Equis ISSN 2398-2977

Muscle: myopathy - fibrotic

Contributor(s): Steve Adair, Helen Herinckx, Prof Derek Knottenbelt, Graham Munroe, Chris Whitton, David Rendle


  • Cause: trauma to muscle, commonly semitendinosus → fibrosis.
  • Signs: restricted movement, uni- or bilateral, abnormal gait Musculoskeletal: gait evaluation, decreased muscle mass Muscle: myopathy - fibrotic.
  • Diagnosis: palpation, ultrasonography Ultrasonography: overview, radiography.
  • Treatment: surgical excision of affected tissues, tenotomy.
  • Prognosis: fair.



  • Trauma.
  • Intramuscular injection.

Predisposing factors


  • Stock chasing; rodeo events; polo.


  • Sudden stop and turn exercises, eg polo, cattle cutting, or struggle from trapped/caught leg → contraction of semitendinosus (and biceps, semimembranosus) against extended stifle and hock → greatest rotatory force borne by semitendinosus (has longest insertion) → muscle tearing at insertion.
  • Injection site abscesses → inflammatory process in muscle.
  • Adhesions between semitendinosus/semimembranosus/biceps femoris may cause muscle restriction.
  • Functional muscle shortening → increased muscle tension → prevents full stifle extension + hock extension during cranial swing phase of stride.
  • Traumatic/degenerative neuropathy → denervation of distal semitendinosus.
  • Repair process → fibrosis → ossification.
  • Other hindlimb muscles may be involved, eg gracilis, semimembranosus, biceps femoris; forelimb cases have been reported.
  • Generally unilateral but may be bilateral possibly as a result of progressive neuropathy.
  • In neonates may be caused by trauma at or soon after birth, which may indicated a congenital cause Foal: congenital deformity 05 - myopathy; in foals it is not usually associated with palpable fibrosis of the distal semitendinosus.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Janicek J et al (2012) Hindlimb kinematics before and after laser fibrotomy in horses with fibrotic myopathy. Equine Vet J 44 (43), 126-131 PubMed.
  • Pickersgill C H, Kriz N & Malikides N (2000) Surgical treatment of semitendinosus fibrotic myopathy in an endurance horse - management, complications and outcome. Equine Vet Educ 12 (5), 320-324 VetMedResource.
  • Bramlage L R, Reed S M & Embertson R M (1985) Semi-tendinosus tenotomy for treatment of fibrotic myopathy in the horse. JAVMA 186 (6), 565-567 PubMed.
  • Turner A S & Trotter G W (1984) Fibrotic myopathy in the horse. JAVMA 184 (3), 335-338 PubMed.

Other sources of information

  • Valentine B A (2003) Mechanical Lameness in the Hindlimb. In: Diagnosis and Management of Lameness in the Horse. Eds: Ross M W & Dyson S J. Saunders, USA. pp 477-478.
  • McIlwraith C W & Robertson J T (1998) Surgery for Fibrotic Myopathy. In: Equine Surgery - Advanced Techniques. Williams & Wilkins, Baltimore. pp 213-215.