First, some basic mechanics:
According to Isaac Newton’s Third Law of Action and Reaction, whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former.
In other words, when your legs push your feet against your kayak’s foot braces (or footrests) they also push your lower back against your seat – and as a result the seat pushes back against your lower back with an equal force.
Your legs have the most powerful muscles in your body, and they constantly generate this force from the moment you sit in your kayak until you get out of it.
The L kayaking position deprives your legs from their natural role, and together with the seat and footrests turn them into a source of ergonomic problems for your back.
All that unnatural pressure is bad for your legs as well, and this is why you suffer from leg numbness, poor circulation and sometime pain and even chronic injuries.