Usually 12x12x1 white pine boards (actual dimensions are a little smaller). Wood boards can break at some point other than the impact point. Wood does not have to be hit in the exact center. When choosing board to break:
- Choose ones with least number of knots (wood will break through a knot not through it). Avoid wood with knots that run along the length of the board. Small round knots are not a problem but avoid large knots in the center of the board.
- Choose ones lightest in weight (less moisture)
- Choose ones lightest in color (less sap)
- Choose ones with rings closest together. Since you are splitting the board along the grain, a board with grain running in narrow bands along the length will be easier to break. The more grain that runs in a vertical direction, the harder the wood will be to break.
Only break in one place, so they must be hit dead center. Plastic interlocking "fingers" that create the board's resistance to breaking are susceptible to temperature. The warmer they are the easier the board is to break. Breaking resistance decreases the more times a plastic board is broken. When re-chambering the hand or foot after a break, the board fingers may dig into the arm or leg and cause injury.