One of the more common questions heard around the cycling traps is - "do I ride when I'm sick?" This has been particularly relevant to me this week - I had a cracker of a head cold last weekend, and I was hesitant to get back on the bike too quickly.
First of all, let's keep in mind where I'm coming from. I am - literally and metaphorically - a C Grade Cyclist. So my advice isn't aimed at someone in the ProTour, or even an A-grader who has a coach that guides them to one or two peak races a season. My perspective about sickness and training is all about your average weekend warrior, who also has a day job and a family to come home to at night.
So - my general rule of thumb is "leave the bike in the garage". There are a few good reasons for this:
1. You can extend the length of your sickness. Training obviously puts your body under stress. A sick body is already under stress, and the training simply knocks out some further defences against the illness. By training on, a week-long head cold can turn into a month-long general illness and fatigue.
2. Your training will be sub-optimal. Sure, you can knock out the mileage and intervals (or whatever) that you or your coach have programmed for yourself. But you will not be able to hit your usual performance marks. Your body will be speed & effort limited by the sickness, meaning your legs will not be able to work as hard.
So these two reasons alone should be enough to put you off training - you won't be able to train properly anyway, plus you'll just extend your sickness time. Instead of being 1-week behind in your preparation, you'll end up more like a month (or more!) behind.
3. You could permanently damage yourself. OK - this is more serious, and won't happen to everybody. But if you push yourself hard while sick, you can really hurt your insides. Your heart - already under pressure, beating at a higher rate to cope with the sickness - will be asked to work even harder while you train. Your lungs too, which are probably gunked up, will be struggling to cope with the respiration workload. If you are unlucky, permanent damage to these organs can result. And that's a lot worse than missing a couple of weeks of training...
Have I scared you yet? That's not the idea, of course. But the key is to change your mindset. Often people think that they are 'tough' for pushing through with their training while sick, and it puts them ahead of the pack. Ironically, the opposite occurs - it often puts them in much worse shape than the sensible cyclist who takes the week off and rests up.
So when do you get back on the bike? Do you have to wait until you are 100% better and sickness free?
Personally, I'll get back on the bike when I'm almost better. I find if I take it very easy - just cruisy, low heart-rate effort, and leaving the bike in the small chainring - it actually assists with the final recovery from the illness. For me, I think it "gets everything moving again". But I must stress the super-cruisy effort though - its not 'training', just rolling the legs over.
In summary - if you are sick, rest!! Pushing yourself back too early will have the opposite effect you intend. Resting up and getting better will have you riding more strongly in the long run.