Swimming
Quick results Engages all major muscle groups, and is a strenuous workout for heart and lungs. Water supports body weight, so there's no stress on joints, and being about 14 times thicker than air, it offers ideal muscle-toning resistance.
Bulking up Only by sprinting laps in the swimming pool will muscle be built to any significant degree. Swimming lengths flat out will give great upper body definition after a while, but it can be pretty exhausting.
Fighting flab Prolonged exercise at lower intensity is better for fat-burning, which is why swimming is so effective - a man weighing around 70-80kg will burn about 750 calories an hour while swimming breaststroke at moderate intensity.
Beating boredom You use both upper and lower body to propel yourself through the water, so it's a perfect total body workout. With the choice of strokes, it offers variety, too, and, if you relax and focus on the rhythm of the stroke, it can have a calming and almost meditative effect.
Cycling
Quick results A great aerobic workout that improves stamina. Low weight-bearing, so can be as joint-friendly as swimming. To get the most out of swimming you need the skill to develop an efficient stroke, but with cycling you just get on and pedal.
Bulking up Cycling in higher gears will build muscle strength in the calves, gluteus, quadriceps and, by standing up out of the saddle, upper and lower back, too. However, it's not so great for building up the upper arms.
Fighting flab The higher the cadence (80-100 revolutions a minute should be achievable) in low gears, the more fat is burned. But it's still not as effective as swimming - an hour's cycling by a 70-80kg man averaging 10-15mph would use 600 calories an hour.
Beating boredom It's multi-functional, getting you from A to B while getting you fit, it's social and the scenery changes. It can also get you to work, so you can avoid the gym - a half-hour daily commute burns about eight calories a minute, or 11kg of fat over a year.
