In previous posts on COD, we spoke about the importance of reactive strength. In particular, we emphasized the role leg and ankle stiffness plays in the production of reactiveness. Ultimately, high levels of reactiveness are predicated by very fast eccentric-concentric muscle actions. These actions impact a variety of movements in tennis, including any type of first-step reaction that involves very little changes in knee, hip and ankle amplitudes. `
But what about movements that have longer ground contact times? For instance, a player is forced into a deep lunge position - perhaps because of a fast low ball or because they’re retrieving a low volley at net. To recover from these types of scenarios requires qualities that extend beyond reactiveness. This is where strength and power qualities come into play. While reactiveness is great when joint angles are small, inertia is low and ground contacts are short, when these parameters are reversed, fast stretch shortening cycle (SSC) abilities won’t cut it.
This post will wrestle with the following notion - tennis is chaotic and unpredictable. While elite players are familiar with both the patterns of the game and the specific footwork patterns required to succeed at the highest levels of our sport, how the body deals with these tasks, is far from predictable. We’ll explore how strength and power qualities can influence these so called ‘other’ movement requirements and why relative strength far outweighs absolute values. In next week's post, we’ll provide a number of exercise examples, along with the specific ways in which they should be implemented, to elicit the most effective responses ON the tennis court.
Strength Training and It’s Role in Stiffness
Before we move away from stiffness (for now), I’ve got some news for you. Apart from specific fast SSC drills - which we covered in last week’s post - there’s another way to improve reactive strength. Weight training. That’s right, research (Brazier et al 2014) has consistently shown that increases in leg and ankle stiffness can occur via a variety of weight training means. This includes Olympic weightlifting, loaded ballistic movements and heavy strength training.
Now when it comes to reactive strength purposes, weight training enhances this quality in a number of ways - from increases in neural drive to changes in tissue structure - while one theory postulates that weight training requires a high degree of co-contraction (simultaneous activation of agonist and antagonist muscle groups). Recall that co-contraction is a precursor to stiffness. Consider a squat for example. If you’ve ever performed this lift at a high intensity, you’ll know that to keep yourself upright (essentially from falling over), it requires a high amount of stiffness - in the lower-extremity, through the trunk and even into the upper limbs.
While various forms of strength training may improve stiffness, these lifts should remain multi-joint/compound in nature. Isolation type exercises like calf raises generally have very little transfer to sport/movement. In fact, these types of exercises could increase injury risk because of compensatory factors - in other words, they don’t provide a uniform increase in strength or power and should be used with caution. Literature supports the use of lifts like cleans, snatches, squats, deadlifts and derivatives of these movements. Those lifts can be accompanied by loaded ballistic movements - like barbell jump squats (with varying degrees of knee/hip flexion). These exercises will provide a much more uniform adaptation in leg and ankle stiffness.
Clearing the Air on Stiffness
I’d like to add 2 key points to this topic before moving on. Firstly, when referring to ankle and leg stiffness, we’re not necessarily referring to lack of mobility/flexibility in this area. While there may be some passive increases in joint/tissue stiffness in these regions, it’s the ability to stiffen these structures ACTIVELY - through proper cueing and specific exercise implementation - which improves reactive strength.
Secondly, too much stiffness isn’t good either. Think of an extremely thick and, sturdy spring. There’s no compliance, no give. It doesn’t have any spring to it whatsoever. That’s definitely not the quality we’re after. We want joints and tissues to have a certain level of compliance, of ‘jump’.
Tennis Players ARE NOT Weightlifters
By this point, it’s important we get one thing straight. Ultimately, we want to become better tennis players, not weightlifters or powerlifters. You won’t need to (and shouldn’t) spend most of your time in the weight room. These various forms of weight training are merely used to supplement specific tennis drills and on-court training. They should be implemented carefully and in very deliberate ways - the aim being to elicit the desired neuromuscular, hormonal and mechanical adaptations. I'm glad we got that out of the way...now let's move on.
Training the Fast SSC Isn't Enough
While the fast SSC is of prime importance to tennis players - from a reactive split-step, to getting set for a fast oncoming ball - it’s not the only important component of a successful change in direction. Power and strength have their place, beyond their stiffness enhancing qualities. But how can we make the distinction? When should we train reactive abilities vs power/strength? Before we can answer this question, we need to know the differences as they relate to elite tennis.
How Movement Velocity and Joint Angles Affect COD in Tennis
Recall from a previous post that there are 3 phases to any COD task. The braking (or deceleration) phase - in tennis, this occurs when tracking down a ball. Next is the planting phase - which occurs either before, during or after we execute a stroke. And lastly, we have the propulsive phase - or the re-acceleration phase, once we complete a stroke. As we saw in a previous post, each has it's own distinct characteristics.
When running down a ball, a player must make a rapid deceleration - which is influenced by entry velocity - in other words, the speed at which a player is moving while tracking down the ball. This entry velocity is dependant on how far a player has to travel to meet the ball. If a player needs to sprint from one sideline to the other, they’ll surely develop more running velocity than if they only had to move a metre or two. Furthermore, if that ball is coming fast and low, the player will be required to get into a lower position - not only to meet the ball at a better impact point but to maintain a low stance. This low stance helps to effectively transition from braking to planting and then from planting to propulsion.
Because this type of shot requires that a player gets and stays low, the angles at the knees and hips (image of Kerber above) will be much greater. High levels of reactiveness won’t help us much here. Our ground contacts - the amount of time your foot is planted before initiating a recovery - are longer (definitely above 250 msecs). Anything above this mark and we’re now looking at a bigger influence from the slow SSC - this component of the SSC is able to absorb and generate more power; it just takes more time. But in these cases, it’s ok, the extra time needed is worth it as it’ll produce a much more powerful movement - and this it what we need.
While we don’t know exactly how long the ground contact time will be, or the amount of force we’ll need to generate, we better prepare the body to deal with a variety of circumstances. There are times when the deceleration phase of a COD places more load on an athlete compared to their body mass - sometimes the forces on the body are even several times greater. Being able to absorb these types of forces off the court, is simply a precursor to prepare the body to meet the demands on the court. This is where strength and power training become more important. Off the court, we can control the angles we train in, the loads we impose, the contraction times and so on. This allows us to target each phase of COD in very specific ways.
COD in Tennis is Different than Other Sports
The thing with tennis, compared to football and soccer, for instance, is that you have this rotationally demanding activity right in between your change of direction - a groundstroke. Depending on the type of shot required - which is based on the tactical scenario in question - the propulsive phase, and it’s contractile requirements, will differ. This is why I am constantly emphasizing jumping activities that are both multi-directional and multi-planar. The videos below are just a couple simple examples of exercises that involve the slow SSC - the objective of these jumps is to control movement in multiple planes (similar to hitting a big forehand and then having to develop force to recover upon landing). It’s what the sport demands, so we must expose the body to everything and anything that it may encounter on the court.
Lastly, in case you were wondering why we do a number of sagittal plane movements, it’s based on the fact that braking and propulsive forces are correlated to both horizontal and vertical ground reaction forces (Spiteri et al 2013). While to the naked eye, it may appear that a player is simply moving in a linear fashion (either horizontally or laterally), in fact, at the joint moments themselves, there is considerable vertical forces that contribute as well. This gives rise to the fact that improving the ability to absorb/produce force and power in the vertical direction - via squats, vertical jumps etc - is a worthwhile pursuit.
Body Mass and Relative Strength - Contribution to COD
In a study by Delaney et al (2015), professional rugby players were assessed in multiple abilities to determine various performance attributes and how they correlate to COD ability. As we learned previously, vertical power production has relevance to COD performance. In this study, when comparing a 505 COD test to a slow SSC vertical jump (40kg CMJ - countermovement jump), there was no significant correlation. While peak power from the vertical jump had no bearing on COD ability, when that peak power was converted to relative peak power (based on body mass), everything changed. All of a sudden, there was large significance between the jump and COD performance.
When it comes to maximum strength in the back squat, the findings are even more telling. Delaney and his colleagues found no correlation between COD and a 3RM max full depth back squat. However, when back squat strength was reported relative to body mass, there was a strong and significant correlation to COD ability. In sub-elite rugby players, relative 1RM back squat strength was the greatest predictor of 505 COD ability! While in female basketball players (Spiteri et al 2015), there was a very strong and significant correlation between relative strength in a half-back squat and 505 COD ability.
What About Female Athletes?
Female athletes seem to consistently show greater correlations in COD ability when compared to relative strength. In the case of the Spiteri study, one contributor to this finding is that a half squat was used for testing instead of a full squat. When it comes to training, this is important for 2 reasons. First, a half-squat elicits angles that are more specific to COD tasks. On a tennis court, it’s more common for players to move and change direction at these smaller angles than very deep ones - like a full squat represents. Second, higher loads can be tolerated in these smaller angles, which further increases relative strength values. More time should therefore be spent training these specific angles, especially around the pre-competitive and competitive phases.
But wait...it’s not uncommon to see players, especially in women’s tennis, get into these deeper ranges. In fact, while these larger angles may be less prevalent, often times in matches, they could come at critical, momentum shifting moments. You know those moments, when you’re running down a wide ball and need to get into a low lunge to receive it. Or when a fast, deep ball forces you to get into a deep squat position, just to have a chance to get the ball back in play. Look at the images below and the angles at the knees and hips - these happen more often than we realize.
COD ability is influenced by a number of factors. Remember that one of those factors was technique. We haven't even touched this topic yet, but it's an important one. For a player to move better on the tennis court - by initiating movement rapidly and changing direction efficiently - they need both the TECHNICAL PROFICIENCY and the CAPACITY. We've been concerned with capacity but we cannot forget about the specific footwork requirements that a player must possess. You may be able to lift a bus, but if you're movement technique is inadequate, it won't help matters. The other side of the coin is equally true - if you're already technically sound, further attempts to improve technique may not be worth the time investment. This is where the coach needs to step in to make an informed decision as to what needs to be trained and when.
In next week's post, we'll provide further examples of exercises that can help improve COD ability - and its distinct phases. It'll also provide us the opportunity to consider certain loading schemes - in other words, how much to lift, for how many sets/reps, at what tempo and so on. These variables is what distinguishes training programs that have a specific target versus those that simply tell us to do 3 sets of 10 - the former taking many parameters into consideration, while the latter lacking in context and specificity.