Having examined many agility dogs, I’ve truly come to appreciate the structure of the canine front limb assembly (specifically the shoulder), and the dysfunction that might occur at this location in agility dogs due to breed and genetics, existing injury, or simply the stress and strain imposed upon these structures from competition and training. I’ve also examined and treated other full and cross-bred herding dogs (Border Collies, Australian Kelpies, Shetland Sheepdogs) in other sports (flyball, obedience, rally-O) and non-sporting dogs (weekend warriors, squirrel chasers, fence line fighters) demonstrating shoulder issues as well (mostly related to hypermobility):
Promising (baseline) research is in the works (we have to start somewhere) with respects to characterizing the cause of shoulder injuries in dogs. While investigating newer studies on sporting related injuries in dogs, I fell upon this study. Pfau et al. (2011) performed a kinetic study of agility dogs jumping over obstacles at high speed (a kinetic study means that the researchers were trying to study the forces involved in creating and changing motion in a moving body). They found that the forelimbs of dogs jumping at high speed over high obstacles experienced an extremely high peak vertical force on their forelimbs (4.5 X their bodyweight!) with acute (steep) landing angles. Not a shock to anyone reading this, but here’s the clincher – this means the average sized male Border Collie (let’s say a fit 20 kg, or 44 lb dog for you standard measurement folks) would be driving 90 kg (that’s 198 lbs!!!) onto their front limb upon landing. Ouch! The authors conclude with a note that this force impact is quite concerning, and more studies are needed to investigate the effects these forces may contribute to injury.
In light of this, a complex etiology likely surrounds shoulder injuries and research (if it could be produced) would be clouded with a number of relevant questions. We have no idea how a dog’s particular landing style may affect injury, although it may stand to reason that a dog landing more forward on their front ends from a rounded position, or a dog jumping an unaccustomed jump height may experience more force on their shoulder assembly. Dogs with steeper shoulder angles and shorter humeruses (ie. less than the desired 30 degrees cited by Zink, 2008), may also be at risk for shoulder problems with a straighter shoulder conformation meaning less essential shock absorption, and altered gait patterns. Arena footing, jump construction/style, and placing likely play a role in shoulder injuries as well. Based on my experience, and the experience of other rehab professionals, certainly dogs with hypermobile shoulders (typical of many herding breeds – more on that later), and dogs with a pre-existing shoulder or forelimb injury would be more prone to problems with jumping resulting from repetitive stress and injury.
The shoulder joint is a complex structure. The shoulder joint (glenohumeral or scapulohumeral joint) of the dog is a ball and socket type joint between the top end of the humerus and the bottom of the scapula) (Miller, 2007). There is a loose joint capsule surrounding the joint which offers little support, so the surrounding joint is held secure by the tendons of various intrinsic muscles supporting the joint, including the deltoid, supraspinatus, infraspinatus, teres major, teres minor, subscapularis, coracobrachialis, biceps brachii and triceps brachii (long head). Other extrinsic muscles act on the shoulder joint as well, and a certain kinetic-chain allows proper movement of the shoulder and forelimb in functional movements, and this can be affected by injury. Although not a breed specific study of herding dogs, dogs predisposed to medial shoulder instability (explained in more detail in another post) have wider shoulder abduction angles than controls (Cook et al, 2005). To visualise abduction think of your dog giving you a hug if they stood up and how the shoulder would have to abduct or move outward to do this. Rehabilitation practitioner anecdotal reports abound with reports of herding dogs with wider abduction angles (symptomatic and asymptomatic). We feel this may be due to breeding and selecting dogs that have increased lateral (sideways) motion through their shoulders in a working-herding situation.
The forelimb of the quadraped dog is naturally evolved as a braking device and the hindlimb of the dog was designed as a propulsive device (Lee, 2011). The stretch-shortening cycle (SSC) involving initial stretching (eccentric loading), then shortening (concentric action) of the forelimb muscles, is a major means of braking and slowing/controlling movement during running, jumping and hopping (Nicol et al. 2006). One might argue that dogs are naturally made to bear more forces through their forelimbs, since hindlimb to forelimb weight bearing is disproportionate (60 – 65% of BW on the forelimbs, and 35-40% on the hindlimbs), however, unlike agility dogs, most dogs in a natural state would choose a more efficient means of moving when possible (jumping less than the average Border Collie training in one agility season with less resultant repetitive stress to the structures of the shoulder, and not pushing through and/or training through neuromuscular fatigue – an inherent weakness of the SSC).
What do I mean when I mention a “hypermobile” shoulder? You may also hear the terms “lax”, “laxity”, or “loose”? When a rehab vet checks your dogs shoulder, they are looking for a few signs that deviate from normal. We feel for trigger points or sensitive points along various parts of the tendons surrounding the joint (and palpate up along the muscles that attach to these tendons) to see “who” (muscle and tendon -wise) might be involved. We then determine your dog’s shoulder abduction angle as well. “Normal” dogs have an abduction angle of less than about 32 degrees, with wider angles equating to more mobility in the shoulder (ie. hypermobility – “hyper” meaning more) giving us bigger angles upwards from 32 degrees. The end feel of this movement, glides of the bones of the joint, and the feel on shoulder range of motion (and presence or absence of pain) is important too, since this will tell us how the joint and surrounding structures are affected, and allow us to make the best treatment recommendation. Dogs with medial shoulder instability are more prone to injury of the structures surrounding and supporting the joint, typically starting with varying stages of fibre pattern disruption along the musculo-tendinous junction, and varying degrees of inflammation and pain depending on the location, duration of problem, and structures involved. More chronic cases have less inflammation, and typically a tendinopathy (not inflammatory that may or may not include calcification) of one or more of the musculo-tendinous junctions.
You might be asking, what types of shoulder problems exist, and how are they treated? Four main conditions of the shoulder are identified from the standpoint of surgical or rehabilitation intervention, and each presents (usually) with a spectrum of severity. Surgery is usually reserved for the most severe cases (medial shoulder instability), with many rehabilitation options available, especially for the tendinopathies:
Medial Shoulder Instability
Teres major Strain
What so you do if you suspect your dog has a shoulder issue? Your dog may be lame on one fore leg, or just seem a little off after a trial or training. Dogs who have issues with both shoulders (bilateral) may show little lameness on either side if both sides are equally affected, or the lameness may appear to affect either side intermittently and you can’t pick out a lame front limb. You may have checked the foot, palpated the legs, flexed the joints and felt nothing. The best advice, is to have your dog assessed by a rehab practitioner such as a CCRT or CCRP, or a veterinarian comfortable with orthopedic assessment (most GP vets do not have training in special tests used to assess the shoulder, so I tend to get referrals for these). Surgeons are generally knowledgeable in assessing, and may or may not refer to a rehab practitioner for rehabilitation consultation in the event the dog can be managed conservatively without surgery.
Caution – Many of my clients will be seen wielding a Theraband at a trial or on training day (and they often get weird looks from their agility colleagues). You will see them doing a specific exercise meant to strengthen the shoulder apparatus, and may be exuberant enough to get a demo of the activity/reasoning and attempt to try it yourself if you suspect your dog has a shoulder issue. Like any therapeutic exercise, I caution doing this without first consulting with a rehab practitioner to ensure you do not mask any clinical signs of instability by doing this. The exercise may help your dog if you cannot see a rehab practitioner or other professional for an accurate assessment, but it does tighten the shoulder just enough that it may be a little difficult for a therapist to feel everything they need to, to obtain a confident functional diagnosis especially in cases of milder instability. Additionally, the dogs prescribed this exercise are often prescribed other exercises, and managed with modalities and therapies prescribed to aid healing and repair of injured tissue, and this is another fundamental aspect of treatment.
Cook JL, Renfro DC, Tomlinson JL, Sorenson JE. 2005. Measurements of abduction for diagnosis of shoulder instability in dogs using goniometry and digital image analysis. Vet Surg. 2005 Sep-Oct; 34(5):463-68.
Evans HE. 2007. Miller’s Anatomy of the Dog. Elsevier Science Health Science Division. pp. 1113.
Lee, D. 2011. Effects of grade and mass distribution on the mechanics of trotting in dogs. J Exp Biol 2011 Feb;214:402-11.
Nicol C, Avela J, Komi P. V. 2006. The stretch-shortening cycle: a model to study naturally occurring neuromuscular fatigue. Sports Med. 2006; 36(11):977-99.
Pfau T, Garland de Rivaz A, Brighton S, Weller R. 2011. Kinetics of jump landing in agility dogs. Vet J. 2011 Nov;190(2):278-83.
Zink, C. 2008. “Knowing All the Angles”. The Agility Advantage. Clean Run Productions, LLC. p. 6-11.
No part or whole of this article may be written or reproduced with express written consent from Dr. Budiselic of Equilibrium VRC Ltd. Copyright 2013.