Tim Henman said recently that the press doesn't understand his temperament. Any suggestion that he has a detached, nerveless disposition - so laid back that he can take defeat or victory with the same fleeting half smile he learnt as a prep school boy at the Dragon School in Oxford - is obviously nonsense. Nonsense not only because his facial expression, his posture, gait, gestures and the way in which his game may suddenly deteriorate, show that he is as highly strung as any Derby winner, but also because athletic success is not obtained without determination and a ruthless competitiveness.

Competition can become addictive. People with an obsessional nature can become as enthusiastic and focused on exercise as a person suffering from anorexia nervosa is on slimming. Someone with anorexia may allow their whole life to be dominated by the desire to lose weight and all their actions become directed to correcting what they see, quite falsely, as obesity.

A preoccupation with physical fitness in a man can be the equivalent of anorexia in a woman: either trait can be found in either sex. As well as a compulsion to exercise, athletes have an obsessive desire to win. This is, in part, the result of inheriting, or acquiring, a type A personality - competitive, thrusting, ambitious, determined and tenacious.

Research published within the last months has confirmed that temperament may be inherited in the same way as red hair - or any other physical characteristic. Both Henman's parents and grandparents were successful games players. Athletes may not only inherit determination and ambition, but also physical traits - good co-ordination, a good eye, an athletic build and effective cardiovascular system.

The subtle ingredient of the physical formula for success is the supply of endorphins. Endorphins are chemicals produced in the body, closely related to the opioid drugs, such highly addictive substances as morphia and heroin. Just as junkies can become addicted to heroin, so may the athlete become addicted to the endorphin, which his body produces when he is stressed while taking part in compettive or challenging sports.

As a result of the tense-making, demanding activity, athletes are on a high - they don't notice the discomfort of aching, over-strained limbs, but only feel great, and sense that they can achieve anything they set out to. Endorphins are found in highest concentration in nervous tissue, including the brain, and are thought to influence the action of the pituitary gland, which nestles at the base of the brain. It is the conductor of the hormonal orchestra, controlling many of the other endocrine glands, thereby affecting both physique and psyche.

Tim Henman's performance didn't only inspire other aspiring tennis players. Henman may have inherited many athletic advantages, but a year or two ago, he didn't look like the world-class performer he has become. The example which he has set us all, of whatever age, is that we, unashamedly idle couch potatoes, could, like him, be fitter if we put our minds to it.

No one is too old to improve physical prowess. There is a lot to be said for brisk walking as the exercise of choice, but just as Henman has had to build himself up step-by-step, so must any ageing couch potato start gradually, and slowly exercise for longer periods each day. A week or two off and 20-30 per cent of everything that has been gained in athletic ability may be lost. After an enforced rest it is important to return gradually to the exercise regime.

To keep the "little grey cells" working they need strenuous exercise: never rest until a forgotten name is recalled, or a fact remembered. Cerebral exercise is not only a matter of reading - that does not test the intellect. The ageing person should be playing chess, doing The Times crossword, sorting out accounts, or writing. This should be done for at least half an hour every day.

If someone can have a daily, brisk walk, and do The Times crossword, they may achieve in later life what they never achieved when they were at school: the age-old concept of mens sana in corpore sano.