Reactive strength has been described as the capacity of an athlete to bear a stretch load and subsequently switch rapidly from an eccentric to concentric muscle action. This strength quality is most important in movements that involve short ground contact phases (<0.250 s) such as running, sprinting, side stepping, jumping and hopping.
Researchers have traditionally assessed reactive strength during a drop jump where an athlete drops from a set height and upon landing performs a vertical jump at maximal effort. This test yields a measure of reactive strength known as the reactive strength index (RSI). Within the literature, RSI has been calculated using two calculation methods: the jump height in a drop jump, generally derived from flight time (time in the air), divided by the contact time (time on the ground) or alternatively the flight time of the jump divided by the contact time.
The difference in calculation method may seem trivial, however this is not the case. When RSI is calculated using jump height as the denominator it signifies a theoretical quantity that represents the predicted jump height that would be achieved with a ground contact time of one second. Consequently, it has the unit m·s-1. When RSI is calculated using flight time as the denominator it represents the ratio of flight time to contact time. It is unitless as it is a true ratio. To make this distinction in calculation method clear it is recommended to refer to the former method as RSI and the latter method as reactive strength ratio (RSR).
These distinctions can have a considerable effect in terms of athlete testing. Data collected in male and female sprint athletes has revealed that RSI and RSR scores are highly influenced by the performance strategy utilised i.e. how an athlete achieves their performance. Jumping higher will typically have a larger effect on RSI whereas jumping with a shorter ground contact time will typically have a larger effect on RSR. From a practical perspective this means that RSI will better at assessing an athlete’s ability to generate an impulse and thus achieve a higher jump height whereas RSR will be better at assessing an athlete’s to tolerate a stretch load and thus achieve a shorter contact time. Coaches need to be aware of this when deciding which reactive strength calculation method they use with their athletes.
A full paper that further explains this concept can be found here.
Robin Healy is temporarily covering a lecturing post in Biomechanics in the Department of Physical Education and Sport Sciences at the University of Limerick. In addition to lecturing Robin is finishing his PhD. Robins current area of interest focusses on the biomechanical specificity of resistance training exercises and their potential transfer to both acceleration and maximal velocity sprinting. Contact Robin via email on email@example.com or view his research profile on Researchgate.