How to Assess Lower Limb Power Output with Dr. Paul Comfort
Dr. Paul Comfort is a Reader in Strength and Conditioning at the University Salford in the United Kingdom. He recently co-edited Performance Assessment in Strength and Conditioning, available for sale here. Dr. Comfort joined us on the CoachMePlus Sports Science Minute to discuss vertical jump testing for lower-limb power output.
I think a lot of our coaches want to know what is the most effective way to assess athletes power development lower limb power development?
Power development all depends on what you want to get out of it. If you’re assessing power out of different weight-lifting exercises, the best option is to use a force platform. A force platform will allow you to use the forward dynamics approach to integrate the force-time data. From there, you can calculate acceleration and velocity and therefore you can calculate power output. Most people use a countermovement jump to assess power output.
But power is actually a very poor measurement to use for most for most jump tests. Now that might sound a bit strange because its reported in the majority of studies and I’ve no doubt reported it in a number of my own studies in the past. You can achieve the same power output from completely different jump strategies. So it is possible to achieve the same power output with a really low force, developed over a longer duration or to develop the same power output with a really short movement time but a really high force. So just by adopting a completely different jumping strategy for something like a countermovement jump, you can end up with exactly the same power output. But more importantly, you can end up with a completely different jump height and for most athletes, jump height is the more important metric. Jump height is what determines whether they have a better or worse performance than their competitors.
So what you’re saying is that power is not the best assessment metric because it can be misleading?
Yes, definitely. Because you can use different strategies during your jump performance and get the same power output. You can get the same power output from different jumping strategies. What we really need to do is identify the strategy that an athlete is using to achieve that power. If power is constant, you would assume that there’s no neuromuscular fatigue, there’s no adaptation. But if the athlete is now going through a shorter movement time and still generating the same power output, we’ve got an improvement in their performance. The same principle applies if their jump height is constant: if someone has a 50 cm vertical jump but they’ve achieved that over half the movement time, you’ve actually got a positive adaptation. The gross metric of jump height is now the same even though the overall output.
So, if I’m a Strength Coach with limited resources, what’s the easiest way to assess vertical jump improvement?
Jump height has the same issues as assessing power output as I said, you can have the same jump height but a shorter or longer movement time. Shorter movement time being a positive adaptation, a longer movement time being negative or an indicator of fatigue if it’s taking you a longer duration to jump the same height. And that would be the same if you’re just using a gross measurement like power.
What you really need, and most people might not have this available, you need a force platform of some kind. There are a lot of force platforms out on the market now which are more affordable. Prices range from $1,200-1,500 for some relatively cheap force platforms up to $38,000 which is ridiculously expensive. However, if you look at some of the manufacturers on the market like Hawkins Dynamics, their fully automated systems are cost-effective and they allow you to look at movement time and jump height together. How long did it take them to create that jump height? What you’re looking at is the reactive strength index modified or flight time: contraction time ratios which are built into a lot of those commercially available force platform systems in their software. If you don’t have a force plate, if you’ve only got a jump mat or some optoelectronic system to assess jump height, the problem is you’re only to get jump height from flight time. It won’t give you the movement time because you’re in contact with the jump mat the entire time.
And if you can’t afford a jump platform?
The other option then so you can still get contact time and flight time to give you the reactive strength index from a drop jump is just doing a drop jump with some form of an optoelectronic system or jump mat. But what you have to be aware of is if you set a 12-inch box and you ask your athlete to drop off that, they very rarely drop off the full 12-inches. They’ll step down part of the way or they’ll jump off it. Unfortunately, you’re not always comparing like with like but if you’ve only got a jump mat system which gives you contact time and flight time, that’s probably the next best option. But you have to be cautious in interpreting that data, observing the athlete, and making sure that they really are dropping from that height and not stepping down from the box.