I'm about to start a training program for rock climbing, cycling, and running. Can you tell me if the lactic acid produced by the aerobic part of my training will affect my weight training for climbing?

— Merrick, Sydney, Australia

The answer depends on how you organize your training. If you're doing your weight training right after a hard aerobic workout, you'll feel the effects. But if your aerobic and weight training are done at separate times, then the effects should be minimal. Keep in mind that not every aerobic workout will result in significant lactic acid accumulations in your blood. The levels of lactate are dependent on the intensity of the aerobic workout: higher intensity equals a higher level of lactate.

Lactate is a byproduct of muscles producing energy without the benefits of oxygen, as happens when your breathing can't keep up with how fast you're trying to move. Due to its association with muscle fatigue, lactic acid has gotten a bad name in exercise circles. And while it's true that the higher your lactate levels, the quicker you're going to have to stop whatever you're doing and rest, lactate does have its benefits, such as being a preferred source of fuel for the heart muscle.

Ideally, your aerobic training will be a mix of training intensities, some days hard, some easy. Besides allowing you to regulate your lactate production, mixing intensities actually improves fitness more effectively than doing every aerobic workout at the same rate. Each level of intensity has its associated benefits. Easy training develops a plumbing system that can get lots of oxygen to the working muscles, while higher intensities enhance the mechanical efficiency and your ability to sustain a faster pace. Doing strength training right after a hard aerobic session will limit how well you'll be able to lift, as the residual fatigue will make lifting the same weight seem harder. To get the most out of all your workouts, schedule your training so that the strength days are scheduled either before your hard aerobic days, or one to two days after.