There is a general consensus in the sports medicine field that cross-country skiing is the ultimate aerobic sport, with Nordic skiers attaining the highest max VO2 levels of all endurance athletes. Looking at this conclusion from a different angle, perhaps I can convince some of you who don't ski of the benefits of this sport.

Several years ago, I noticed that I was constantly plagued by nagging injuries. It seemed as if I was constantly in rehab. So I started thinking about how I might help other aging skiers avoid the pitfalls that had tripped me up.

But then the roof fell in. It seems I didn't realize how severe constant rehab could be. In the past year and a half, I've had five major surgeries and one bout of staph infection. My life has became a constant cycle of hitting rock bottom after surgery, then slowly building back up.

At first I was completely surprised at how fast my body could deteriorate. My muscles seemed to shrink before my eyes. After one surgery, I found that I had literally forgotten how to walk. My muscles seized up after 100 yards first time out and I had to limp back home.

 Now I'm not looking for sympathy, and in some ways consider myself very lucky. After all, I'm still here to write this column. My purpose in divulging a small bit of my recent medical history is to lay the groundwork for the discoveries I made in rehab over the past 18 months.

For me the most visible consequence of prolonged inactivity due to my medical layoffs was muscle shrinkage. But the more significant shrinkage was in my red blood cell count. A normal hematocrit range for a male is 41 to 53 percent. (Hematocrit refers to the percentage of one's red blood cells.) Mine, from blood loss during surgery, was 25 after each of the medical procedures. I was able to get it back up to about 34, which was still low, between surgeries. But when each subsequent medical emergency struck, it was back to ground zero once again.

So, as you can imagine, starting aerobic activity again was no picnic. At first it was just short walks, and I do mean short. During this stage, any moderate hill brought me to a standstill.

Eventually my red blood cell count rose to a level that allowed easy biking and, finally, roller skiing. That is when I made some discoveries that wouldn't have been possible had I been at normal physical capability.

Easy biking did the trick
Biking was the best way for me to build up aerobic capacity and leg strength after being knocked down to ground zero. Why? First, the easier gearing on my mountain bike. Second, there are plenty of places with flat terrain, such as the rail trails, that allowed me to bike easily. Even on gradual terrain, the granny gear made biking up most hills fairly easy. I also had an indoor exercise bike as an option.

Roller skiing and skiing on snow are far more taxing activities on the aerobic and muscular systems than biking, even when biking hilly terrain. Yet I found I could return to form fairly quickly on a bike.

I gradually increase the gearing on a hilly loop I have used it for years after the snow melts and before the mountain bike trails open. My goal each spring is to bike that series of hills in 2:7, meaning the middle gear on the front and the seventh one down in the back. During the summer I'd move into even harder gears on the occasional neighborhood workout. But during my period of constant rehab, I was able to get up to a 2:7 gear on the hills, even with a depressed hematocrit level.

But roller skiing? That's a different matter. While I was able to tear around on the bike for 90 minutes, my first roller ski workouts were limited to 10 to 15 minutes and then only on moderately rolling terrain.

Skiing proves more demanding
Why the huge difference between biking and skiing? Exercise physiologists claim that the reason aerobic capacity is so high among cross-country skiers is that, while skiing, the heart must pump the blood to both the lower and upper body. As I learned in my weakened state, the threshold jump from biking (using my lower body only) to skiing (upper and lower body) was just too much for my depressed aerobic system.

 In fact, last winter I was never able to ski up longer hills without stopping. Roller skiing for more than 20 minutes continuously was literally impossible. Yet before the snow hit I was able to easily hammer my hilly bike loops with no duress.

While an elite skier or a fit citizen racer might not experience that big of a difference between the demands of biking and skiing, in my weakened state it was obvious that cross-country skiing was the much more intense aerobic sport. So for anyone desiring the ultimate exercise for aerobic fitness, skiing is the ticket.

Striding vs. skating
Surprisingly, I also found a fairly large difference in how the lower body muscle groups are taxed whether striding or skating on skis.

Back when I was in top shape, I literally noticed no muscular differences between the two disciplines, other than the initial soreness (where the thigh meets the torso and in the calves) for the first few striding days. But at ground zero physical condition, it was a very different story.

Because my muscles were so weak, I noticed immediately that different parts of the thighs complained and were sore longer as I alternated days between striding and skating. But most significantly, the glutes had a much higher involvement while skating.

One of my surgeries required a stent in the lower abdomen to repair an aneurysm. Unfortunately, that repair forced the surgeons to tie off an artery to my right glute area. And did I notice the effect. Claudication happens when the blood supply to a muscle is insufficient and the muscle cramps. The surgeon told me that eventually new vessels would form and I'd be back to "normal."

The first time out on roller skis after the aneurysm repair, my right cheek seized up in less than 200 yards. And that was from open field skating on a flat pavement. I was forced to stop and start for the entire 10 minute workout. Striding the next day was a different story. I was immediately able to roller ski about 10 minutes before my cheek muscles froze up on a gradual hill.

Why the difference between skating and striding? First, it was obvious that different muscle groups were in play. I now understand why ice skaters have such pronounced gluteus maximus development. Second, much of classical skiing involves a pure double pole in fast conditions on the flats. When skating, the legs are usually in constant motion with no rest. Naturally, with more stress on the lower body, claudication occurs sooner.

This leads to a tip for the average skier. If you are tired while skating on flat stretches, or your legs are shot from the last climb, double pole parts of these faster sections to allow your legs to recover for the next hills. While the elite don't need the rest, I've never been in good enough racing shape not to have to rely on double pole relief during competition or even hard workouts.

Striding at low intensity is easier
Because the upper body acts alone while double poling, it is one of the reasons that striding can be done at an "easier" aerobic level than skating. The other reason is that hills can be negotiated at a slower rate. You literally can walk up a hill while striding, given the proper kick wax. But skating, with glide, is a different story. There is a threshold level of muscular involvement and even at the slowest pace. For a fatigued skier, the thigh and butt muscles will tend to seize up. Boy, did I find that out this past winter.

Another thing I noticed in my weakened state is that when gliding during skating, my leg angles were fairly constant. This is another reason skating is more stressful on the lower body. This leads to an isometric hold position which I found to be surprisingly stressful. When striding, the glide leg is in constant motion. The foot/leg pushes forward for more glide then gathers for the kick. This little motion is just enough to avoid the tiring isometric hold of skating.

Conclusions
1) Biking is an ideal way to start rehab from ground zero. In fact, biking provides a way to gradually increase the difficulty of your workouts by shifting to harder gears on the hills. Plus biking provides an excellent recovery day from a hard bout of skiing, especially skating.

2) Since skiing is much more taxing on the aerobic system, it pays to roller ski to get in shape before winter so you can truly enjoy the early snow days.

3) If you are serious about the sport, striding and skating are different, so you ought to practice both. Alternating between the two disciplines is a good way to avoid overtraining as the muscle groups involved are slightly different.

4) Don't be afraid to stop and start when you're out of shape. It is far more productive to stop and recover so you can continue to ski with decent form. To stagger around with lousy technique is not helpful.

If you take up skiing, I hope you find it to be fun. But if you only start for the fitness benefits, you may find that flying around on skinny skis under your own power is surprisingly enjoyable. There are muscular benefits from this aerobic strength sport, so don't be surprised if you begin to see bigger muscles and less flab.

Lee Borowski is a past USSA Nordic Coach of the Year, Badger State Winter Games Athlete of the Year and the coach for several junior, senior and collegiate skiers of the year. He has also coached many master skiers who have won both national and world championships. Borowski is the author of several books and articles, and producer of four videos on cross-country skiing technique. He runs the website thesimplesecrets.com. To order Borowski's "NEW Simple Secrets of Skating" or "The Simple Secrets of Striding," demonstrated through footage of Olympic and world champions, and available on VHS and DVD, send $25 plus $1.75 shipping to Lee Borowski, 4500 Cherokee Drive, Brookfield, WI 53045. Wisconsin residents add $1.27 tax.