The Deltoideus muscle (Deltoid) is an intrinsic muscle found in the thoracic limb (forelimb).
Intrinsic muscle = muscle origin + insertion are within the thoracic limb (forelimb).
Anatomy Review: Bony landmarks
Canine scapula
Top Clinical Tip: Always palpate using mindful Therapeutic Touch techniques to explore and assess the canine musculoskeletal (MSK) system. Avoid using gross touch linked to pressure as this will result in false positives and be a negative clinical experience for the dog. LESS is always MORE!
The lateral scapular muscles of the thoracic limb are;
- Deltoid: superficial + easily palpable
- Supraspinatus: deep + mainly covered by the muscles; Trapezius (cervical fibres) + Omotransversarius
- Infraspinatus: deep + mainly covered by Deltoid (scapular part) plus the thoracic fibres of Trapezius
- Teres Minor: deep + covered by Deltoid + Infraspinatus muscles
Muscle attachments
Skeletal muscle attachments; muscles are attached to bone or cartilage by connective tissue attachments which may be a cordlike structure (tendon) or a flat sheet like structure (aponeurosis). Some muscles attach direct to the periosteum of the bone and this is known as fleshy attachments.
Skeletal muscle origin + insertion in the limbs: The origin is the more proximal point (fixed point of attachment) to the muscle insertion (movable point of attachment), which is therefore more distal to the origin.
The canine Deltoid muscle consists of two portions; the scapular and acromial parts and both are superficially placed.
The scapular part is covered by the superficial fascia. It's located between the spine of the scapula and the proximal half of the humerus in the forelimb.
Both parts of Deltoid unite and attach with; a part tendinous, part direct muscle attachment to the bony landmark called the deltoid tuberosity.
Lateral surface of the Deltoid muscle
From the spine of the canine scapula arises an aponeurosis, providing attachment for muscle fibres (myofibres). This aponeurosis blends with the deeply placed Infraspinatus muscle and also covers more than half of the acromial part of Deltoid muscle.
Distal to the shoulder joint, it becomes a more tendinous sheet with two distinct tendinous processes which penetrate deep (medially) into the main part of the muscle.
Medial surface of the Deltoid muscle
The medial surface of both the scapular and acromial parts of Deltoid has an aponeurosis which is thin distally as it attaches to Deltoid tuberosity.
A bursa is occasionally found between the tendons of the acromial part of Deltoid and the Infraspinatus muscle.
Clinical Tip: Deltoid muscle injuries are often confused with a lateral Head of Triceps muscle injury. Use Therapeutic Handling and Touch to carefully palpate these structures and the fascial gutters between the muscles, to accurately locate and analyse the scapular portion of the Deltoid muscle.
Innervation of Deltoid Muscle
The Brachial Plexus (C6 - T2) gives origin to the nerves which supply the thoracic limb (forelimb).
The Deltoid muscle is innervated by the Axillary nerve arising from the Brachial Plexus as a branch from the combined C7 + C8 nerves (C = cervical nerves)
Nerve Route: leaves axillary space caudodistal (below) to the Subscapularis muscle and proximal (above) to the Teres Major muscle.
Motor innervation: shoulder flexors- Deltoideus, Teres Major, Teres Minor (acronym DTT)
Cutaneous innervation: dorso-lateral aspect of the brachium.
Action of Deltoid Muscle
The Deltoid is a shoulder flexor working as part of a group of canine shoulder flexors.
EMG activity in several speeds of canine motion, flexing the shoulder joint, evidences the neuromuscular function of Deltoid as a shoulder joint flexor.
Clinical Tip: It's important to understand the possible biomechanical influences a harness can have on the dog's natural balanced motion. Well fitted Y-Shaped Harnesses optimise the dogs movement, whereas poorly fitted harnesses or styles across the the dog's chest clearly restrict forelimb protraction and limit efficient canine movement.
We explore in-depth the wide range of Therapeutic Uses of the Y-Shaped Harness to improve the quality of each dog's natural balanced stance, postures and motion in Hub 4 as our first in our NEW series videos: Canine Clinical Tools.
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