Later this month, I’ll be speaking at the Women’s Tennis Coaching Association Conference in NYC. The presentation will focus on 'Change-of-Direction Ability in Tennis'. To get prepped for this topic, for the next several weeks, I’ll be sharing an in-depth look at each underlying quality that makes up this ability. There are still some admission spots open, so If you’d like to attend the event, visit the WTCA site or the conference Facebook page. Hope to see you in NYC!  

Change of Direction and Reactive Strength: Underlying Theory

The ability to respond quickly and efficiently to an oncoming shot, is perhaps one of the most important qualities a tennis player must possess. This ability is predicated on a number of factors including anticipatory skills, perception skills - picking up cues from the other side of the net, judging the ball appropriately etc. - along with physical qualities. Reactiveness is one of those physical qualities. It enhances a more efficient split-step and a more explosive recovery. Why is that important? Because the player that can initiate a better first step AND recover powerfully after they execute a shot, has a clear advantage on all subsequent shots in a rally. 

This post will tackle reactive ability - also referred to as reactive strength in the literature -  and it’s importance to successful COD ability. This includes both the initiation of a change of direction from a split step AND the recovery phase after a shot. This is a 2-part series - the first part will highlight why this quality is important, what reactive strength is and how to test it. The second part will explore the research on reactive strength and it's influence on COD while also providing a practical progression to improve it. 

At this point, I’d like to reiterate that COD and agility are not one and the same (for more on this topic and some added context, please review this article). Recall, agility has perceptual and cognitive requirements along with COD requirements AND is based on the presence of a stimulus. COD, on the other hand, is pre-planned (i.e. no stimulus needed) and it’s requirements are all rooted in physical qualities. We're exploring the reactive strength, one of the sub-categories that influences COD. 

A 'Different' Look at COD and Agility 

Before we get into the details as to what reactive strength really is, I’d like to share a hypothesis. Remember that COD is based on a pre-planned movement sequence while agility is in response to a stimulus. If we assume that statement to be true, recovering after a shot is pre-planned right? Well...not exactly. I like to think of agility and COD existing on a continuum. Some movements are more pre-planned than others. A second serve recovery, for instance, is more pre-planned than a first serve recovery. Why? The serve is slower, which provides a player with more time to make a pre-planned movement towards the appropriate recovery location. On the other hand, a first serve is faster and gives a player less time to recover and hence less time to respond to a 1st serve return. This takes time away to execute a ‘truly’ pre-planned recovery.

This however, is still just a hypothesis, but one that is grounded in scientific rationale. For example, researchers have categorized different sports (along with specific movements in each of those sports) into a different degrees of open and closed skills. Like the tennis serve. This stroke is often referred to as mostly closed in nature because a player has total control over it. But that’s not always the case. There may be times when the execution of a serve is altered based on the environment - wind, sun, a noisy crowd. While typically, a forehand would be considered an ‘open’ skill. But at the elite level, the patterns of play are so ingrained that players often know, ahead of time, where a particular shot is heading. Of course this depends on the tactical situation.

In the end, whether we classify a particular scenario as pre-planned (COD) or in response to a stimulus (agility), doesn’t seem to be relevant. Research (Nimphius 2016) suggests that training COD in a planned, controlled environment, DOES in fact improve both COD AND agility. That's the point here. Concentrating our efforts on the physical sub-components that make up COD can - and likely will - improve all forms of agility and COD related tasks. Which is why it's a worthwhile pursuit. 

What's Reactive Strength Again?

I've written briefly about this topic in the past but let's explore it in more detail. Remember the stretch shortening cycle (SSC)? In particular, the fast component of the SSC? To recall, its basic premise is that a rapid change from an eccentric contraction to a concentric contraction, will ultimately produce a highly explosive movement. When looking at the the fast SSC component of the lower extremity, we’re dealing with actions that have very short ground contact times - i.e. the foot doesn't stay on the ground for very long and no heel contact. To accomplish this, force must be generated in a very short amount of time (250 msecs or less). Why is any of this important? Essentially, reactive strength is defined as an individual’s ability to utilize this eccentric-concentric coupling (Young 1994). In tennis, when split-stepping and initiating the first movement towards the oncoming ball, a shorter ground contact time would theoretically allow us to initiate a faster first step and hence to be on the ball faster.

Another important component that helps achieve a shorter ground contact time is minimal flexion of lower-body joints, in particular, the knee and ankle complexes. This is important in tennis when the distance a player travels is short, the ball is traveling fast and players don’t have a whole lot of time to react. Keeping the amplitudes at these joints small, while still being able to develop enough force to make an explosive movement, is critical for successful COD. This quality, as we’ll see, is highly trainable.

This reactive strength ability possesses one particular sub-component - called stiffness - which enhances the use of the fast SSC. Let's understand how stiffness enhances reactiveness. 

Joint Stiffness

During explosive actions, there is energy exchange between the various tissues of the lower-extremity, including muscles, tendons and ligaments. This transfer of energy is highly influenced by stiffness. As the term implies, this literally means a stiffening of the targeted musculature and surrounding tissues, both in an active and passive sense. Stiffness in the lower-extremity can be seen as either total leg stiffness - which includes the ankle, knee and hip joints - or simply ankle stiffness. Stiffness in the ankle is of primary importance due to the activation and rapid contraction (from eccentric to concentric) of the triceps surae musculature - the 2 heads of the gastrocnemius along with the soleus (image below).

 
 

A stiffer spring is theorized to rapidly release stored elastic energy. This is exactly what we’re looking for when initiating movement or recovering after a shot. And it begins in the ankle. If the ankle cannot stiffen, not only will our ground contact time be higher but energy will be ‘leaked’. This results in diminished stiffness at the knee and hip - both of which are influenced by triceps surae stiffness. Think of the kinetic chain - power, force etc. is always generated from the ground up. If at some point there’s a broken link, the rest of the chain suffers. That’s how important ankle stiffness is in generating movement and changing direction!

How Exercises and Cueing Affect Stiffness

Researchers (Arampatzis et al 2001) have found that acute changes in stiffness do in fact occur. What this tells us is that having the correct intent will help achieve stiffness AND that morphological changes in the muscle-tendon complex are not entirely necessary to achieve stiffness. Proper exercises and cues, therefore, have a profound affect on establishing this quality with athletes.  That said, long-term passive and active morphological adaptations in the muscle-tendon and surrounding tissues may occur. While researchers have yet to confirm this, passive ankle stiffness in elite tennis players, shouldn’t be seen negatively.   

One final note on the notion of proper exercise prescription and cueing. What the aforementioned study revealed was that when drop jumps (a form of training that can enhance stiffness, as we’ll see in next week's post) were performed by athletes, two things happened. First, there was a greater pre-activation of muscles of the ankle complex BEFORE they hit the ground. And second, co-contraction - the ability to contract both the agonist and antagonist musculature simultaneously - increased. Both of these factors increase stiffness. This simply highlights the importance of deliberately training stiffness/reactiveness off the court, so that on the court, an increase in pre-activation and co-contraction, can occur automatically.

How do we Test Reactive Strength? 

Reactive strength is measured via the Reactive Strength Index (RSI). RSI is determined through a drop jump test. An athlete steps off a box and upon landing, attempts to jump both as explosively as possible AND as high as possible (video below). RSI takes into account the ground contact time (in seconds) along with the jump height (in metres). So if an athlete were to have a ground contact time of 0.175 s (175 msecs) and a jump height of .4 m (40 cm), their RSI would be 2.29 m/s.

 
 

Testing reactive strength has garnered a lot of interest over the last several years. There are a number of reasons for this. Here are the 3 most common:

  1. Establishing appropriate box height for drop jumps. Knowing when an athlete is under the 250msec threshold is critical as anything above this number, means we’re no longer using the fast SSC component.

  2. Motivation. Having a number available for athletes to see, acts as a biofeedback motivational tool that encourages them to increase arousal and beat their previous scores.

  3. Monitoring. There’s a body of evidence that suggests that RSI can provide insight into neuromuscular fatigue. Weekly monitoring of this quality can provide coaches with information regarding early signs of overtraining and/or burnout.

Traditionally, this test was limited to research settings as it required the use of expensive equipment - force platform or jump mat. But recent advancements in technology have allowed coaches and athletes easy access to these types of metrics. PUSH - a sport tech company from Canada, have developed a wearable device that can derive RSI, along with other metrics. In the video above, I use a PUSH Band to carry out the test. The image on the right (below) provides a screenshot of what the PUSH app showcases (not a bad score for a non-athlete, but improvements needed). Eamonn Flanagan, the lead S&C coach for the Irish Institute of Sport, has studied this topic extensively and provides the following RSI thresholds and recommendations on how to interpret the results (image below left). 

RSI Thresholds and Recommendations

RSI Thresholds and Recommendations

No Way to Test? Here's What to Look For.

If working in a high-performance environment, I strongly encourage coaches to invest in one of the above technologies. That said, many of you may not have the resources or access to equipment such as a jump mat or PUSH band. If that’s the case, there are still ways a coach’s eye can help identify whether an athlete has developed the appropriate amount of stiffness or whether an appropriate box height is being used.

According to Flanagan (2008), here are a couple things to look out for. For starters, when it comes to ground contact times, coaches may observe an athlete’s ability to stay on the balls of their feet, rather than having their heels hit the ground during the jumping phase. Next, make sure there is very little flexion at the hips and knees - they should flex just enough to produce the desired explosive action. And lastly, the ankle complex should resemble a spring - one that is more on the stiff side but that still has ability to uncoil. This is perhaps the most important feature as it will enhance the other contributing factors. Look at the video below for an idea of what stiffness should look like. 

 
 

Keep in mind that these are important factors to look out for when the aim is to increase fast SSC abilities/stiffness. If the focus is on maximizing power output, these observations would no longer apply.

Still to Come...

In next week’s post on reactive strength, we’ll look into some of the research that highlights it’s effectiveness to improve COD speed along with another interesting finding - it’s potentiating effects. There’s a growing body of evidence suggesting that performing highly reactive movements before competition can augment COD and explosive power. I believe that’s something worth exploring, especially if it’ll provide a competitive edge. Next week's post will also feature a number of exercise examples while providing a framework for how to organize and progress this quality with both developing and elite players. 

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