This is an older post which has been updated on Sunday Oct. 1, 2017. Over the course of the next several weeks, I'll be exploring some of the fundamental sport science basics along with training principles that should help guide programming and training design. These principles are derived from research by sport science pioneers - Siff, Zatsiorsky, Verkhoshansky, Bondarchuk, Francis, Issurin - along with some of the more recent experts in the field - Haff, Cormie, Niumphuis, Pfaff, McMillan, Stone, Plisk, Young - of which some are scientists and others are coaches. Furthermore, for the better part of a decade, my aim has been to apply these principles into tennis specific settings - working with elite juniors in the UK, Canada and pro players from various parts of the world. Finally, I'd like to note that while the principles are guided by science, some of what you'll read is my interpretation of that science, and how it relates to tennis specific tasks.
How do different athletic qualities fit into the program of a tennis player? This is a complex question but one that deserves an answer. With information being so readily accessible these days, there are countless videos of players doing all kinds of things off the tennis court. But let me ask you this: just because a top 100 or 50 player is doing X or Y, does it mean it’s effective? Is it driven by some underlying scientific basis? Often times, it’s not. It’s a regurgitation of someone else’s training or a drill that was once seen before. If you’re a player, and someone is telling you to do squats on a stability ball…or ladder drills to develop agility and change-of-direction (COD) ability...seek alternatives as these are merely gimmicks that have little transfer to the aforementioned performance qualities.
So how does a player increase power, explosiveness, first step ability and racquet head speed? Before we can truly understand how these different abilities make their way into the training program of an elite tennis player, we must first understand a basic principle of sport science...the force-velocity (F-V) relationship - aka the F-V curve.
What follows is a brief description of the F-V curve, how it impacts tennis and what training methods have been proven to improve various components of the curve.
Sport Science Basics - The Force-Velocity Curve
The force velocity relationship underpins all muscle contractions and joint movements. It states that muscle force and velocity are inversely related. When contraction force is high, velocity is low and vice versa (Figure 1). For instance, lifting very heavy, like a 1RM back squat, produces very high forces, at very low velocities. On the complete other end of the spectrum you have high velocity movements and low forces - like sprinting at max speeds. Hill (1938) has been attributed for the discovery of the force-velocity relationship, almost 80 years ago. Today, strength & conditioning coaches and sport scientists use the F-V curve to guide training programs, as an upwards shift in the curve will improve muscle function and athletic performance. How so? The basic premise is this - shift the curve upwards and at any given velocity, you can now produce more force. Or put another way - at any given force, you can produce more velocity.
In theory, this is what the curve looks like...along with associated training qualities:
Here’s an example in tennis. During the tennis serve, the shoulder internal rotators can reach velocities of 3000 degrees/s (that’s fast!). The internal rotators must produce up to 200 N (Newtons) of force to reach these speeds. Now, let’s say you’ve gone through specific velocity training (the velocity/low end of the F-V curve) of the rotators and have increased the velocity output of that motion. You’re now able to reach 3250 degrees/s for the same given force output. This is a velocity specific adaptation. What does this do? It increases your power output for that motion (NOTE** - basic physics, Power = Force x Velocity - in Figure 1, the blue curve represents the power curve; the peak of this curve being a blend of force and velocity).
How Does the Force-Velocity Curve Affect Tennis?
Every stroke and movement in tennis doesn’t fall perfectly into one specific part of the F-V curve. Remember, the F-V curve applies to muscles and joints. We should therefore look at it as part of a continuum. Within one tennis stroke, many (and perhaps all) parts of the curve could be involved as human movement is complex (contraction types, speeds & angles are different and are affected by many variables). In any case, here’s a brief breakdown of the various parts of the F-V curve and where certain movements in tennis fit in.
Maximum Strength and The Force-Velocity Curve
Maximum strength can also be referred to as absolute force or absolute strength. It’s concern is with maximum force generation of a muscle, or a group of muscles. To develop maximum force, you need to lift maximum loads - this doesn’t mean doing a 1RM every time you're in the gym but it does mean lifting somewhere above 85% of your 1RM. You’ll find max strength at the very top of the F-V curve, where higher forces are generated at concomitantly lower velocities.
In tennis, max strength is critical to both absorb high forces and to generate high forces. When referring to the absorption of forces, the most common scenario in tennis is deceleration. The higher the running speed before setting up for a ball, the faster will be the rate of deceleration and the more force the lower body must absorb. Eccentric strength is vital in this scenario. If you think about decelerating when tracking down a ball, you can associate that with the deceleration phase of a heavy squat. Strength adaptations are joint specific, contraction specific and speed specific. Believe it or not, deceleration in sport and the lowering phase of a squat have similar characteristics. There are even cases when more than 2-3 times a player's bodyweight is acting on them during deceleration tasks...if they can't handle these loads in the gym, they surely won't have the ability when it comes to the tennis court.
Another example of force absorption in tennis is landing after a serve. When serving, there are large forces and torques (another term for force but in a rotational manner) created. Kovacs & Ellenbecker (2011) observed that these forces can be as much as 2 times your bodyweight. If we don’t have the eccentric strength to land efficiently on our front leg after the serve, that force will crush us and impede the next movement - and that movement is an explosive recovery to organize oneself for the oncoming shot.
Next is the generation of high forces in tennis. Let’s run with the previous serve example. Once you absorb the landing forces after a serve, to initiate movement, high forces are required. There is a large inertia (or resistance) that’s acting on a player at this point, and the only way to move explosively is to generate force. Two other examples include the re-acceleration after a shot (especially when deceleration forces are high… like running for a tough wide ball) and the very early stages of a big groundstroke - planting the foot and using ground reaction forces.
It is critical to remember that the higher the inertia (or resistance) the more important max strength becomes. Research also suggests that you cannot have high levels of power without first being relatively strong (Cormie et al 2011)! Stronger athletes have greater neuromuscular characteristics - greater size of type 2 muscle fibres, enhanced neural recruitment, superior inter & intra muscular coordination and so on. Because of these neural factors, max strength potentiates the remainder of the F-V curve. That means that at any given velocity, you’ll produce more force and hence, more power (remember...P = F x V).
Strength-Speed and The Force-Velocity Curve
When compared to max strength, strength-speed refers to the generation of high forces at higher speeds. This is generally seen in explosive strength exercises like the clean & jerk or snatch (Olympic weightlifting movements). Again, here, the loads must be quite high (between 80%-90% of 1RM) which puts this quality just below max strength on the F-V curve.
In tennis, strength-speed qualities are important during various parts of groundstrokes and serves. More specifically, the leg drive and take off phases of these strokes. Another example is movement qualities. After the initial acceleration phase, high levels of explosive strength need to be generated to propel the body in the direction of the oncoming ball. Remember, at different moments of a movement, the characteristics of a muscular contraction differ. Within an individual muscle, one fibre may be acting in a fast concentric manner while another may be acting in a slower concentric manner. This is based on the length-tension relationship - in other words, when a muscle fibre is very long or very short, it cannot produce a lot of force while somewhere in between those lengths, the highest forces (and muscle contraction speeds) are produced. We will explore this relationship in another post.
Strength-speed is primarily developed through Olympic weightlifting movements. These movements generate higher power outputs compared to traditional strength training exercises. This is true because the bar (and body) are propelled throughout the entire acceleration phase of a lift. In other words, there is no deceleration until you have to catch the bar (either on your shoulders in the clean OR overhead in the snatch). This deceleration helps with braking forces in tennis, especially when the requirement is to decelerate VERY quickly.
Furthermore, Olympic weightlifting exercises have similar movement kinetics (joint angles, recruitment patterns etc) to movements that occur in sport. This includes jumping, accelerating and changing direction - all important qualities for tennis.
It's never too early to start prepping young juniors with Olympic weightlifting movements. Here's a video example of an 11 yr old performing snatches with a light load.
We ask our junior players to hit serve after serve and groundie after groundie...but when low load, low volume, technical lifts are implemented into their training, they're considered unsafe?!?! A note to parents and coaches, that line of thinking is why injury rates are on the rise in junior tennis...
Speed-Strength, Reactive Strength and the Force-Velocity Curve
One quick thought before we move on to speed strength. To better understand the difference between strength-speed and speed-strength (I know it can be confusing), just remember that the first word in each of the terms is the quality that’s being emphasized a bit more AND both refer to what many people call, power. So strength-speed uses heavier load power training and the adaptations will be more on the strength side (because of higher forces). While speed-strength is more on the lighter side of power training and the adaptations are more speed focused (because of higher velocities). With speed-strength, we’re now starting to move our way down the F-V curve and into ranges of 30-60% of 1RM.
In tennis, the acceleration phases of serves and groundstrokes require high levels of speed strength. In other words, you must be able to produce very high velocities under lighter resistances. Bigger shots and serves will be seen if one can produce higher velocities at a given force, which is the goal of this type of training. Speed-strength is primarily targeted through medicine ball (MB) exercises, loaded jumps, and other ballistic type movements (lighter load Olympic lifts work too).
Furthermore, speed-strength qualities are important during the use of elastic/reactive strength on most change-of-direction (COD) actions, strokes, split-steps etc. This is where our plyometric training and reactive abilities come into action. These exercises are seen further down on the F-V curve as there is no external resistance added (i.e. unloaded).
Again, the further down the curve you go, the more sport specific the exercises become - so when it comes to speed-strength, in some cases, there is still an added resistance to the exercise, but because it’s lighter, you’ll be able to produce more velocity. The adaptations are similar to those seen with strength-speed (neural) along with an increase in RFD (rate of force development).
Essentially, speed-strength can be lightly loaded (barbell jumps etc), plyometric focused (either with upper body MB work or lower body jumping drills) and plyometric/reactive (fast SSC jumps...which I've written about before).
Lastly, at the bottom of the F-V curve, we have maximum speed. This is emphasized during training drills that can generate very high speeds. An example is certain phases of a throw or a sprint (but not all phases and muscle actions). Some sport scientists argue that loads are usually less than 30% of 1RM - but I believe this is too difficult to quantify as movements here are more specific to sport, and occur at higher velocities.
In tennis, max speed can be seen through arm speed (and hence, racquet speed) on all serves and groundstrokes. The highest velocities occur just before contact. Also, high speeds can occur with the rotational components of these shots, especially during (and just after) the acceleration phases.
Max speed, to an extent, can also occur during sprinting movements in tennis. More specifically, when running down drop shots, serving & volleying, retrieving wide balls, on the run etc. It’s nearly impossible to truly reach top speeds in tennis, but high running speeds still have a place in the training of elite players for the previously mentioned scenarios.
Furthermore, research has proven that increases in power and force in high-velocity movements occur with high-velocity training. Meaning that the F-V curve at the velocity end will shift upwards (Figure 3 above).
I know that was a lot to digest but knowing the basics provides us with a foundation when it comes to programming. Coaches can start looking at each drill, exercise etc. from an F-V perspective. Why? Because each area of the force-curve, starting from highest force and moving downwards, potentiates the subsequent area of the curve. Look at figure 4 above - when training incorporates all areas of the curve, there's a shift upwards and to the right. That means, at any force, there will be an increased velocity response and vice versa. For sure, in tennis, more time will be spent on the bottom areas of the curve. But I believe - and the science backs it up - that athletes should be hitting all areas of the curve during training. When it comes to tennis, this type of multi-faceted approach has yet to find it’s way into the training regimes of most players. From a performance perspective, failing to target specific areas of the curve may hinder a player’s ability to move more explosively, hit bigger shots and remain injury-free.