Nevertheless, I do have one rule that I follow assiduously: Start all workouts gently.
Whether it be a run, a bike, a weightlifting session, Crossfit conditioning workout, or whatever...it is vital to start easily and slowly build the intensity. This is non-negotiable.
Everyone has heard that you need to warm up before exercising. The problem is that many don't actually do it and many don't quite know what a warm up should be.
Rather than get confused about what warm-up to do...simply adopt the habit of beginning workouts slowly and easily. If it's a run...start with a jog....much slower than the pace you will eventually be settling into. If you're a beginning runner, start your runs with a walk. If you're a biker start with easy spinning. If your lifting weights, do a bit of aerobic work like a brisk walk or easy jog...and then start your lifting with very little weight on the bar. If you are doing a Crossfit workout, do some easy aerobic work to get your heart rate up AND then do the exact movements you'll be doing in the workout, except with less weight and fewer reps.
An effective warm-up for any aerobic sport is usually accomplished by 10-20 minutes of easy, continuous exercise where your effort/heart rate is gradually increased. Stretching does not constitute a warm-up. It begins with a very easy pace and gradually increases to the point where, at the end of the warm-up period, you are at your basic training pace.
Besides preventing injury and keeping you safe, starting gently makes your workout many times more enjoyable. For runners...it makes it possible to get in the “zone” and achieve a “runner’s high.” Without a proper warm-up you actually send your body into a mild state of shock. Eliminating or rushing a warm-up can result in abnormal heart function and poor blood pressure response...among other things.
You'll know if you warmed up properly by how you feel after the warm-up. If you do the warm-up correctly, you will be raring to go at the end of the warm-up period. You should feel noticeably better than when you started. You'll notice an increase in body temperature, improved range of motion and increased energy. Picking up the intensity will feel good...natural. You won't have to force
The Kenyan runners live by the start easy principle. Their workouts begin at a pace barely faster than a walk and progress gradually from there. Even though they will sometimes finish workouts at paces faster then 5mins per mile, they'll start out at 9-10 mins per mile. No lie. Anyone who has ever stood at the start of the Falmouth Road Race and observed the top Kenyan runners before the race, knows this to be true. I figure if it's good enough for them, it's good enough for me.
So start slow and let the workout come to you. Try it. You'll like it. It's how the pros do it.
PS Here's a good article by running coach Tom "Tinman" Schwartz on warming up and cooling down: http://220.127.116.11/articles/wac.pdf