{"id":1851,"date":"2026-06-26T14:25:21","date_gmt":"2026-06-26T14:25:21","guid":{"rendered":"https:\/\/adrencoder.com\/blog\/uncategorized-en\/reactive-strength-index-rsi-what-it-is-how-it-is-measured-and-how-to-improve-it\/"},"modified":"2026-06-26T14:25:21","modified_gmt":"2026-06-26T14:25:21","slug":"reactive-strength-index-rsi-what-it-is-how-it-is-measured-and-how-to-improve-it","status":"publish","type":"post","link":"https:\/\/adrencoder.com\/blog\/en\/vertical-jump-en\/reactive-strength-index-rsi-what-it-is-how-it-is-measured-and-how-to-improve-it\/","title":{"rendered":"Reactive Strength Index (RSI): what it is, how it is measured and how to improve it"},"content":{"rendered":"

He Reactive Strength Index (RSI)<\/strong>, or reactive strength index, is one of the most valuable metrics for evaluating an athlete’s explosive performance. In short: it measures how much the athlete jumps in relation to the time it takes to do so. A high RSI means that the athlete produces a lot of force in a very short time, which directly translates into a better vertical jump, greater sprint speed and more efficiency in any explosive action.<\/p>\n

Unlike simply measuring jump height, RSI captures the stretch-shortening cycle (SSC) quality<\/strong>: that physiological mechanism that allows elastic energy to be stored when landing and released explosively to jump again. To measure RSI you need to know two pieces of information: jump height and ground contact time. Devices like the ADR Jumping<\/strong><\/a> They calculate both variables automatically and show you the RSI in real time.<\/p>\n

What is the Reactive Strength Index (RSI)?<\/h2>\n

The RSI was developed in 1995 by Young as part of the Strength Qualities Assessment Test (SQAT)<\/em> from the Australian Institute of Sport. Its formula is simple:<\/p>\n

RSI = Jump height (cm) \u00f7 Contact time (s)<\/strong><\/p>\n

An athlete who jumps 40 cm with 0.20 s of contact has an RSI of 2.0. Another that jumps the same height but with 0.30 s has an RSI of 1.33. The first is more reactive, although it jumps just as high.<\/p>\n

According to Brooks (2026)<\/a>, the RSI provides a unified mechanical framework<\/strong> to evaluate the function of the SSC objectively, being especially useful to identify neuromuscular deficits and design individualized plyometric training programs.<\/p>\n

\n\n\n\n\n\n\n\n
Variable<\/th>\nDefinition<\/th>\nHow to obtain<\/th>\n<\/tr>\n<\/thead>\n
Jump height (cm)<\/strong><\/td>\nVertical distance traveled by the center of mass from takeoff to the highest point<\/td>\nCalculated from flight time<\/td>\n<\/tr>\n
Contact time (s)<\/strong><\/td>\nTime the foot remains in contact with the ground between consecutive jumps<\/td>\nMeasured directly with contact platform or infrared system<\/td>\n<\/tr>\n
RSI<\/strong><\/td>\nRatio between jump height and contact time<\/td>\nAutomatically calculated by the device<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

The stretch-shortening cycle: the physiological basis of RSI<\/h2>\n

To understand the RSI you have to understand the SSC. It is the mechanism by which muscles and tendons act like a spring: they are loaded during the braking phase (eccentric) and release that energy explosively in the propulsion phase (concentric), generating more force than in a pure concentric contraction.<\/p>\n

There are two types of SSC with different practical implications:<\/p>\n

\n\n\n\n\n\n\n\n\n
Feature<\/th>\nSSC slow<\/th>\nfast SSC<\/th>\n<\/tr>\n<\/thead>\n
Contact time<\/strong><\/td>\n> 250ms<\/td>\n< 250ms<\/td>\n<\/tr>\n
Typical exercises<\/strong><\/td>\nCMJ, squat jump, long jump<\/td>\nDrop jump, rebounds, sprint, hurdle jumps<\/td>\n<\/tr>\n
Dominant mechanism<\/strong><\/td>\nActive muscle contribution<\/td>\nMuscle-tendon stiffness + elastic energy<\/td>\n<\/tr>\n
Does the RSI measure it?<\/strong><\/td>\nNot directly (\u2192 RSImod)<\/td>\nYes (\u2192 Classic RSI with drop jump)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n
\n

The RSI measures the fast SSC.<\/strong> An athlete may have an excellent CMJ and a mediocre RSI. They are different qualities that require specific training.<\/p>\n<\/blockquote>\n

How is RSI measured? Protocols and tools<\/h2>\n

To calculate RSI you need to accurately measure jump height and contact time. These are the main options:<\/p>\n

\n\n\n\n\n\n\n\n
Tool<\/th>\nMeasure height<\/th>\nMeasure contact time<\/th>\nCalculate RSI<\/th>\nPortability<\/th>\nCost<\/th>\n<\/tr>\n<\/thead>\n
force platform<\/strong><\/td>\n\u2713<\/td>\n\u2713<\/td>\n\u2713<\/td>\nLow<\/td>\nvery high<\/td>\n<\/tr>\n
Mobile app (video)<\/strong><\/td>\n\u2713<\/td>\n\u2717<\/td>\n\u2717<\/td>\nHigh<\/td>\nLow<\/td>\n<\/tr>\n
ADR Jumping<\/a><\/strong><\/td>\n\u2713<\/td>\n\u2713<\/td>\n\u2713 (automatic)<\/td>\nHigh<\/td>\nEconomic<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

Most used RSI tests<\/h3>\n