The next time I post what I've been doing, this workout will be in there, so I thought an explanation was in order.
The first time I encountered this workout was in examining the training of some world-class 800m runners. I thought it was a typo. Years later, whenever I see it online, there's inevitably a comment from someone saying that "This must be a mistake. It doesn't sound reasonable." Here's a typical Greyhound workout:
10x100m with 5 meter recoveries
See? You immediately wonder if I meant 50 meters or maybe 5 minutes. 5 meters looks wrong.
Think of a dog (a greyhound) running along the fence of an enclosed yard. It runs as fast as it can, suddenly encounters a fence and has to slam on the brakes. Then it turns around and runs the other direction as fast as it can. It does this until it's exhausted or gets distracted. That's what the workout is like.
It's usually done on a track where the 100m race is marked. One runs a sprint in lane 1, turns into lane 2 and sprints back, turns into lane 3 and so on. The beauty of the workout is in the stopping and turning. Having to rapidly decelerate stresses the same muscles you use in sprinting, but in a completely different way, switching from concentric loading to eccentric loading. Runners tend to slow gradually in training; about the only time one runs like this is in a cross-country race where there's a hairpin turn at the bottom of a hill [I have a great story that involves doing that; if we ever meet, ask me about it]. The pivots at the end of each sprint require some agility and work the smaller "balance" muscles, which are also under-used by most runners. It is a completely different workout than running 100m hard/ 100m easy.
Energetically, short sprints use the creatine phosphate/ ATP shuttle. You can regenerate creatine phosphate in less than 5 minutes (usually) and your normal muscle stores are enough for about 45 seconds. Thus the first 2-4 sprints in this workout are mostly depleting the creatine phosphate stores; after this, one goes into lactic acid training - the sprints become more difficult to do and are generally slower as one fatigues. Eventually, after 10-12 "greyhounds," one is slowing enough that the ability to run anaerobically is ending and one should stop before turning it into something else.
It's a tough workout. Invariably, after 5 sprints I find myself thinking that there's no way I can do more than 6. I still end up doing 8-10. It's a workout you don't want to do often enough to get good at it (!) - doing it frequently tends to teach one the "bang" start used by some sprinters (which is good if you're a football player, not for most runners) and then one slows over a much longer distance more gradually; it's the short stop that's important.
Ultra Loony update
2 days ago