The M310P is used for leg strengthening exercises and lower body workouts. 

The M310P is a padded strap with Velcro® closure and a metal D-ring. Interchangeable.

Comes in the following sizes:

Small: 14-16in (35.6-40.6cm)
Medium: 16-18in (40.6-45.7cm)
Large: 18-21in (45.7-53.3cm)
X-Large: 23-26in (58.4-66cm)