By rights these knees, which have carried McGrath to the heights of his profession, should have carried him off the field years ago. They have been so badly injured during his 12-year career that it is a wonder, at the age of 34, he can continue to play at all.
As McGrath embarks on a new football season, which will include service for both Villa and the Irish in European competitions, his knees are often too painful for normal training. He overcomes this with the help of a physiotherapist who has devised an alternative way of keeping him fit.
McGrath's professional career started comparatively late - and almost ended immediately. He was spotted in his native Dublin by a Manchester United scout when he was 22. He was still in the United reserve team when he suffered the first of his injuries, a twisted knee during a tackle. It took him two- and-a-half months to get over the operation.
He got into the first team the following season, then was injured again. During the seven years he spent with United, he had only one season without injury and eight operations on his knees.
Injuries like his are known simply as 'cartilage problems' and are common among professional footballers. But McGrath stands out for the number of times he has suffered them, and the severity of his associated arthritis.
When the knee is twisted at an unnatural angle during a tackle, there is usually damage to the meniscus - the pad of cartilage in the knee which acts as a shock absorber between the femur in the upper leg and the tibia in the lower leg. The damage varies from the meniscus being slightly chipped to being torn in half.
The chipping injury is the most common and can be cured fairly simply with an arthroscopy. This involves inserting a thin 'sight line' into the knee with keyhole surgery so that the specialist can look inside and locate the floating fragment of cartilage which lodges in sensitive areas of the joint, causing pain and swelling. The fragment is flushed out with water squirted down the same thin tube, or sucked out by a miniature vacuum with a cutting edge on the end.
When the meniscus is badly damaged, however, it often has to be removed with major surgery. Many footballers can continue without it, although its absence leads to arthritic changes in the knee which are likely to shorten their careers.
Without the natural shock absorber to act as a cushion between them, the ends of the femur and tibia rub against each other and gradually wear away the layers of hyaline cartilage which coat the knuckle ends of the two bones. There comes a time when bone grinds against bone and the discomfort makes it impossible for the footballer to continue playing.
In McGrath's case, the left knee has survived three arthroscopies, the meniscus is still in place and he has little trouble with it. The main problem is in his right knee which has undergone three arthroscopies and two major operations, resulting in the removal of all the meniscus.
The whole of this medical process took place while he was still with Manchester United, who then put him up for sale. Aston Villa paid pounds 400,000 for him in 1989 after he had been examined by the club surgeon, El Safty, who decided his knees were still up to the rigours of modern football.
Since his arrival at Villa Park five years ago McGrath has needed no further surgery and has not missed a match because of knee problems. Two seasons ago he played in all 42 league matches - a feat achieved by only a handful of professional footballers each year. It was the season in which he received one of the greatest accolades of all - the Professional Footballers' Association's Player of the Year award.
But it has been at a cost. The arthritic condition in his right knee has been ever-present and he is usually in some pain. If it is particularly bad, he will take two paracetamol on match days plus an anti-inflammatory drug the day before, the day of the game, and the day after.
He is often excused the seven to eight hours a week of normal training and running expected of his team mates. Instead he is handed over to the club physiotherapist, Jim Walker, who arranges a special fitness programme which ensures that the painful grinding of his femur and tibia is confined to matches. It is almost certainly a unique situation for a world-class player.
He spends five hours a week on a cycling machine and does leg muscle exercises for 10 minutes of every waking hour when this is practical. The latter strengthen the hamstring and quadricep muscles in his upper leg, and this reduces the amount of body weight carried directly by his knee. 'Whenever my knees are sore I know that I should not be doing normal training and Jim arranges the alternative for me,' he says.
'I know when I need to train and do just enough to keep ticking over. When we are due to play I don't even go on the cycling machine the day before, but do warm- up exercises instead.'
'Paul really is walking a tightrope and I therefore always listen carefully to what he says,' says Mr Walker, who was a professional footballer for 15 years. 'If he says his knees are sore and he should not be training, I believe him and we work in the gym instead.
'It is a feat to be playing Premier League and international football at Paul's age, even without his arthritic problems. The fact that he does so is due to his enormous enthusiasm. There are not many players like him.'
McGrath is more diffident: 'In spite of my injuries, I believe that I am a lucky player. I keep thinking about how many years I will not be able to play football once I am too old, and that makes me determined to play on for as long as I can.'
Paul McGrath's autobiography, 'Ooh] Aah] Paul McGrath', is published by Mainstream at pounds 12.99.