Injury to the peroneus tertius muscle affects the stay apparatus of the hindlimb and disrupts the reciprocal action of the stifle and hock joints. This condition may also develop when a horse is kept in a full-limb cast for an extended period. The most characteristic diagnostic feature is the ability to extend the hock and flex the stifle simultaneously. The horse is lame but usually able to bear weight on the limb. The affected hindlimb exhibits a jerking motion as it is brought forward. Conservative treatment consisting of prolonged rest (usually 4 mo) is indicated; the prognosis is favorable.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVS, MRCVS; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD; Jean-Marie Denoix, DVM, PhD, Agregé; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS