The Hammacher Schlemmer Institute commissioned a panel of swimmers to test swim fins and these proved to be the best because of the superior thrust and momentum they generated. Unlike rubber fins, these are made from molded silicone that is denser than water, helping swimmers keep their feet below the surface for proper form and faster swimming. Swimmers lauded the fins' ability to cut through the water with little resistance, leading one panelist to say they provided "...the best speed with no effort or fatigue," while lesser fins required vigorous kicking that quickly left legs exhausted. The fins have v-shaped rails on the blades that help to channel water along the blades as you kick, reducing unnecessary drag. Scoring highest for features, the fins have dedicated left and right feet, rounded foot cavities, and heel straps with ankle cutouts, eliminating the chafing and pinching common to traditional fins. Treads on the soles ensure confident footing, and the asymmetrical blades are short yet wide for improved mobility when walking on land. Each size has a specific blade-to-foot area ratio to ensure optimal thrust. Men's sizes M (8-9), L (10-11), or XL (12-13); Women's sizes S (7-8), M (9-10), or L (11-12).