Why is Ground Contact Time Important in Plyometrics
Well everyone, it is that time again! The Olympic s have come, and we suddenly have found ourselves fans of sports we hardly knew existed (or at least did not care very much about until now).
The Olympic Games however are certainly a great tradition and it is always a good experience to watch people at the top levels in any competition, whether that be swimming, weightlifting, handball or table tennis. What I would like to focus on for this particular article, however is two sports which we are all quite familiar with: Track and Field, and Basketball.
Introduction to Ground Contact TimeI am under the assumption that the average reader for this type of article is interested, primarily in increasing their vertical leap, and it would be a fair remark to say that, yes, vertical leap is important in basketball and also track and field (mostly in the jumps and throws).
For this analysis in particular we are going to focus on the difference between jumping in basketball players, and track and field athletes. In doing so, you may realize what you might be able to do in order to help yourself become better in your given jumping sport of choice, even if it is not basketball or track and field.
Before I really get into the research though, I would like to discuss, quickly, the importance of ground contact time in sport. It is one thing to be strong, but it is completely different to be both strong and fast. Most of us are aware of this concept, although perhaps some examples might help you really determine why this is important.
Figure 1: King James shows what
Standing Vertical Jump: .5 seconds
Depth Jump off of 24" Box: .2-.4 seconds
High Jump Takeoff: .14-.2 seconds
Long Jump Takeoff: .12 seconds
Sprinting: less than .10 seconds
As you can see, there is a trend that, the faster the movement, the lower the ground contact time must be. Research has shown that generally speaking, athletes with a greater percentage of fast twitch muscle fibers will do better in events which require the lower ground contact times.
Athletes in sports which do not require low ground contact times, such as volleyball, can get away with not being so blessed in the fast twitch realm, and studies have shown that international level volleyball players with running vertical leaps well into the 40" realm, are only around 50-55% fast twitch muscle.
Without getting too far into the details of fast and slow twitch cross-bridging action, you can still get up quite well even if you might not think you are blessed in the "fast twitch" realm. The only difference in this case, is that you are genetically geared to spending more time on the ground to do so.
Perhaps an enlightening article in this area is that of research done by Stafanyshyn and Nigg et al (1998). In their study, they took 4 basketball players and 4 elite long jumpers (research has shown that track and field jumpers have very high fast-fiber ratios) and had them perform single leg jumps, the basketball players performing for height on a basketball hoop, and the long jumpers performing pop-ups into a pit.
In this study, the long jumpers spent about half the time on the ground that the basketball players did. The long jumpers spent between .14 and .17 seconds on the ground, while the basketball players spent up to .35 seconds for their jumps, which is double the time that the long jumpers spent. (Remember these are both off of one leg).
I have also confirmed the difference in ground contact time in work I have been doing in the biomechanics lab of the university which I now study. My research project on depth jumps involved athletes from a variety of sports, but mostly track and basketball. Although not the focus of my study, I have noticed that the track and field jumpers get off the ground significantly more quickly than the basketball players in a two legged depth jump.
I would say the average ground time for a track jumper is about 70% of the time that a basketball player took to get off the ground. The interesting thing was though, that the highest jumper in my study was a basketball player. This player also took the second longest out of everyone on the ground, or in other words had by far the longest ground contact time (This player was 6'3.5" and jumped over 2 guys to dunk in a slam-dunk contest 4 days later).
Like I said before with the volleyball example, you can certainly "get up" spending a long time on the ground.
Why Stress Ground Contact Time?So what is ground contact time important for then? Well two things really. The first is quickness in team sports. The ability to get up "quickly" is just as, or perhaps even more important than the ability to get up very high. Scrambles for the ball in jumping sports are random and quickness off the ground is extremely important!
Players with short ground contact times have the advantage when it comes to getting up in the air quick for a rebound, block, or whatever the case might be. Secondly, a low ground contact time will allow athletes to utilize the elastic element of their musculature more efficiently, which is paramount in activities which draw on short ground contact times. Have you ever watched a top Olympic long jumper "muscle" a takeoff? Probably not, they make it look smooth and efficient.
Something that I tend to enjoy watching in track meets is the throwers doing their warm-ups, in fact I think I should video it sometime, because it is just such a stark contrast to the jumpers (at least the good ones). The biggest differences I notice are in the skipping drills they do, as the arm motions are overemphasized and the times they spend on the ground are just massive.
Of course they jump well off two legs, and particularly from a standing start, but don't even ask them to jump off of one and do a good job at it (in most cases). An athletic coach who I respect greatly, Vern Gambetta, has categorized the two types of strength which highlight these differences between the jumpers and throwers. These two types of strength are called "explosive strength" and "elastic strength".
Training explosive strength is what we are probably most familiar with, the Olympic lifts, jumps up to boxes, weighted jumps, and the like. Elastic strength is that which draws into the realm of low ground contact time. These exercises include bounding, hurdle hops, sprinting, and anything else where the athlete spends less than two-tenths (roughly) of a second on the ground.
Figure 2a and 2b: An Olympic weight lifter displays great 'Explosive Strength', whilst the long jumper demonstrates superior 'Elastic Strength'
Research Says?Research has demonstrated this need for specificity in the ground contact times of training. An Australian sport scientist, Young, completed a research study in 1999 which showed the need for specificity in ground contact time in jump training (in this case it was depth jumping).
In his study, he had athletes jumping from their own individualized box-height in a two-leg depth jump for height. One group simply jumped as high as they possibly could without regard for time spent on the ground around 400ms), while the other time tried to jump as high as they could while keeping the ground contact time to the lowest level possible (around 200ms).
The outcome of the study showed that the group which did not attempt to minimize their ground contact time failed to improvein any type of variable, including standing vertical jump, while the minimal ground contact time did manage to improvetheir single leg jumping ability by about 2 inches (4cm), and also improvetheir reactive ability as measured by a depth jumping test.
So in short, even if you don't care much about one leg jumping, it still can pay to keep the ground contact times in your plyometric activities short.
Practicality!So what is the practical application of all this? In short, if you want to be a springy and powerful athlete, particularly off of one foot you really have to work hard on utilizing your elastic elements in your body's movement tissues.
This can be done by, as stated above, working hard to keep ground contact time low in plyometrics, as well as choosing plyometric activities which tend to keep ground contact time lower.
When you do depth jumps (or any other "free" 2- leg plyometric) for height and don't think about the ground contact time at all, you can end up spending a lot more time on the ground than what is good for you.
My advice is that If you feel that your single leg jumping ability needs improvement, then spend more time doing hurdle hops and bounding (both are low contact time), or if you are lucky enough to have access to some sort of force plate or "just jump" mat in your plyometrics, use that and keep track of the time you spend on the ground.
A good ground contact time is .25 seconds or less, although depending on how you are built, this time can vary. I can also tell you that in depth jumping, a depth jump over a hurdle has a significantly lower ground contact time than if you are jumping up for a target, so if you don't have some sort of ground contact measuring device, or even if you do, this can be a valuable practice.
The second important thing to remember is to maintain the elastic elements in your body. Although I don't have a research study to support this, I can tell you that top level track coaches will often have their athletes do a long series of low amplitude jumps that are focused on low ground contact time, in order to maintain their bodies reliance on the tendons to stay heavily involved in the movement, and not just the muscles.
An example of this type of work could be anything from 10x30 meters of single leg jumps covering about 2 feet per jump while maintaining good posture and a stiff knee or simply, jumping rope on one and two legs. of course this shouldn't make up the majority of the training program, but it can be implemented in a training program a one or two times a week if elastic needs are not already being met.
A case where you probably wouldn't need to use this would be if you were playing some sort of team sport a few times a week, in which case you would already be applying plenty of low-ground contact times within that game.
ConclusionSo there is it, a look at ground contact time in plyometrics without trying to overcomplicate things too much. Remember though, that relative strength is key in any jumping program, so make sure you don't just go for low contact time plyometrics. If you are looking to get quicker and more powerful off of the ground, you will need to get stronger as well.
Also, and this is just as important as any plyometrics and strength work, don't neglect practice of your primary sport movement. If you are a basketball player and want to jump better off of one foot, make sure you spend some time practicing your layups and dunks off of one foot! Remember these points and you have just unlocked least one "secret" to having a both big jump off of one leg, and improved quickness off of the ground, best of luck!
ReferencesStefanyshyn, D. & Nigg, B. (1998) Contribution of the lower extremity joints to mechanical energy in running vertical jumps and running long jumps. Journal of Sport Sciences, 16, 177-186.
Young, W. (1994) specificity of jumping ability and implications for training and testing athletes. Proceedings of the National Coaching Conference, Canberra Australian Coaching Council. 217-221.
Young, WB., Wilson, G., & Byrne, C. (1999) A comparison of drop jump training methods: Effects on leg extensor strength qualities and jumping performance. International Journal of Sports Medicine, 20(5) 285-303
Zatsiorski, V. & Kraemer, W. (2006) Science and Practice of Strength Training, 2nd Edition, p.27, Human Kinetics Publishing, Champaign IL.
About the AuthorJoel Smith is a CSCS and USTAF level 1 certified coach at the University of Wisconsin-LaCrosse, USA. He specializes in jumping events and is currently completing his masters degree. His thesis is focusing on depth jumping variants. You can read more of his work at jumpcoach.blogspot.com or enjoy some of his fantastic videos at www.youtube.com/jumpscoach
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