Gardening shouldn’t require a recovery day.
Kneeling on paving puts roughly four times your body weight through each kneecap. A centimetre of foam changes the arithmetic.
Twenty minutes of weeding should not cost two days of sore knees. When it does, the problem is not fitness. It is pressure.
The arithmetic
Kneeling concentrates your weight onto two small contact patches. On a hard surface, the pressure through each kneecap is roughly four times body weight. Foam under two centimetres compresses flat within a minute and stops helping.
Why thicker is not always better
Past about five centimetres, the pad starts behaving like a cushion: you sink, you wobble, and you use your back to compensate. Four centimetres is the range where the foam is still firm enough to keep you stable.
The other half of the fix
Height. A pad raises you four centimetres, which is four centimetres less bending for every weed you pull. Over an afternoon that is a few hundred centimetres of movement you do not make.
What to ignore
Knee pads sold on "memory foam" alone. Memory foam is designed to conform under body heat, which is the opposite of what a kneeling pad needs.