It may feel like a luxury to have an hour to work out, but it's what the American College of Sports Medicine recommends for maintaining optimum fitness, three to five days a week.

On the days you can steal an hour, use it to work your cardiovascular and muscular strength. "It's enough time to stress your muscles and include a proper warm-up and cool-down," says Juan Carlos Santana, director of the Institute of Human Performance in Florida, US. "And when you exercise for an hour, it will also help you burn more fat post-exercise - for up to eight hours," he says.

OPTION 1: train circuits

If you have a lot of energy and are up for a challenge, Schoffstall suggests creating a six- to eight-move circuit that combines strength and cardio training as well as stretching. After a 10-minute warm up that includes dynamic flexibility (see Option 1 in the 10-minute workout) and core work, begin your circuit working from the chest down. Work your chest, back, shoulders, biceps, triceps and legs in that order. (To start with basics, try chest presses, back rows, shoulder presses, biceps curls, triceps kickbacks and then side lunges.) Then complete a set of jumping jacks or jog on the spot for five minutes. If you plan to do some of the standing moves with one leg or on an unstable surface, it's good to work your legs last, because they're a big muscle group and can better endure fatigue, Schoffstall says. And when you switch from one opposing muscle group to another, you're actively stretching one muscle while strengthening another. For instance, when you do a bent-over row to strengthen your back, you're also actively stretching your chest. Aim to complete three sets of the circuit in an hour.

OPTION 2: advance your circuit

For the more advanced, increase the intensity of your circuits by doing a minute of cardio in between each exercise. Your whole workout morphs into a giant cardio session, since your heart rate will be raised throughout. The short cardio bursts can actually enhance your aerobic endurance and help prepare you for everyday life. After all, life is a series of intervals, Santana says, because you do everything in bursts.

OPTION 3: listen to your heart

Weekends are great for slow, long-distance cardio sessions, according to Dr Cummings. Not only will you relieve stress, you'll also burn fat. "These days are usually the most beneficial for fat-burning, because when you're working in your low-aerobic threshold, your body burns carbs and fat for fuel. They're sustained energy sources,"Dr Cummings says. Do a five- to seven-minute dynamic warm up, go for your 45-minute excursion, and then stretch for the next five to seven minutes. Shoot for a pace that allows you to talk, or check your pulse and aim for 60 per cent of your maximum heart rate (220 minus your age). Then add a short abs routine. Try four-way abs to work them from all angles: do 25 reps each of regular, oblique (both sides), and reverse crunches. Next, flip over and do 10 Supergirls: clasp your hands under your chin and lift your shoulders off the floor a few centimetres, then release. And now? Time to hit the shower.