Muscle of the Month: Infraspinatus
September's Muscle of the Month is the Infraspinatus, a muscle which works closely with August's muscle the Supraspinatus. These 2 muscles run parallel to each other within the shoulder group, read on to find out more.
The Infraspinatus is located in the forelimb of the dog on the upper arm, and caudal (closer to the tail) part of the shoulder. Similarly to the Supraspinatus, it is a spindle shaped muscle, round and thick, with a fusiform structure. All of the muscle spindles run parallel in the belly of the muscle (middle portion).
The muscle's origin, where it begins, is on the infraspinous fossa, the area at top of the shoulder blade, and also down the scapula spine, the thin bony spine that runs down the centre of the scapula. The muscle extends down the length of the shoulder blade, crosses the shoulder joint and inserts (end point of attachment) onto the humerus.
What does it do?
As mentioned in last month's blog, the shoulder joint can move in multiple directions, but the main movements produced at the joint are flexion and extension. This allows the forelimb to extend forward in front of the body, and flex up towards the body, bringing the limb backwards. This action at the shoulder is essential to produce free flowing movement, and a good stride length.
The main function of the Infraspinatus is to flex the shoulder, and also to abduct the forelimb, bringing it away from the midline of the body. The muscle also plays a role in stabilising the shoulder joint as part of the rotator cuff. Therefore, the infraspinatus is at work whilst your dog is on the move because in a normal stride pattern the shoulder flexes up and the forelimb retracts (moves backwards), before the shoulder is extended and the forelimb protracts (moves forwards). Activities that require the limb to be moved away from the body, for example navigating weave poles in agility, also works the Infraspinatus.
Can the Infraspinatus become injured?
All muscles can be prone to injury and the Infraspinatus can be strained, overused, or under used and weak. There is one injury that is more specific to this muscle and that is Infraspinatus Muscle Contracture.
This injury is not often seen, but it is more likely to be a problem in highly athletic dogs, for example dogs that train or compete in canine sports. When a dog is running, the shoulder is fully extended ahead of them, and then the Infraspinatus contracts to flex the shoulder and move the body forward. When a dog's activity is high energy or if they have been running for a longer period, for example during a hunt, the muscle can be overworked. This causes tears in the muscle fibres, which will heal via scar tissue formation, if not correctly addressed. Scar tissue is not as functional as muscle tissue, so the Infraspinatus tightens, contracts and becomes firm. This restricts the movement of the shoulder, changing the way they walk and run.
It is thought that Infraspinatus Muscle Contracture may be linked to a lack of prior training or conditioning, before high level activity is carried out. This highlights the importance of keeping our dogs at peak fitness if we do want them to take part in sport, plus making sure they are correctly warmed up before hand. This is something a physiotherapist will be able to advise you on.
If an injury does occur, surgery is the chosen solution. Following this, various modalities can be used to achieve more functional healing of the muscle. Starting with icing of the area to reduce inflammation, followed by range of motion exercises for the shoulder. I also perform stretches for the affected muscle to elongate fibres and prevent scar tissue formation, as well as massage to break down any adhesions that form. This would be followed with a strengthening programme to build muscle mass, and return the limb to its previous condition.
For more information please get in touch, see you for the next 'Muscle of the Month'.