If the 400 metres is the so-called man-killer, the middle distanceevents of 800m and 1500m present the most complex set of demands on runners.

They need to have the pure speed to contend with fast-paced races; the savvy to run at a pace that will keep them in the race without burning out before the end; the stamina to have something left in the tank for a finishing kick; and the tenacity and mental strength to contend with other racers' tactics and to push aching muscles past the point where common sense tells them to quit.

According to Canadian 1500m specialist Leah Pells, "You must be ready to react, to change pace, to stay on your feet when it gets rough. The last lap can really hurt physically, and basically that's what we train for, to be able to sprint when you are already tired. To run a good 1500m, an athlete must be very aggressive and really want to fight hard for it."

Relaxed, efficient form is extremely important in middle distance events, and not just to enhance stride rate and length of stride. Fatigue can make runners get sloppy, which will slow them down, but good form is self-perpetuating -- it will mean they don't have to work as hard to keep the pace, so they'll conserve energy for a strong finishing kick.

Watching the world's premier 1,500m runner, Hicham El Guerrouj of Morocco, you can see what smooth, contained form and economy of movement can do. He doesn't run so much as glide effortlessly, at least on the surface, through his races, and when he shifts into his kick, there are few signs of strain on his impassive face. That's partly because speed is manufactured by making muscles contract faster, and you do that by pumping them full of oxygen -- and economical form conserves oxygen.

The middle distance events are very tactical, and those tactics can unfold gradually or in an instant. Because the races are still relatively short and run quickly, there just isn't the time to make amends for a tactical error or falling asleep at the switch, especially in the 800m.

"You really do want to be careful," explains Canadian 800m Olympian Zach Whitmarsh. "Go out too fast, and you will likely pay for it over the last couple hundred metres. But it's just as dangerous to get caught up in the pack and lose momentum and find yourself unable to make up the gap between yourself and the leaders."

There are two main kinds of tactics in middle distance events: taking an early lead and basically challenging the rest of the field to catch you; or more commonly, settling back in the field, remaining very alert to the amount of race remaining and where you are relative to the leaders, and choosing the right moment for your finishing kick. Typically, runners who plan to finish with a strong kick to overtake the field will follow on the heels of another runner to use his or her slipstream, which can significantly reduce wind resistance, which again conserves energy for that kick.

In races in which runners are more concerned with placing well than posting fast times, the races become extremely tactical, with many changes in the pace and runners reluctant to take the lead or go out too fast. That's one reason why world records are rarely set in Olympic finals: the important thing is the medal, not the prestige of a fast time. But the more tactical a race, the riskier it becomes.

"Tactics can backfire," says CBC track analyst Geofff Gowan. "In tactical races in which the pace is very slow, the distances from front to back is very short, so there's the ever-present danger of tripping over the person ahead of you (which has put more than one medal favourite out of contention in the past), getting an elbow in the ribs or getting boxed in. The big decision in the tactical races is when the big kick will start. When there is a slow start, you know there will be a kick -- you just don't know who or when."