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One of the mysteries of being physically active is when the same workout feels great one week and lousy the next. Why does a routine 4-mile run or hourlong yoga class suddenly morph into the last leg of an Ironman triathlon?

Bad workouts happen to everyone, from professional athletes to office workers trying to carve out 30 minutes at the gym.

Coaches and academics can identify many factors to explain this mystery. In short, much of what you eat, drink and even think about can affect how you exercise.

Peter Thompson, a longtime track and field coach who has worked with athletic novices and Olympians, says one factor that can slow regular exercisers is inadequate recovery time.

"Training does not make you fitter. It's the recovery and adaptation for training that makes you fitter," says Mr. Thompson, a native Brit who lives in Eugene, Ore. "So you should train to recover, not recover to train."

Mr. Thompson recommends that people rank their energy level each day on a scale from 1 to 10. If they never rank themselves an 8 or 9, they're working out too hard, too often or both, he says.

For most of us, excessive channel-surfing is a bigger problem than overtraining. But we don't always prepare for workouts as predictably as we set our DVRs.

Failing to eat enough, especially before a taxing workout, can leave people feeling heavy-legged, says Stella Lucia Volpe, chair of the department of nutrition sciences at Drexel University in Philadelphia.

The idea is to provide your body with fuel that it can use quickly. The closer you get to exercising, the simpler and more carbohydrate-centric the food should be. Dr. Volpe recommends eating an apple or a small piece of toast with peanut butter, for example.

"Even some coffee before is absolutely fine," she says. For part-time coffee drinkers she cautions to start with a small cup, because it can stimulate the gastrointestinal tract.

Some people can't eat before exercising without getting nauseous. For them, Dr. Volpe advises eating a light snack of yogurt or cereal the night before. If you're trying to lose weight, weigh yourself once a week to make sure your caloric intake isn't overtaking your activity level, Dr. Volpe says.

Alcohol, though it can have health benefits in moderation, can be a stealthy workout disrupter. The American College of Sports Medicine, an organization of 50,000 exercise-science professionals, recommends avoiding alcohol for 48 hours before an athletic competition.

Teetotaling isn't necessary if you're just cycling your usual loop. But the quantity or type of alcohol you drink could help explain why that loop sometimes feels tougher.

"If you normally drink wine but then the night before you have some brandy, your body might react to that," Dr. Volpe says. "Maybe you won't sleep as well."

Alcohol is one of several things that can pull water from the body. Dehydration can make your body less efficient at cooling itself, making exercise feel more labored.

Sleep also can be dehydrating, particularly if sleep with your mouth open, says Chris Winter, a neurologist in Charlottesville, Va., who consults pro athletes on how to manage their slumber. That could explain why a morning workout leaves you feeling especially parched if you usually work out later in the day.

The effects of poor sleep also can linger for as long as 48 hours. A deficit of as little as one or two hours can throw people off, Dr. Winter says.

A bad night's sleep can have ripple effects, such as altering what and when you eat, he says. Sleepy people tend to crave fatty, carb-laden snacks, which can disrupt energy levels and lead to overeating. Even if you're not hungry, Dr. Winter says, "you may find yourself walking by Gladys's desk and helping yourself to her Hershey's Miniatures seven or eight times a day."

Lost sleep and snacking could prompt you to skip a workout. But the best thing to do after poor sleep is return to your regular routine as soon as possible, the doctor advises.

While everyone is a bit different, "I tell people, the best thing you can do for your sleep is to exercise the same time every morning," Dr. Winter says.

One important thing to remember as you labor to hoist the kettlebell you were lifting with ease last weekend is that bad workout days are normal. Emily Rand, a semiprofessional middle-distance runner who works out under Mr. Thompson's guidance, says she has a few workouts every month that feel worse than expected. In most cases, she says, she never figures out why.

"I always say if you're working out five days a week, you're going to have one good day, one bad day and three kind of normal days," says Stephen Ball, an associate professor at the University of Missouri department of nutrition and exercise physiology.

What about those days when you're perfectly hydrated and fed, you've had a good few days of sleep and limited your alcohol intake to a few sips? Why are you still dragging?

Simple monotony can cause a bad training day, Dr. Ball says. Serious weightlifters in particular can spend several hours each week in the same confined area and often do the same routine, he says. That's a recipe for boredom and lackluster performance.

Often there are multiple exercises or machines that can work the same muscle group, Dr. Ball says. Instead of plopping down and doing the same old bench press, try using dumbbells, a chest-press machine—or try changing the angle of the bench, Dr. Ball says.

To stave off sameness, you can also try varying the length of rest periods between sets, he says.

If you're still stumped as to why a workout went badly, think about your life away from exercise. Are you preoccupied with a looming project at work? Are you feuding with a rebellious teenager? Mr. Thompson, the running coach, says he coached one athlete whose workout performance dipped for six months after a broken engagement.

From 2000 to 2010, Mr. Thompson coached the cross-country running club at England's academically intense University of Oxford. Those athletes, like many people, "had this kind of mistaken belief that they could compartmentalize the stress in their lives."

Instead, their personal worries sometimes would encroach on their performance.

"I would encourage them to go to a comedy club once a week," he says.

http://online.wsj.com/articles/the-ways-...y&mod=e2tw