What is the difference between RSI and mRSI?

RSI stands for Reactive Strength Index.

• RSI is the ratio of Flight Time & Ground Contact Time
• RSI = Flight Time / Ground Contact Time
• Time To Take-Off (TTT) can be substituted for Ground Contact Time if the movement being performed is a Countermovement Jump.
• There is no unit for RSI.

mRSI stands for Modified Reactive Strength Index.

• mRSI is the ratio of Jump Height & Ground Contact Time
• mRSI = Jump Height / Ground Contact Time
• Time To Take-Off (TTT) can be substituted for Ground Contact Time if the movement being performed is a Countermovement Jump.
• There is no unit for mRSI.

Practical Application

• RSI and mRSI are typically used as a way to measure an athlete's reactive strength ability. These metrics helps answer the questions...
  • How elastic is this athlete?
  • How effectively do they use their stretch-shortening cycle?
  • Do we need to work on this quality?
• Assessments of reactive strength include...
  • Fast Explosive Strength: drop jump, countermovement rebound jump, and multi rebound jump.
  • Slow Explosive Strength: countermovement jump
• What is a good mRSI value?
  • This figure below shows RSI on the righthand sign, however Eamonn is using the equation Jump Height / Contact Time. These values are best used when analyzing mRSI during a drop jump off of a 30cm box.
    

    This figure above is an except from the Train With Push blog post titled 'THE REACTIVE STRENGTH INDEX REVISITED - PART 2 BY EAMONN FLANAGAN'

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