When push-points are absent, but the space is too narrow for sidewinding, such as in tunnels, snakes rely on concertina moving. In this mode, the snake braces the back part of its body against the tunnel wall while the front of the snake extends and straightens. The front portion then flexes and forms an anchor point, and the back part is straightened and pulled forwards. This mode of moving is slow and very demanding, needing up to seven times the energy of laterally undulating over the same distance. This high cost is due to the repeated stops and starts of portions of the body as well as the need to use the muscles to brace against the tunnel walls.