In modern sports science, one of the most practical ways to understand explosive athletic capability is through the Reactive Strength Index (RSI)—a metric designed to quantify how efficiently an athlete uses the stretch-shortening cycle (SSC). While RSI has long been a staple in field-based assessments, visualizing its components in a two-dimensional quadrant model—the Reactive Strength Quadrant (RSQ)—offers a more nuanced interpretation.
This model, especially when paired with technology like the Plyomat switch mat, allows coaches to evaluate how well and how quickly an athlete produces force, giving context to individual test results and guiding more targeted training decisions.
Reactive strength is best understood as a blend of skill (software) and capacity (hardware)—two interconnected systems that allow the body to rapidly and efficiently utilize the stretch-shortening cycle.
This is the athlete’s neuromuscular coordination and timing ability—their capacity to fine-tune stiffness, anticipate ground contact, and reflexively manage transition speed. It includes: - Proprioception & Anticipation: Recognizing timing and preparing tissues for impact - Pre-activation: Stiffening muscles before landing - Stretch Reflex & Neural Drive: Enhancing concentric output via rapid reflex loops - Motor Control: The brain’s ability to regulate when and how force is applied
This refers to the mechanical and structural qualities of the muscle-tendon system that support fast and forceful movement: - Tendon Stiffness & Elastic Recoil: Ability to store and return elastic energy with minimal hysteresis - Muscle-Tendon Interaction: Quasi-isometric muscle action while tendons lengthen and recoil - Rate of Force Development: Speed at which force can be applied concentrically after landing
Together, these components determine how well the athlete can load, store, and release elastic energy during a plyometric task.
When both software and hardware are trained appropriately, the athlete can minimize ground contact time and maximize flight time—hallmarks of high reactive strength.
Reactive strength is the neuromuscular capacity to dynamically and efficiently exploit the stretch-shortening cycle—via optimal pre-activation, controlled stiffness, rapid eccentric-to-concentric transitions, and minimal energy loss—to maximize force output in a short time frame.
It relies on: - Tendon elasticity for energy storage and recoil - Pre-activation and stiffness modulation to minimize energy leakage - Fast eccentric-to-concentric transition to recapture elastic potential - Reflexive and coordinated timing to support quick responses
First introduced by Flanagan & Comyns (2008), RSI is calculated as:
RSI = Flight Time / Ground Contact Time
It serves as a practical field measure of reactive strength by assessing how much vertical force (via jump height) is generated in how little time (contact time). A high RSI indicates both fast and forceful movement, often reflecting strong tendon recoil, efficient neural drive, and stiffness control in fast SSC (stretch-shortening cycle) activities.
However, RSI is a proxy, not a direct measurement of tendon mechanics or neuromuscular function. Two athletes may have the same RSI but with very different contact times or jump heights, implying different reactive strategies:
These athletes share the same RSI score but reflect different strengths: one favoring elastic return, the other favoring speed of transition.
To bring clarity to RSI outputs, we can plot Ground Contact Time (X-axis) against Jump Height (Y-axis), placing athletes into one of four quadrants:
RSQ gives context to RSI results by showing not just how reactive an athlete is, but how they express that reactivity. It allows coaches to:
|
Quadrant |
What to Train |
Example Drills |
|
🔴 Strength-Dominant |
Faster force application, reduce amortization |
Drop jumps, contrast training |
|
🔵 High Reactivity |
Maintain capacity, fine-tune tissue control |
Depth jumps, pogo hops, shock method |
|
🟡 Low Reactivity |
Build stiffness, motor control, foundational posture |
Marching drills, rope skipping |
|
🟢 Speed-Coordination |
Enhance vertical force and concentric impulse |
Loaded verticals, sprint drills |
Recent studies provide physiological backing for RSI and RSQ models:
Reactive Strength Index is not just a number—it’s a gateway to understanding how athletes produce force in time-constrained settings. The Reactive Strength Quadrant adds an additional layer by showing how that output is generated.
When interpreted correctly, RSI + RSQ: - Help refine plyometric programming - Support load management strategies - Guide athletic profiling and rehab timelines
Every athlete is reactive. The real value lies in identifying where they are reactive, and designing the pathway toward where they need to be.