You all know that I believe in a well-balanced workout regimen with both cardio and strength training. I must confess that when I am training for an endurance event, I tend to skimp on the strength training and sometimes due to poor or over scheduling (the kids need to get to swim class or we have run out of milk, bread, and eggs all on the same day) I am forced to skip it all together. Professor of Kinesiology William Kraemer, Ph.D., reminds us that this is not a good idea; you need to keep on using the weights!
Running, when done for periods of time over 30 to 45 minutes can actually break down muscle. This happens when the glycogen stores (aka muscle fuel) is depleted and the body looks for other sources of fuel sometimes scavenging muscles for energy. Aside from properly fueling (like sipping a sports drink every 10 to 15 minutes) yourself on long runs and rides, you need to rebuild muscle with strength training. Kraemer suggests 60 minutes to 90 minutes a week, which I find can be hard to fit in. I try to fit in squats and wall sits (during TV commercials) whenever I can as well as focused strength-training workouts even if they are short.