Establishing Point B: Reactive Strength

What is Reactive Strength and How to Train for It

To continue in our series on Establishing Point B, which focuses on the four fundamental physical capacities any athlete should possess to achieve an optimal level of high performance is the quality known as Reactive Strength.

Reactive strength is not a new term, but definitely might not be in the lexicon of many coaches due to a misunderstanding of what it means, the mechanisms behind it and how to properly train for the emergence of it.

What is Reactive Strength?

The simple understanding of reactive strength is related to athleticism. Athleticism is one of those qualities that is easily observed but not so easily explained. We often hear of athletes being more “athletic” than their peers. Often this athleticism is related to the athletes ability to quickly change direction, quickly accelerate or to get off the ground quicker while jumping that from a visual perspective makes them stand out. Often, this concept of athleticism is aligned with the quality of agility. Herein lies one of the training misconceptions regarding reactive strength, as the use of speed ladders, cones and other implements does not create a stimulus to influence reactive strength. When we observe these athletes we must understand that there is a similarity amongst all athletes that display athleticism, which is a high degree of reactive strength, or more specifically the ability of the athlete to rapidly change from an eccentric contraction to a concentric contraction. It is the athletes ability to absorb forces in minimal time and to very quickly use those forces to generate maximal force in another direction that makes the athlete “pop”.

Reactive strength is the ability to rapidly change from an eccentric contraction to a concentric contraction.

Reactive Strength Quantified

As with other strength qualities reactive strength can be measured. It is quantified through the reactive strength index, which is a simple ratio involving the two metrics of jump height and ground contact time. The index is calculated by dividing the height jumped with the ground contact time. For example, an athlete jumping 50cm (0.5m) with a contact time of 200ms (0.2s) would score an RSI of 2.5 units. The RSI can be improved by increasing jump height or decreasing ground contact time.

The difficulty with the RSI is the equipment needed to calculate it, the most important (and expensive!) being the ground contact mat. This is a barrier for most coaches to understanding the RSI and reactive strength.

The Role of Reactive Strength in Explosiveness

Inherent in reactive strength this lies an element of explosiveness or explosive strength. In fact many may associate athleticism with purely explosive strength, although this provides only part of the puzzle. Explosive strength is a special strength that is a component of speed strength whereby there is an accelerative force that allows the athlete to move in either direction, or to jump. Explosiveness is what is termed in strength training terms the rate of force development. The rate of force development and the rate of force application are simultaneously outputs of the nervous system and are based to a large extent on maximal strength. For this reason understanding athleticism purely from a rate of force production perspective only implies a concentric effort (the next article in the series will focus on this, as it is the fourth quality any athlete should possess and therefore train to reach an optimal level).

As mentioned above, reactive strength is the ability to move from an eccentric to a concentric quickly. This implies that there is a lengthening of tissues and then subsequent contraction of tissues at a specific rate of force production that allows the athlete to display athleticism. It then may be more appropriate to think of explosive strength as a special strength quality that is a summation of both reactive strength and rate of force development.

Looking at the posterior leg tissues during the performance of an explosive jump as an example, we would notice an eccentric contraction initially of the posterior leg as there will be relative dorsiflexion of the ankle joint. Then there would be a subsequent concentric contraction of the same region to produce an amount of force quickly into ankle plantarflexion that would allow the athlete to quickly get off the ground and into the air. This ability to quickly. explode off the ground is only available to the athlete that displays high reactive strength ability as well as a high rate of force production and discharge.

Explosive Strength = Reactive Strength + Rate of Force Production

(CNS Output) (Tissue Specific Output). (CNS Output)

It is therefore evident that an athletes explosiveness or their athleticism is a combination of both a tissue specific capacity as well as capacity of the central nervous system.

While this may seem obvious, there are many details that are often overlooked in our understanding of reactive strength and its role in explosiveness which leads to improper training of this tissue specific quality.

Perceived Mechanisms of Reactive Strength

In the literature on reactive strength there are two main mechanisms that have been associated. One is the stretch shortening cycle (SSC) and the other is tendon stiffness.

Stretch Shortening Cycle

The basis of the stretch shortening cycle actually defines reactive strength. To review briefly the SSC refers to the loading (eccentrically) and unloading (concentrically) of tissues during the performance of movement. The action of the SSC is perhaps best analogized as a spring-like mechanism, whereby compressing the coil causes it to rebound and therefore jump off a surface or in a different direction. Obviously increasing the speed at which the coil is compressed or how hard it is pressed down both will result in the spring jumping higher or farther. Both of these things will increase the rate of loading which will often mean the spring will jump higher or farther. Again, this analogy brings together the rate of force production and reactive strength.

One of the most interesting and important aspects of the SSC is what is termed the amortization phase. Your mortgage has an amortization phase. The key to your mortgage is to reduce the amortization and pay off your house. In the emergence of strength amortization refers to the time between the loading and unloading phases and is a huge key to understanding reactive strength and its impact on high performance. In this instance as with a loan, the shorter the amortization phase the better

The other main component of the SSC discussion is the fact that there are two types. This is explained by the fact that some movements are much faster than others (e.g. sprinting vs. walking), and will therefore change the rate of loading and subsequently the speed of the SSC. Consequently, the SSC has been separated into two categories based upon the duration of the SSC:

1. Fast-SSC: <250 milliseconds

2. Slow-SSC: >250 milliseconds

Athletic movements that have shorter ground contact times, a decreased amortization rate, would require a faster SSC. Those movements that occur slower and have greater ground contact times incur a slower SSC.

Tendon Stiffness

It has been discussed that the stiffness of a tendon has a great impact on reactive strength. This relates to the traditional neurophysiological model of athleticism whereby muscles spindles and Golgi tendon organs work together to create power output. This is based on reflexive activations of each receptor that has been hypothesized to increase motor unit recruitment and therefore increase the muscular output. This muscular output must then be “sent” through a stiff tendon to then create an explosive movement and therefore an output of reactive strength.

The Absolute View of Reactive Strength

All of the literature surrounding reactive strength, understanding its mechanism and its training come from a narrow understanding of soft tissues and how they interact and the role of the CNS. This misunderstanding totally negates the impact of connective tissue, both inter and intra-muscularly and how the architecture and behaviour of this tissue at all muscles levels has a profound impact on reactive strength. It is our view at Absolute that reactive strength is a tissue specific capacity that must be adequately trained to maximize the neurological manifestations of it.

The Role of Inter and Intra-muscular Connective Tissue

Briefly, connective tissue exists throughout a muscle at all levels and is continuous at all levels. It has been labeled differently with respect to its levels to parse that there is depth to it, however there is unity in that what is termed endomysium at one level is perimysium at another level and epimysium at another. Furthermore, this continuity extends to other muscles of the same compartment (and beyond) such that the hamstring compartment is a unified compartment of connective tissue that permeates all levels. It must also be mentioned that this tissue goes in the same direction. These are important details in that it also means that due to the continuous nature of the connective tissue it will behave the same (or at least very similarly) at all levels while under load. This is very important as it is this behaviour that will determine reactive strength.

The connective tissue that weaves through a muscle and within muscles of a compartment is a continuous structure that behaves similarly at all levels. In systems terms, it would be appropriate to say that the connective tissue system is fractal in nature (a repeated similar pattern at all levels of the system that creates an ongoing feedback loop). In the diagram above the unity of CT can be observed ‘A’. Also the CT of the perimysium can be observed ‘D’ and ‘E’.

It should be apparent that reactive strength is the representation of fast SSC function, therefore the amortization phase between a rapid change between an eccentric to a concentric must be short. This requires that the potential energy stored during the eccentric must then be quickly used kinetically to generate a quick and forceful movement. As a rhetorical question, what tissue is best suited for this?

Behaviourally, connective tissue is designed specifically for this purpose, as it has been repeatedly shown that it is a specific energy absorbing and dissipating tissue.

The Role of the Central Nervous System

It must also be mentioned that due to the output of reactive strength bearing on fast SSC function, the role of the CNS may be overstated. When looking at the neurophysiological mechanism of spindle feedback reflex mechanisms (and to a lesser extent GTO mechanisms), these occur too slowly to be a predominant mechanism of reactive strength as explosive movement in sport happens much quicker.

This does not mean that the CNS does not have an effect on reactive strength, it just does so indirectly. Recall, that reactive strength is part of explosive strength. Explosive strength is also a product of rate of force production which is highly dependent on maximal strength capacity which is entirely a neurological mechanism of strength.

The role of the CNS in reactive strength is negligible as it has no bearing on the role of connective tissue in the emergence of it. The neurological capacity of absolute strength (as discussed in a previous article) does have an indirect effect.

Gearing and its Effect on Reactive Strength

The dynamic interaction of muscle and connective tissue lead to what has been termed Structural Gearing. Gearing is the mechanism whereby in muscles dynamic changes in architecture during contraction can allow muscles to partially circumvent force velocity constraints to fiber performance. This change in architecture occurs at the perimysial level of the fascicles. During a contraction fibers can “rotate” (essentially change their angle) and the pennation angle can increase. As a result the shortening velocity of the muscle is not necessarily equal to that of the fibers.

For fast SSC contractions, the connective tissue must stiffen at length throughout the perimysial level so the the fascicles can generate a “pull” on it to allow for an output force to be produced. With this occurring at each fascicle by rotating in unison it allows for a very rapid output which occurs far quicker than the CNS can account for. This forms the mechanism reactive strength. This can be observed on the diagram above labelled ‘C’

Training For Reactive Strength

Training for reactive strength has a broad base in the strength literature. The obvious means of training for it would be with the use of plyometrics. Specifically to train fast SSC function these plyometrics would need to have short ground contact times, have smaller displacements of all joints of the lower limb and would fall under the more “dynamic” category. Simple examples would be ankle bounds, ankle hops and short knee arc shallow depth jumps. These types of plyos focus more specifically on springiness and reactivity and will result in overall higher intensities during training than slow SSC plyos.

Herein lies a training conundrum with respect to training reactive strength that is missed in most (if not all) training programs. Plyometrics have been shown to have minimal effect on tissue specific properties but have great effects on the neurological mechanisms of strength. Reactive strength is a tissue specific capacity and must be appropriately trained to maximize the neurological effects of fast SSC based plyometrics.

Tissue Specific Training for Reactive Strength

It is our view at Absolute that it is important to build the behaviour of connective tissue stiffness (and therefore energy absorption and dissipation) as a method of building the emergence of reactive strength.

The intent of connective tissue training for reactive strength is to build load bearing capacity as this is the foundation. In essence create efficiency of energy absorption and dissipation. To do so the programmer must consider the rate of force absorption, the amount of load to be absorbed and where in the range of motion this needs to occur. In addition, because reactive strength is a component of explosive strength the velocity of the training stimulus must also be a consideration. Dynamic efforts are then required.

There is a progression to training for reactive strength. to begin to prepare the connective tissue at length and impart a force to improve energy efficiency would be the use of end range ballistic isometrics accounting for length of tissue, the amount of force imparted as well as over what time frame. This is a very easy way to begin reactive strength training.

Moving from there, it is our experience at Absolute that one of the most effective means of training connective tissue with all of these considerations would be the use of overspeed eccentrics. With the use of overspeed eccentrics the programmer can account now for the force and velocity imparted into the connective tissue and where within the range it is occurring (all the way to end range length or at various points within the available range. of length). This can finely tune the adaptive process of connective tissue at various lengths that most commonly occur in the athletes performance.

Other Articles in This Series

Programming: Establishing Point B

Establishing Point B: Absolute Strength

Establishing Point B: Joint Function and Health

The next and last part of this series, will focus on the capacity of rapid force generation.