So, a few weeks back I had my silly shin tweak. I refuse to call it an injury, because in my mind, an injury is something that FORCES you to take time off. A tweak or a niggle is something where you don't NEED to take time off, but choose to do so, so as to avoid injury.
[yes, this distinction is being overly anal, but I do it for a reason. My big goal for this season is to avoid injury, and I don't want to put myself in a position where I don't back off because I don't want to admit something's wrong. If I call it a tweak when it starts, instead of denying it I respond immediately and prevent it from evolving into an injury. See how that works? You cut back by choice, before you have to.]
So, as I was saying, I tweaked my shin. And I cut back on my running and skipped my hard workouts for a month. And when I came back, I had really lost very little fitness. Pool-running was part of this, but I also give a great deal of credit to my use of the arc-trainer.
So what's an arc-trainer? Well, it looks like a variant of the elliptical, but with even less dignity. Here's a picture. It's a bit intimidating at first, but pretty easy to figure out how to get on (step onto the steps; grab the handles). Then once you get on, you program in time, incline, and resistance, and go on your way (well, so to speak).
So, it's like an elliptical, but different. I really don't know how to explain the difference, except to say that the arc-trainer at a low incline (I set on 3) has a motion that is much more like running than the elliptical. Same path traveled by the feet, same stride length, and same exact action of glutes versus quads. Below is a promotional marketing video from Cybex that hopefully sets this forth a bit better.
Particularly interesting is the point made about the excessive stress on the knee from the elliptical (versus the arc-trainer). From the video, it looks like the elliptical mimics the stride of an inefficient overstriding runner, while the arc-trainer's stride is much closer to correct running form -- thus the greater risk of hurting something while on the elliptical (this is likely why avid runners also cannot STAND the ellliptical). In my experience, the only real difference between the arc-trainer and the treadmill is that the former is non-impact, and so your calves and shins aren't doing the same work they would be doing while running -- fantastic if you're battling shin or calf issues.
To my mind, this makes the arc-trainer a better substitute for running than pool-running for both easy runs and tempos (more later on why intervals don't work as well). Pool-running tends to have a slightly different leg motion than land running, and also works the hip flexors far more than the glutes, leading to some imbalances when you return to land. Additionally, due to the effects of being in the water, pool-running heart-rates tend to be 15-20 beats lower than land, while the heart-rates one can achieve on the arc-trainer are nearly exact to those you hit while running. In fact, I simply equated arc-trainer miles to running miles by assessing how long I "ran" at a certain HR. 145-150 BPM for me is about 8:15 pace, so 32-33 minutes at that pace was ~ "4 miles"
As long as you're cleared for weight bearing, you can mimic close to the exact running motion by a) setting the incline to low and b) ignoring any handles that may exist and instead stabilizing your torso and swinging your arms as you would while running. It feels darn close to running (with the added benefit that you get to watch TV).
During my "tweak time", I used the arc-trainer to sub for easy runs, tempos, and the second part of long runs -- I just used perceived effort as my guide for how hard to go (upping the resistance to match), and a tempo on the arc-trainer felt nearly EXACTLY like a tempo on the track (and with nearly the same heart rate, to boot). For long runs, which my coach generally has us do as progressive effort workouts, I would do the first part of the long run by running easy outside for as far as I felt was safe. Then I'd drive immediately to the gym, hop on one of these, and after a quick warm-up I'd start pushing the effort to mimic the harder work of the final part of the long run.
By following this plan, I was able to keep up with my coach's scheduled workouts almost exactly. The one workout I DIDN'T use the arc-trainer for was intervals. For those, I relied on pool-running, simply because I found that the arc-trainer became unstable at the effort level I associate with hard intervals. Not enough that I thought it was going to flip over, but the machine would definitely start to move across the floor, creating a bit of a hazard.
So yeah, 4 weeks of substituting the arc-trainer for easy running and tempo effort, and pool-running for intervals, and I came back fine. Happy ending. But... there are two caveats about using the arc-trainer.
1) They can be hard to get access to. They're expensive and take up a good amount of floor space, so most of the gyms that have them just have a few. And, they are extremely popular for the silliest of reasons -- basically the calorie count on the machines is ridiculously inflated (supposedly I burn 1200 calories an hour on one of them) and so people desperately trying to match calories in versus calories out flock to them, because they "burn the most calories for the least effort...." (I leave it to the reader to assess the logic of that statement).
2) Because the arc-trainer is SO good at preserving every aspect of fitness except lower leg strength, there's a real risk that one gets reinjured by coming back to running too quickly. Your cardiovascular fitness is there, as is the quad and glute strength. But, if you've spent more than a week or so on one of these things, you've doubtless lost a lot of strength in your lower legs, and you need to carefully and gradually transition from the arc-trainer back to full time running to avoid a new injury (*ahem* tweak).
[Final disclaimer - I own no stock in Cybex, and have never had ANY communication with them regarding this or any other product. I just like the machine a lot]