interpreting results, to tips for keeping your data steady.

What RSI actually measures

RSI (Reactive Strength Index) is a value that looks at jump height relative to ground contact time. The calculation is simple.

RSI = Jump height (m) ÷ Ground contact time (s)

The key here is that it isn't about height alone. Even for the same jump height, an athlete who stayed on the ground longer has lower reactivity, while one who touched down briefly and sprang up has higher reactivity. So RSI looks not at "how high you jump" but at "how quickly you react and jump high."

This ability ties to how well the muscles and tendons turn landing impact into elastic energy and reuse it — in other words, how well the stretch-shortening cycle (SSC) is used. Because it underlies almost every sporting movement — acceleration off the start, changes of direction, re-jumping after a landing — it's a metric well worth a team-sport coach's attention.

The information a coach gets from RSI falls into two broad streams.

  • Reactive strength level: An athlete who keeps or raises jump height while shortening ground contact time has an edge in explosive first steps and quick changes of direction. A low RSI means force is leaking away at landing — a sign there's something to work on in plyometrics or base strength.

  • That day's fatigue state: Watching how much RSI drops across the later reps of a continuous jump tells you whether the neuromuscular system is tired. On a good day, RSI holds up well as the reps go on; on a fatigued day, it sags noticeably toward the end.

RSI catches what's easy to miss if you only look at jump height. If height is unchanged but ground contact time has grown longer, reactivity is getting worse.

Which jump to use: measurement types

PoinT GO measures RSI in several ways to fit your purpose. Pick the one that suits the situation.

Measurement type

How you jump

When to use it

Continuous jump

Rhythmic, repeated jumps in place

Regular condition checks, checking fatigue resistance

Drop jump

Drop from a box, then jump maximally the instant you land

When you want a precise look at short contact and fast reaction

Single-leg continuous / single-leg drop (L/R)

Measured one leg at a time

Left-right balance comparison, return-from-injury decisions

If time is tight, a single continuous jump is plenty. It gives RSI rep by rep, so you can see reactivity and that day's fatigue together. If you want to zero in on just the fast reaction of springing up the instant you land, use a drop jump; if you're concerned about left-right balance or return-to-play, add a single-leg test — combine them as needed.

The steps for testing with PoinT GO

The field workflow is simple. The steps are the same whether on the app (connect) or the web (coach-web).

  1. Attach the sensor: Fasten the 15g PoinT GO sensor firmly to the top of the shoe (the instep) with the body strap. For a single-leg test, attach it to the instep of the leg being measured. A loose mount makes the values wobble, so hold it snug.

  1. Select the test type: Choose from continuous jump, drop jump, or left/right single-leg. For continuous jump, set the target rep count; for drop jump, set the box height.

  1. Get ready: Press "Ready" and settle into position during the short countdown. The sensor sets its baseline from this still position.

  1. Measure: For a continuous jump, jump rhythmically. Ground contact time, flight time, and RSI appear on screen in real time for each rep, and it ends automatically once you reach the target count. For a drop jump, drop from the box and spring up the instant you land.

  1. Save: Check the results and save them, and they're recorded to the athlete's history.

PoinT GO captures the landing and takeoff moments with a 9-axis IMU wearable sensor, and its proprietary algorithm — optimized for exactly this — calculates the rep-by-rep RSI on its own. Abnormal attempts, like an awkward step or bounce, are filtered out automatically, so the coach doesn't have to decide "let's drop this rep" one by one. PoinT GO handles which signals it uses and how it calculates, and the coach can focus purely on reading the numbers that come out.

How to read the numbers

With RSI, reading the values you measured matters more. Here's how to look at what appears on screen.

Judge ability by peak RSI and average. Take the day's representative ability from the peak RSI or the average of the well-executed attempts. A few reps may be off-rhythm, so rather than being swayed by a single value, base your read on the attempts that came out well — it's more stable.

Judge consistency by the coefficient of variation (CV). CV tells you how scattered the attempts were. A small CV means the day's measurement was consistent, so you can trust it; an unusually large one is a sign the rhythm wavered or the condition was unstable, so reset the conditions and measure again.

Judge that day's condition by the fatigue decay rate. For a continuous jump, the decay rate shows how much RSI drops toward the later reps. This is an objective signal of neuromuscular fatigue. If an athlete who usually holds steady suddenly sags a lot in the later reps one day, recovery may be incomplete or the training load too high. In that case it's good for the coach to check on sleep and training load together. The decay rate isn't a fixed good/bad cutoff — it's a value you read against that athlete's own normal.

Gauge a rough position by the grade. PoinT GO automatically assigns a grade on a 5-level scale matched to the test type. It's handy for getting a sense of where an athlete stands the first time you measure them. That said, more important than the absolute grade is always that athlete's own change. Whether RSI is up from last month, or sliding as the season goes on, is far more useful for coaching decisions.

Compare left and right. Measuring left and right RSI separately with a single-leg test lets you see balance. If one side is noticeably lower, it can be a sign of injury risk or incomplete recovery, so it's worth referencing for return-to-play decisions or preventive care. Measuring with both legs only can let the stronger side mask the weaker one, so if balance is a concern, measure single-leg.

One thing worth noting: RSI is calculated from movement values — jump height and ground contact time. It doesn't directly measure the force pushing against the ground (N) the way a force plate does. That makes it a strong metric for reading reaction speed and efficiency rather than the magnitude of force itself.

Keeping your data steady

When the measured values wobble, the interpretation wobbles too. Just taking care of these in the field makes the data far steadier.

  • Measure after a warm-up. Doing reactive jumps with a body that isn't loosened up lengthens contact time and lowers the records — and raises injury risk. Measure after a light warm-up.

  • Measure under the same conditions. Time of measurement, degree of warm-up, drop height, and continuous-jump target count should be kept about the same each time for comparisons to mean anything. Especially when the purpose is fatigue monitoring, measure on the same weekday at the same time each week.

  • Spring up as if "the floor is hot." Deeply folding the knees at landing lengthens ground contact time and lowers RSI. Tell the athlete to touch down as briefly as possible and spring up fast.

  • Measure in the condition that fits your purpose. If the purpose is assessing ability, avoid a fatigued state like the day after training. Conversely, if you're looking at that day's fatigue, measure under fixed, consistent conditions and compare against the norm.

Scaling up to the whole team

Measuring one person and managing a whole team are different jobs. This is where coach-web (the web dashboard) shows its strength.

  • Per-athlete trends: Lay out an individual athlete's RSI history in time order and track how much it's risen or fallen against the baseline. An athlete whose RSI has dropped noticeably below normal may need a one-on-one check-in.

  • Team comparison and best records: Gather records by test, including RSI, at a glance. Handy for comparing between athletes or setting targets.

  • Regular measurement routines: Build a measurement program that includes RSI, assign it to athletes, and run it as a routine measured under the same conditions each week.

The flow is: measure with the app in the field, and that data gets organized team-wide in coach-web. Since the app and web share the same algorithm, the same jump gives the same result wherever you measure it.

App and web: when to use which

Both do the full job of measuring and analyzing. The difference isn't capability, it's the situation.

  • In the field or on the move, the connect app: With just a phone you can measure right away, and it's great for cycling through many athletes. It's the tool people reach for by default at the gym or on the field.

  • To manage a team systematically, coach-web: This is where you organize teams, permissions, and programs on the desktop, and compare and analyze the data you've built up. You can also connect a sensor on the web and measure live (desktop Chrome or Edge).

Usually the natural combination is to measure in the field with the app and review it gathered together on the web.

Frequently asked questions

Q. Should I measure with a continuous jump or a drop jump?

It depends on your purpose. A continuous jump gives RSI rep by rep, so you can see reactivity and that day's fatigue together — good for regular condition checks. A drop jump is what you use when you want a precise look at just the fast reaction of springing up the instant you land. If you have to pick one, match it to the sport's characteristics; but if weekly tracking is the goal, a continuous jump is easier.

Q. Jump height is the same, but RSI dropped. Why?

It means ground contact time got longer. Even jumping the same height, staying on the ground longer means reactivity has fallen. Fatigue may have built up, or the athlete may be folding the knees too deeply at landing. This is exactly why RSI catches changes in condition more sharply than jump height does.

Q. Can RSI show left-right balance too?

Yes. Measuring left and right separately with a single-leg continuous or single-leg drop lets you compare balance. If one side is noticeably lower, it can be a sign of injury risk or incomplete recovery. Measuring with both legs only lets the stronger side mask the weaker one, so if balance is a concern, be sure to measure single-leg.

Q. How often should I measure RSI?

If the purpose is assessing ability, once every 4-6 weeks is about right. If the purpose is monitoring condition during the season, measure once a week under the same conditions (time, warm-up, drop height, target count). Here the key is to look at change against the baseline rather than the absolute value.

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