Spatial Relationships for Muscular Origin and Insertion

The spatial relationships of muscle origins and insertions help in understanding how these muscles interact with the skeletal system to produce movement.
1) Points of Muscular Origin
The origin of a muscle is the fixed attachment point that remains relatively stationary during muscle contraction. In this diagram:

• Coracoid Process (Scapula)
  • Biceps Brachii (Short Head) originates from the coracoid process, a small bony projection on the anterior scapula.
  • Coracobrachialis also originates from the coracoid process.
  • Spatial Relationship: Both the biceps brachii (short head) and coracobrachialis originate closely together on the coracoid process, making this a critical attachment site for anterior arm muscles.
• Supraglenoid Tubercle (Scapula)
  • Biceps Brachii (Long Head) originates from the supraglenoid tubercle, a region just above the glenoid cavity of the scapula.
  • Spatial Relationship: The long head of the biceps brachii runs laterally along the humeral head through the intertubercular groove before merging with the short head in the upper arm.
• Humerus
  • Brachialis originates from the anterior surface of the humerus, spanning the middle to distal third of the bone.
  • Spatial Relationship: Unlike the biceps brachii, which originates from the scapula, the brachialis originates directly from the humerus, making it a pure elbow flexor without involvement in shoulder movement.

2) Points of Muscular Insertion
The insertion of a muscle is the attachment point that moves when the muscle contracts. In this diagram:

• Radial Tuberosity (Radius)
  • Biceps Brachii (Both Heads) insert onto the radial tuberosity of the radius.
  • Spatial Relationship: The radial tuberosity lies lateral to the ulna, allowing the biceps brachii to supinate the forearm in addition to flexing the elbow.
• Medial Humerus
  • Coracobrachialis inserts onto the medial surface of the humerus.
  • Spatial Relationship: Since the insertion is on the humerus itself, the coracobrachialis acts primarily as a shoulder flexor and adductor.
• Ulnar Tuberosity (Ulna)
  • Brachialis inserts onto the ulnar tuberosity.
  • Spatial Relationship: The ulna remains stationary during supination/pronation, which makes the brachialis a pure flexor of the elbow joint, independent of forearm rotation.

Summary of Spatial Relationships

1. Origins:
   • Coracoid process: Common origin for both the biceps brachii (short head) and coracobrachialis, located anteriorly on the scapula.
   • Supraglenoid tubercle: Origin for the biceps brachii (long head), located above the shoulder joint.
   • Humerus: Origin for brachialis, making it distinct from the other muscles that originate from the scapula.
2. Insertions:
   • Radial tuberosity: Biceps brachii inserts here, allowing elbow flexion and forearm supination.
   • Medial humerus: Coracobrachialis inserts here, affecting shoulder movement.
   • Ulnar tuberosity: Brachialis inserts here, making it a pure elbow flexor.

These spatial relationships help in understanding how each muscle contributes to movement, stability, and function in the upper arm and forearm.

The problem is somewhat complicated by the fact that the spatial relationships in both types of movement remain the same. The points of muscular origin and insertion, therefore, are alike, and yet the condition of the intervening muscle differs. Much confusion on this question has arisen through the popular comparison of a muscle to an elastic band. The analogy holds only to a limited extent. If the property of elasticity, by virtue of which a body returns to its original shape when the force responsible for its preceding alteration ceases to act, were inherent in the muscle, any stretching of a muscle would be accompanied by a constantly increasing resistance, equal exactly to the force responsible for the stretching.
This is the principle upon which all

1. spring balances, (see Figure 1)
2. sling-shots, and
3. similar devices

are based. It is entirely opposed to the principle of relaxation which is discussed in the module on Properties of Muscles, and also to that of coordination, discussed in module 9.

The same opposition is found when the distance between the points of origin and insertion is decreased. If elasticity were an inherent muscular property, the muscle would contract directly with diminution of this distance, whether the movement be made actively or passively. But muscular contraction involves work, and work is always the result of an expenditure of energy. Hence, we should have the muscle, in passive movement, doing the same amount of work as in active movement. It is a condition flatly opposed to the law of least effort .