- On avril 27, 2016
Plyometric jump training is one of the most effective methods for developing speed and power in many sports. But how do you know if your jump training is effective and safe for your athletes? In Part 1 of this 2 part series we will discuss why plyometric training is a great way to improve the speed and power of your athletes, and the keys to performing it effectively and safely.
Common Misconceptions about Jump Training
Whilst jump training is recognised by many coaches as a great way to train power and speed, a lot of trainers are reluctant to use plyometric drills with their athletes, as many people associate Plyometric training with high injury risk. I have had many coaches say to me “Yeah we’d like to use more plyos, but it’s just too risky, especially in-season”.
Some coaches may also be concerned about using plyometric training with young athletes, or with athletes who are recovering from injury. Only around 10 years ago, standards were publicised in the industry stating that before even commencing a plyometric training program, athletes should be able to squat twice their body weight. Thankfully recent research has actually shown plyometrics to be completely safe for use with developing athletes.
Understanding Rate of Force Development (RFD) and the Stretch Reflex
In 1992 researcher Dietmar Schmidtbleicher explained that there is a time course for muscles to develop force during explosive movements.
To reach maximum force takes approximately half a second (500 ms). Maximal force is what we term STRENGTH. However in many sporting activities, the movements happen too quickly for an athlete to generate maximum force. For example, in sprinting, typical ground contact times during acceleration are around 100-120 milliseconds, and once they are at top speed, it is even less (around 80-90 milliseconds). So in sprinting, maximum force doesn’t even come on the radar as being important to performance.
How quickly someone can produce force is what we (the industry) call POWER. In the muscles, this is determined by how quickly the muscle can generate force, which is what we call RATE OF FORCE DEVELOPMENT.
In 1997, Fleck and Kraemer published a pivotal training study demonstrating that combined power and strength training was superior to strength training alone, ESPECIALLY when it came to fast movements like sprinting and changing direction. Whilst both strength and combined strength/power training had similar effects on maximum strength, combined strength and power training had a HUGE effect on the initial rate of force development. With strength training only, there was little or no change in the part of the curve that ACTUALLY MATTERED when it came to sprinting.
So whilst getting stronger has its benefits, if you want your athletes to get faster, proper use of plyometric training is critical.
So what happens during these short fast movements that is so special and needs to be targeted during training in order for these changes to occur?
There are a variety of factors discussed in the literature, but the main consensus is that in movements where the contact time is less than 250 milliseconds, a phenomenon known as the stretch reflex (stretch shortening cycle) boosts your muscles ability to produce force and power. In simple terms, think of it as stretching a rubber band quickly and letting it go. A combination of factors is involved, but the end message is simple – training needs to be specific. If sprinting involves ground contact times of 90-120 milliseconds, then power training movements with a similar time frame will be beneficial.
As for what contact times to target during jump and other plyometric training, as with any type of training there is a continuum, and you should use a range of exercises. Some should focus on the initial part of the “plyometric zone” at around 100 milliseconds, and some should focus on the other end where more force is generated, but still not up to maximal strength (around 200 milliseconds is good).
When are Plyometrics Safe to Use?
The simple answer is always. As long as they are done safely. This involves choosing the appropriate exercises, teaching correct technique and so on. But most importantly, safe plyometric training is simply about controlling the force generated.
Let’s take another look at that curve. When athletes get injured during plyometric training, it is because they are trying to generate too much force, rather than focusing on generating less force quickly. Often it’s a competitive thing – you put 3 drop jump boxes of various heights in front of an athlete and what happens – they want to go straight off the highest box. And some coaches also think that the higher the box the better the gains – but for this kind of training that’s not the case.
The key is to stay within the “safe zone” for your athletes, where they are focusing on producing the right combination of force and speed of movement, instead of just force.
Take a simple example. Stand on one leg and hop 5 times – just with your body weight. Focus on getting off the ground as quickly as you can (minimal contact time). This is very safe – even a 2 year old can do it. But what happens if you hold a 20kg plate and try to do it? You contact time will go up, you will need to produce more force to get off the ground, and your injury risk goes up. Also, you will most likely move out of the “plyometric zone” and not be even using your stretch reflex any more.
Plyometric jump training is a fantastic way to train power and speed, and “convert” strength from the gym into usable power out on the playing field. Whilst many people are fearful of using plyometrics from an injury risk perspective, if plyos are performed safely the risk is actually very low, and these exercises can even be used with kids and athletes who are in rehabilitation from injury. The key is quality control – making sure that the contractions are short and power focused, and not moving out of the “safe zone” for explosive movements, which research suggests is between 100 and 250 milliseconds.
The next challenge is to put this theory into practice. In the next article in this series, we will explain how to implement plyometric training safely and effectively using simple and objective monitoring tools.
Schmidtbleicher D (1992) Training for power events. In: Komi PV (eds) Strength and power in sport. Blackwell, London, pp 381–395
Fleck, S.J. and Kraemer, W.J. (1997) Designing Resistance Training Programs. 1st Edition. Human Kinetics, Champaign, Ill.