Key Takeaways
- The origin is the less movable (proximal) attachment point of a muscle, typically closer to the midline or trunk
- The insertion is the more movable (distal) attachment point that moves toward the origin during contraction
- Agonist (prime mover) is the muscle primarily responsible for a movement; antagonist opposes that movement
- Synergists assist the agonist in performing a movement; fixators stabilize nearby joints
- Concentric contraction shortens the muscle (lifting phase); eccentric contraction lengthens it under tension (lowering phase)
- Isometric contraction generates force without changing muscle length (holding position)
- The rotator cuff consists of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis (SITS)
- The hamstrings group consists of three muscles: biceps femoris, semitendinosus, and semimembranosus
Muscle Origins, Insertions & Actions
Kinesiology is the study of human movement. For massage therapists, understanding how muscles produce movement through their attachments and actions allows for more effective assessment and treatment.
Origins and Insertions
Every skeletal muscle has at least two attachment points:
- Origin: The less movable (usually proximal) attachment point. Typically closer to the trunk or midline of the body. During contraction, the origin remains relatively stationary.
- Insertion: The more movable (usually distal) attachment point. Moves toward the origin during contraction.
Memory tip: Think of the origin as the "anchor" and the insertion as the "mover."
Exceptions
In some movements, the typical origin-insertion relationship is reversed. This is called reversed muscle action. Example: When doing a pull-up, the arms (insertion) remain relatively fixed while the torso (origin) moves toward them.
Muscle Roles in Movement
| Role | Definition | Example (Elbow Flexion) |
|---|---|---|
| Agonist (Prime Mover) | Muscle primarily responsible for producing the movement | Biceps brachii |
| Antagonist | Muscle that opposes the agonist; must relax for movement to occur | Triceps brachii |
| Synergist | Muscle that assists the agonist in producing the movement | Brachialis, brachioradialis |
| Fixator (Stabilizer) | Muscle that stabilizes a joint so the agonist can work efficiently | Rotator cuff stabilizing the shoulder |
Understanding Muscle Relationships
- The agonist-antagonist relationship is movement-specific: the biceps is the agonist for elbow flexion, but the antagonist for elbow extension.
- Reciprocal inhibition ensures that when the agonist contracts, the antagonist relaxes reflexively.
Types of Muscle Contraction
| Type | Description | Muscle Length | Example |
|---|---|---|---|
| Concentric | Muscle shortens while generating force | Shortens | Lifting a dumbbell (biceps curl up) |
| Eccentric | Muscle lengthens while generating force | Lengthens | Lowering a dumbbell slowly |
| Isometric | Muscle generates force without changing length | Unchanged | Holding a dumbbell at 90 degrees |
Clinical relevance: Eccentric contractions are associated with greater muscle damage and delayed onset muscle soreness (DOMS). Massage can help with recovery from eccentric exercise.
Key Muscle Groups — Origins, Insertions & Actions
Rotator Cuff (SITS Muscles)
The rotator cuff stabilizes the glenohumeral (shoulder) joint:
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Supraspinatus | Supraspinous fossa of scapula | Greater tubercle of humerus | Initiates abduction (first 15 degrees) |
| Infraspinatus | Infraspinous fossa of scapula | Greater tubercle of humerus | External (lateral) rotation |
| Teres minor | Lateral border of scapula | Greater tubercle of humerus | External (lateral) rotation |
| Subscapularis | Subscapular fossa (anterior scapula) | Lesser tubercle of humerus | Internal (medial) rotation |
Quadriceps Group (Anterior Thigh)
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Rectus femoris | Anterior inferior iliac spine (AIIS) | Tibial tuberosity (via patellar tendon) | Extends knee, flexes hip |
| Vastus lateralis | Greater trochanter, linea aspera | Tibial tuberosity | Extends knee |
| Vastus medialis | Linea aspera (medial) | Tibial tuberosity | Extends knee |
| Vastus intermedius | Anterior shaft of femur | Tibial tuberosity | Extends knee |
Hamstrings Group (Posterior Thigh)
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Biceps femoris | Ischial tuberosity, linea aspera | Head of fibula | Flexes knee, extends hip, laterally rotates knee |
| Semitendinosus | Ischial tuberosity | Medial tibia (pes anserinus) | Flexes knee, extends hip, medially rotates knee |
| Semimembranosus | Ischial tuberosity | Medial condyle of tibia | Flexes knee, extends hip, medially rotates knee |
Upper Back & Shoulder
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Trapezius (upper) | Occipital bone, ligamentum nuchae, C7 | Lateral clavicle, acromion | Elevates scapula, extends neck |
| Trapezius (middle) | T1-T5 spinous processes | Acromion, scapular spine | Retracts scapula |
| Trapezius (lower) | T6-T12 spinous processes | Scapular spine | Depresses scapula |
| Rhomboids | C7-T5 spinous processes | Medial border of scapula | Retracts and downwardly rotates scapula |
| Levator scapulae | C1-C4 transverse processes | Superior angle of scapula | Elevates scapula, laterally flexes neck |
Hip Muscles
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Gluteus maximus | Posterior ilium, sacrum, coccyx | IT band, gluteal tuberosity of femur | Extends and laterally rotates hip |
| Gluteus medius | Lateral ilium | Greater trochanter of femur | Abducts hip, medially rotates hip |
| Gluteus minimus | Lateral ilium (deep to medius) | Greater trochanter of femur | Abducts and medially rotates hip |
| Piriformis | Anterior sacrum | Greater trochanter of femur | Laterally rotates hip; when hip is flexed 90, abducts the thigh |
| Iliopsoas | Iliac fossa (iliacus) + T12-L5 vertebrae (psoas major) | Lesser trochanter of femur | Primary hip flexor |
| TFL (tensor fasciae latae) | ASIS, anterior iliac crest | IT band → lateral tibial condyle | Flexes, abducts, medially rotates hip |
Lower Leg and Foot Muscles
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Gastrocnemius | Medial and lateral femoral condyles | Calcaneus (via Achilles tendon) | Plantarflexes ankle, assists knee flexion |
| Soleus | Posterior tibia and fibula | Calcaneus (via Achilles tendon) | Plantarflexes ankle (works independently of knee position) |
| Tibialis anterior | Lateral tibial condyle, interosseous membrane | Medial cuneiform, first metatarsal | Dorsiflexes ankle, inverts foot |
| Peroneus (fibularis) longus | Head and upper fibula | First metatarsal, medial cuneiform | Plantarflexes ankle, everts foot |
Forearm and Hand Muscles
| Muscle | Location | Action |
|---|---|---|
| Wrist flexors | Anterior forearm (medial epicondyle) | Flex wrist and fingers |
| Wrist extensors | Posterior forearm (lateral epicondyle) | Extend wrist and fingers |
| Pronator teres | Medial epicondyle, coronoid process | Pronates forearm |
| Supinator | Lateral epicondyle, proximal ulna | Supinates forearm |
Clinical relevance for massage: Tennis elbow (lateral epicondylitis) involves the wrist extensors at the lateral epicondyle. Golfer's elbow (medial epicondylitis) involves the wrist flexors at the medial epicondyle.
Which muscle is the agonist (prime mover) for elbow flexion?
The four rotator cuff muscles are remembered by the acronym SITS. Which muscle does the "S" at the beginning represent?
During the lowering phase of a biceps curl, the biceps is performing which type of contraction?
Which of the following is the only quadriceps muscle that crosses both the hip and knee joints?
Match each muscle role to its definition.
Match each item on the left with the correct item on the right
The more movable attachment point of a muscle, which moves toward the origin during contraction, is called the ___.
Type your answer below
Which three muscles make up the hamstrings group?
Which of the following rotator cuff muscles perform external (lateral) rotation of the humerus? (Select all that apply)
Select all that apply
Which muscle is the primary hip flexor?
Tennis elbow (lateral epicondylitis) involves inflammation at the origin of which muscle group?
The piriformis muscle is clinically significant because when it is tight or in spasm, it can compress the: