In traditional safety footwear, the toes are protected by a 200-J toe cap, however, the metatarsals - the long bones connecting the back of the foot (the tarsus) and the toes (the phalanges) - are left unprotected. For trades where there is a risk of heavy objects falling, it's not only your toes that are exposed to potential impact, the top of your feet are too. It's therefore recommended that workers wear safety shoes that incorporate metatarsal protection.
In EN ISO 20345, the additional requirement M states that a safety shoe must feature metatarsal protection that is resistant to 100 joules of impact, equivalent to a weight of 20 kg falling from a height of 50 cm.