The 1st compartment contains abductor pollicis longus and extensor pollicis brevis. The 3rd compartment contains extensor pollicis longus. A muscle together with its tendon and bony attachments form an extensor or flexor unit. The other three are the triceps, anconeus, and extensor carpi radialis longus. The extensor digitorum indicis and extensor digitorum communis are within the 4th compartment. Occasionally united at its origin with the extensor digitorum longus. The body maintains the balance between the tone of flexor and extensor muscle groups. Treating these extensor or flexor group of muscles in isolation to relax can be difficult. The radial collateral ligament is attached to the lateral epicondyle below the common extensor tendon. Supernumerary tendons are common in the fourth extensor tendon compartment. Triceps is the major extensor and brachialis and biceps the major flexors. A common injury to the extensor carpi ulnaris is tennis elbow. The extensor plantar reflex is usually absent. The gluteus maximus is the main extensor muscle of the hip. The extensor muscles are inhibited to prevent competition between the extensor and flexors.