Why You Need to Know the Drop Height When Interpreting RSI in Drop Jumps

Drop jumps are used to assess a person’s reactive strength; they drop from a box, land on the ground, react to the landing and jump upwards.

Reactive Strength Index, or RSI, is commonly used to assess drop jump performance and is calculated using one of the following two methods:

RSI = flight time ÷ contact time

RSI = jump height (flight time) ÷ ground contact time

In simple terms, a higher RSI suggests that a person can jump relatively high compared to the time they spend in contact with the ground. Because of this, RSI is used as a marker of reactive strength, plyometric ability, and the capacity to rapidly absorb and reproduce force.

But RSI should never be interpreted in isolation.

Two individuals can achieve the same RSI in different ways. One may jump higher but spend longer in contact with the ground. Another may not jump as high but spend less time on the ground. The RSI value can be the same, but the underlying strategy is different. If we only look at the overall RSI value, we miss important information regarding how the person is achieved it. Flight time and contact time should always be considered alongside RSI.

However, there is another key variable that is often overlooked:

Drop height. The height you drop from is not just a minor detail. It determines the whole drop jump challenge.

Force plate drop jump metrics are derived from vertical ground reaction force and time using the underlying principles of impulse and momentum (Imp-Mom). Momentum is the product of an individual’s body mass (kg) and velocity (m/s). During a testing session, an individual’s body mass will not change meaningfully, therefore their momentum is dictated by their velocity. Vertical velocity is calculated using the equation √2gh, where g = gravity and h = height. Since gravity does not change either, the key determinant of velocity is drop height. A greater drop height means gravity accelerates the person for longer, so they will have greater vertical velocity when they contact the ground.

Note that mass does not affect velocity. Gravity does not care about mass, we all accelerate to the ground at the same rate. Carrying extra mass like dumbbells will not make us drop faster! For a given body mass, greater contact velocity means greater downward momentum. That momentum has to be decelerated during ground contact before the person can reverse direction and jump upwards.

Higher drop height → greater vertical velocity at contact → greater downward momentum → greater deceleration challenge.

This is important because impulse is the integral of force over time and is equal to the change in momentum. Therefore, to reduce momentum to zero at the bottom of the drop jump, the person needs to produce an eccentric impulse that is equal and opposite to the momentum at contact. If the momentum is larger, the individual has to produce more force, take more time, or a combination of both to produce the required eccentric impulse.

This directly affects ground contact time.

If the drop height is increased, an athlete may need longer on the ground to safely and effectively decelerate before rebounding. If jump height stays the same but contact time increases, RSI will decrease. This does not necessarily mean the person has become less reactive, it means the task was more challenging.

This is why interpreting RSI values, contact time, and jump height without considering drop height can be misleading. A drop height of 10cm or 15cm is not the same challenge.

But why would this matter if the person is dropping from the same height box?

The box height doesn’t necessarily match the drop height. If the person rolls or steps off the box on one leg but hops off it on the other leg, the drop height can be very different. Manipulating the drop height might be a conscious or subconscious strategy to reduce the challenge and ‘cheat’ the test. Without knowing the drop height, we do not know the size of the challenge we gave them.

Drop height is measured by the force plates using Imp-Mom. Since the eccentric impulse will equal the momentum at contact, vertical velocity at contact will be eccentric impulse ÷ mass. Drop height can then be calculated from vertical velocity using the formula h = v²/g.

This is particularly relevant in rehabilitation and return-to-sport testing. A lower RSI may reflect poorer reactive strength or a larger eccentric demand. RSI, contact time and jump height alone cannot tell us which of these is happening.

The first question should be: What was the drop height?

Drop height provides the context for the whole task. It tells us how much downward momentum the athlete had to decelerate before producing the jump. RSI and its constituents should always be interpreted alongside drop height.

The message is simple:

The higher the drop, the greater the downward momentum. The greater the downward momentum, the greater the eccentric impulse required to decelerate it.

If we do not know the drop height, we do not fully understand the RSI.

In drop jump testing, the drop height is not just part of the protocol. It determines the entire challenge.