Sometimes you come across information on the web that is…questionable? I don’t want to throw any particular person under the bus so I won’t provide the links to these quotes. However, I’ve been coming across some sites lately with some interesting quotes on ground contact time (GCT – the amount of time your foot spends on the ground during one step) such as:
GCT and cadence:
“Increasing your cadence reduces the amount of ground contact time”
“A higher turnover rate reduces the amount of time the foot spends on the ground, also known as ground contact time”
GCT and injury:
“Injuries occur only when there is ground contact. So if you reduce the ground contact time, you reduce the risk of injury.”
“The shorter your ground contact time, the less chance for injury”
So, are these statements tue?
Very briefly, GCT is measured by the amount of time one foot spends on the ground during one gait cycle. In general, GCT decrease as speed increases also, heel strikers generally have longer GCT and GCT usually increase as fatigue sets in. Is it true that cadence has a strong relationship to GCT? Yes. And is it true that faster runners generally have short GCT’s? Yes. However, if the goal is to reduce GCT for everyone, that seems like a silly idea to begin with. Reducing GCT comes at an increased metabolic cost according to many studies. This is great if you’re a sprinter, but a long distance runner? Maybe, but it depends on a few things.
But I digress…
Part 1: Increase your cadence to reduce your GCT
With respect to the first idea – that a faster cadence always reduces the ground contact time, that just simply is not true. Below, I submit a video of two different patients running approximately the same speed. I measured both of their cadences and they are both just over 180. However, I also measured the GCT, and this is what happened: (keep in mind, I wasn’t using high tech accelerometers/force plate etc. so it’s not incredible accurate, but it’s obvious in the video where the differences are)
Before we go any further, we all need to pause for a moment and reflect on how cool the intro to that video was!
OK, moving on… there are many examples of what we saw in the video, but the point remains – a faster cadence doesn’t always mean less ground contact time for everyone. Not all runners have the same GCT at the same speed. Not all runners have the same GCT at the same cadence. As cadence increases not all runners decrease the GCT at the same rate.
If you don’t understand that, read the part below on the blue writing.
One step length is measured as the distance between one foot fall and the opposite foot fall.
One stride length is measured as the distance between foot falls of the same foot (2 consecutive step lengths equals one stride length)
Cadence is the number of steps per minute
One step length includes the ground contact time of one foot PLUS the flight time until the opposite foot touches the ground. This means that I could potentially increase my cadence simply by reducing the flight time. By reducing the flight time, the step length would be shorter and cadence would be increased (assuming the same speed). However, I never changed my ground contact time.
There are lots of speed walkers who have high cadences and no flight phase of gait, therefore very long GCT. In fact, here is a video of a champion speed walker. His cadence is 206, and guess what? It’s all ground contact! So let’s not say that increasing your cadence always decreases GCT.
Part 2: Reducing the GCT reduces your chance of injury
Back to the two runners in the video and the questionable claims made on some other websites – the second idea that reduced GCT will reduce the chance of injury. This assumes that the bottom runner in the video will have less chance of being injured. Hmmmm…..which of the GCT’s in the video above is better for reducing injury?
Unfortunately, I have been unable to find any literature demonstrating that shorter GCT is associated with less injury. I did a few different PubMed searches and was unable to come up with anything. However, one article (thanks Blaise!) examined 130 female and 70 male runners and found that amongst the male runners, a shorter GCT was associated with an increased risk of injury.
A causal relation? No, but an association at least. They attributed the shorter GCT to increased leg stiffness (“leg stiffness” in this case refers to the stiffness of the leg spring, not the flexibility of the leg). The authors go on to say ” To the best of our knowledge no research has been done to examine the effect of leg stiffness on the incidence of (running related) injuries” If anyone knows differently, I’d love to hear from you.
Since one study is just that – one study, I won’t take it as the end of the argument, but it was a pretty well designed study.
So with respect to GCT and injury, one could argue either way: a shorter ground contact time could mean that you are landing closer to the center of mass and then lifting the trail foot quicker. This should result in less sagittal plane acceleration/deceleration and thus less risk of injury. However, a shorter GCT also means that you must be generating the same amount of force over a shorter time period which would mean less force dissipation and a stiffer leg. This would increase the rates of force and thus, increase the rate of injury.
So in the end, it’s irresponsible to say that a faster cadence always equals less ground contact time and it’s also irresponsible to say that shorter GCT reduces injury. We do know that faster runners generally have shorter GCT’s than average mom’s and pop’s. Does that mean the average Joe should emulate the short GCT?
Again, if you have any thoughts on this, I’d love to hear from you.