A firm foam cushion that makes flat ground slightly unstable. Stand on it on one leg, squat on it, or put a knee or a forearm down during floor work - the surface gives just enough that the small muscles around the ankle and hip have to keep working.
The hardness is deliberately moderate: soft enough to be unstable, firm enough not to bottom out under a full bodyweight. It is made of rubber, measures 30 x 20 x 6 cm (11.81 x 7.87 x 2.36 in) and weighs roughly 300 g, so it stacks against a wall or slides under a bed between sessions.
Compact 30 x 20 x 6 cm block, about 300 g
Moderate firmness for balance and stability drills
