Thermoskin hyperextension elbow features x cross Velcro locking straps. Combining these straps with the medial and lateral hinges provides a bracing mechanism which stops hyperextension at zero degrees. This support may be suitable for those suffering from medial elbow instability or elbow hyperextension ailments. Measure around elbow joint with arm extended. Sizing guide: small; 9-10 1/4", medium; 10 1/2-11 3/4", large; 12-13 3/4", X-Large; 14-15 3/4", XXL; 16-17 3/4"