You want that svelte physique. You want to bench-press 100 pounds. But swim another lap? Do another rep? No way. You're tired. It feels bad. Left to your own devices, you'd surely stop.
It is a sad fact of human nature that even if we have realistic goals and a will to succeed, it is often impossible for the average person to make himself bench-press an extra 25 pounds or bike up one more hill.
For many of us, the reason is pretty elementary. Psychologists point out that the human species is hard-wired to like rewards - immediate rewards. It can't be denied that in the short term, physical exertion is kind of unpleasant, and the lusted-for rippling abs or four-minute mile are far-off and intangible.
Exercise, for most people, "is not fun, or interesting, and the activities may be too challenging", says Christina Frederick-Recascino, psychology professor at Embry-Riddle Aeronautical University in Florida.
"We're not innately geared toward engaging in those activities. When you have an external force that pushes you to engage in them, who can remind you of the outcomes you wish to achieve, that becomes the motivating factor."
Jeff Galloway, renowned Atlanta-based running coach and former Olympian, thinks that for others the culprit may be something else: intense feelings of self-doubt. "It's not just beginners," he says, but also star athletes "who have some deep insecurity packages that are part of our human nature".
That's where the power of the personal trainer or fitness instructor comes in, seemingly magically propelling us far beyond what we think are our physical limits. When working out solo, it's often too easy to skip the last 10 minutes of a run or bail on the final 10 push-ups, telling ourselves we're tired, we have to go to work, or a million other excuses.
But a fitness pro with the right stuff can squash those excuses and make us drop and do 20. Yet it's not as simple as perky, upbeat words interspersed with the occasional "Woo-hoo".
The best personal trainers are able to tap into the motivational centres of our brains, and tailor their one-on-one messages to their clients' and students' individual needs, says Kate Hays, a Toronto-based sport psychologist. One person may respond to an in-your-face drill sergeant style, another may find the best squats are inspired by well-timed humour. The idea, Hays says, is to ask, "What is the thing that is going to be the most effective with this person?'"
And the key for a client is to figure out what techniques, and what trainers, work for you.
And what motivation. Short-term goals, Frederick-Recascino says, are just that, and shouldn't be the only ones contemplated.
Things such as fitting into a pair of jeans or getting in shape for a reunion are great for fuelling the fire day to day but don't really get at the heart of why we need to stay fit for the rest of our lives.
"There has to be some meaning to it for it to be long-term," she says, along the lines of being able to play with the kids, or cutting the risk of diabetes. It behooves anyone undertaking an exercise programme to put some thought into why they're doing it before they hoist a barbell.
The confidence exuded by a good trainer can motivate a client. "I tell people that they've proven they have the structural integrity to do this, and don't be afraid, get in there and try it," says Brian Thurston, a trainer at The Sports Club/LA in Beverly Hills. "More often they're able to do it, and then some."
Add to that a trainer's implied expertise in exercise and physiology, and it makes for an even more powerful motivational force than some guy you asked to spot you.
And it can't be denied that there's something in our nature that makes us work hard to win our teacher's approval.
Says Frederick-Recascino, "There's the feeling of, ?I don't want to let this person down'."
Such a toolbox of tricks is all very well, but sooner or later people will want to see results.
For that reason, trainers often keep copious notes of their clients' workouts to chart their progress and show them how much they've advanced, especially when confidence is lagging.
Seeing those figures in black and white, says Stephen Kraus, a San Francisco-based performance consultant and spokesman for the National Academy of Sports Medicine, "is really beneficial because people can see change pretty quickly".
Even without a trainer whispering in your ear or a class filled with ecstatic exercisers, it's possible to add some extra drive to a workout, says Kraus. He suggests setting short and long-term goals, and adding visuals - literally, as in travel brochures or catalog photos, keeping them in mind when the urge to ease off comes on. "It adds an emotional impact," he says, "and makes it more tangible - this is why you're going to the gym."