Speedo SMAO Swim Training Paddles Scalloped outer edges to create greater surface area to enhance propulsion, thus the swimmer pulls more water during sprint drills Lateral edge openings and bottom flow channels to allow more resistance, greater buoyancy and increased smooth movement through the water Thumb garage reinforces proper thumb position to allow greater control on stroke recovery phase, adjustable soft silicone straps for hand stability Soft silicone, and wide enough to keep the paddle securely in place Size Chart: SMALL = Female (Moderate Resistance), MEDIUM = Male (Great Resistance), LARGE = Big & Tall Males (Extreme Resistance)