Muscle Anatomy and Actions

These flashcards cover various muscles, their origins, insertions, actions, and attributes.

Tibialis Posterior

Origin: posterior side of the tibia and fibula; Insertion: navicular, cuneiforms, cuboid, and metatarsals 2-4 on the bottom of the foot; Function: powerfully inverts the foot (turns it inwards) and assists in plantarflexion (pointing the foot down), crucially supporting the arch of the foot during weight-bearing activities; Type: deep muscle with a high proportion of slow-twitch fibers for endurance; Primary role: Stability and Power (for arch support and push-off).

Flexor digitorum longus

Origin: posterior surface of the tibia; Insertion: plantar surfaces of the distal phalanges of toes 2-5 (tips of the four smaller toes, excluding the big toe); Function: primarily flexes (curls) the four smaller toes and assists in plantarflexion of the foot (pointing the foot), contributing to grip and push-off; Type: deep muscle, tends to have a mix of fiber types for repetitive action; Primary role: Speed/Agility (fine toe control) and some Power (for push-off).

Gastrocnemius

Origin: posterior surfaces of the medial and lateral condyles of the femur (back of the thigh bone above the knee); Insertion: posterior surface of the calcaneus (heel bone) via the Achilles tendon; Function: a powerful plantarflexor of the foot (pointing the foot down, essential for jumping, running, and walking) and also assists in knee flexion (bending the knee); Type: superficial muscle with a significant proportion of fast-twitch fibers for explosive movements; Primary role: Power (jumping, propulsion) and Speed/Agility (running).

Piriformis

Origin: anterior surface of the sacrum (front of the triangular bone at the base of the spine); Insertion: superior aspect of the greater trochanter of the femur (top prominent bump on the thigh bone); Function: primarily externally rotates the thigh (turns the leg outwards) and also assists in hip abduction (pulling the leg away from the body) when the hip is flexed, crucial for hip stability; Type: deep muscle, generally slow-twitch dominant for postural support; Primary role: Stability (hip external rotation).

Gluteus Minimus

Origin: outer surface of the ilium (outer surface of the hip bone); Insertion: anterior aspect of the greater trochanter of the femur (top bump of the thigh bone); Function: powerfully abducts the thigh (pulls the leg outward away from the body) and internally rotates the thigh (slight inward turn), crucial for stabilizing the pelvis during walking and single-leg stance; Type: deep muscle, primarily slow-twitch for endurance and stabilization; Primary role: Stability (pelvic stabilization, hip abduction).

Semitendinosus

Origin: ischial tuberosity (sit bone of the pelvis); Insertion: medial surface of the proximal tibia (inner upper shin); Function: extends the thigh (moves the thigh backward) at the hip and flexes the knee (bends the knee), also internally rotates the flexed knee, contributing to propulsion; Type: superficial muscle with a mixed composition, leaning towards fast-twitch for powerful movements; Primary role: Power (hip extension) and Speed/Agility (knee flexion).

Biceps Femoris

Origin: long head from the ischial tuberosity (sit bone) and short head from the linea aspera (back of the thigh bone); Insertion: head of the fibula; Function: extends the thigh (moves the thigh backward) at the hip (long head only) and powerfully flexes the knee (bends the knees), also externally rotates the flexed knee, vital for sprinting and powerful leg movements; Type: superficial muscle, generally fast-twitch dominant for explosive power; Primary role: Power (hip extension, knee flexion) and Speed/Agility (running).

Gluteus Maximus

Origin: posterior gluteal line of the ilium, sacrum, and coccyx (back of the pelvis and tailbone); Insertion: gluteal tuberosity of the femur (back of the thigh bone) and the iliotibial (IT) band (side leg band); Function: a powerful extensor of the thigh (moves the leg backward), crucial for standing up from a seated position, climbing stairs, and kicking backward, also an external rotator of the hip; Type: superficial and generally fast-twitch dominant, capable of generating immense force; Primary role: Power (hip extension, climbing, jumping).

Tensor Fasciae Latae

Origin: anterior superior iliac spine and outer lip of the iliac crest (front edge of the hip bone); Insertion: iliotibial (IT) band on the outside of the thigh; Function: abducts the thigh (moves leg outward), flexes the thigh (moves leg forward), and internally rotates the thigh (slight inward turn), primarily contributing to hip and knee stability through the IT band during gait; Type: superficial muscle with a mixed fiber composition, important for both dynamic movement and sustained tension; Primary role: Stability (hip and knee stabilization) and Speed/Agility (hip flexion, abduction).

Rhomboid Major

Origin: spinous processes of thoracic vertebrae T2-T5 (upper back spine bones); Insertion: medial border of the scapula (inner border of the shoulder blade); Function: retracts the scapula (pulls shoulder blades together towards the spine), elevates the scapula (slight upward movement), and downwardly rotates the scapula, essential for good posture and shoulder girdle stability; Type: deep muscle, predominantly slow-twitch for sustained postural control; Primary role: Stability (scapular stabilization, posture).

Latissimus Dorsi

Origin: spinous processes of T7-T12, thoracolumbar fascia, iliac crest, and inferior three ribs (lower back and hip area); Insertion: intertubercular groove of the humerus (front top of the upper arm bone); Function: powerfully extends the arm (pulls arms back), adducts the arm (pulls arms down towards the body), and internally rotates the arm (pulls arms inward towards the midline), important for pulling activities, climbing, and swimming; Type: superficial muscle with a significant proportion of fast-twitch fibers for powerful pulling actions; Primary role: Power (pulling, climbing, swimming).

Trapezius

Origin: external occipital protuberance, nuchal line, and spinous processes of C7-T12 (back of the head and entire upper spine); Insertion: lateral third of the clavicle (collarbone), acromion, and spine of the scapula (shoulder top and blade ridge); Function: a large, superficial muscle with multiple actions including elevating the scapula (shrugs shoulders), retracting the scapula (pulls blades together), depressing the scapula, and rotating the neck/head, critical for upper body posture and movement; Type: superficial muscle with mixed fiber types, varying per region (e.g., upper trap for shrugging, middle/lower for stabilization); Primary role: Stability (scapular control, posture) and Power/Speed (shrugging, head movement).

Deltoid

Origin: lateral third of the clavicle and acromion and spine of the scapula (collarbone and top of the shoulder blade); Insertion: deltoid tuberosity of the humerus (outer middle of the upper arm); Function: a large triangular muscle with three heads: anterior fibers flex and internally rotate the arm, middle fibers abduct the arm (lifts arms to the side), and posterior fibers extend and externally rotate the arm (lifts arms backward), enabling a wide range of arm movements; Type: superficial muscle with mixed fiber types, allowing for both sustained activity and powerful lifts; Primary role: Power (lifting, throwing) and Speed/Agility (versatile arm movements).

Teres Major

Origin: posterior surface of the inferior angle of the scapula (bottom corner of the shoulder blade); Insertion: medial lip of the intertubercular groove of the humerus (front inside part of the upper arm); Function: adducts the arm (pulls arms down towards the body) and internally rotates the arm (pulls arms inward), also assists in extension of the arm, often called the 'Lat's little helper'; Type: deep muscle, mixed fiber types for powerful arm movements; Primary role: Power (arm adduction, extension, internal rotation).

Pectoralis Minor

Origin: anterior surfaces of ribs 3-5 (upper ribs); Insertion: coracoid process of the scapula (hook-shaped bump on the shoulder blade); Function: depresses the scapula (pulls shoulder blades down) and protracts the scapula (pulls shoulder blades forward), assisting in forced inspiration and contributing to shoulder stability; Type: deep muscle, often slow-twitch dominant for postural stabilization; Primary role: Stability (scapular stabilization, posture).

Serratus Anterior

Origin: lateral surface of ribs 1-8 or 9 (side ribs); Insertion: medial border of the scapula (inner edge of the shoulder blade); Function: protracts the scapula (moves blade forward around the rib cage), crucial for pushing actions (e.g., punching) and preventing 'winging' of the scapula, also upwardly rotates the scapula; Type: superficial muscle with a mixed fiber composition, important for both sustained stabilization and dynamic pushing; Primary role: Stability (scapular stabilization, preventing winging) and Power (pushing).

Rectus Abdominis

Origin: pubic crest and pubic symphysis (front of the pelvis); Insertion: xiphoid process and costal cartilages of ribs 5-7 (lower ribs and bottom of the sternum); Function: flexes the trunk (bends body forward, as in a sit-up) and compresses the abdominal contents, contributing significantly to core stability and forced exhalation; Type: superficial muscle, mixed fiber types for both dynamic movement and sustained abdominal tension; Primary role: Power (trunk flexion) and Stability (core compression, posture).

External Abdominal Oblique

Origin: external surfaces of ribs 5-12 (sides of ribs); Insertion: linea alba, pubic tubercle, and anterior half of the iliac crest (top of pelvis and center seam of abs); Function: compresses the abdominal wall, flexes the trunk, and unilaterally rotates the trunk to the contralateral side (twists and bends torso), playing a key role in rotational movements and core bracing; Type: superficial muscle, mixed fiber types for dynamic rotational power and core stability; Primary role: Power (trunk rotation, flexion) and Stability (core compression).

Sternocleidomastoid

Origin: manubrium of the sternum and medial third of the clavicle (top of sternum and inner collarbone); Insertion: mastoid process of the temporal bone (bone bump behind ear) and lateral half of the nuchal line; Function: unilaterally rotates the head to the opposite side and laterally flexes the head to the same side (turns head to the side), bilaterally flexes the neck (helps nod the head forward) and assists in forced inspiration, vital for head positioning; Type: superficial muscle with a mixed fiber composition for both rapid movements and sustained head posture; Primary role: Stability (head posture) and Speed/Agility (head turning).

Orbicularis Oculi

Origin: nasal part of the frontal bone, frontal process of the maxilla, and lacrimal bone; Insertion: surrounding skin around the orbit (circular around the eye); Function: closes the eyelids (palpebral part for gentle closing like blinking) and tightly closes the eyelids (orbital part for winking or squinting), essential for eye protection and facial expression; Type: superficial muscle, predominantly fast-twitch for rapid blinking and fine movements, but also capable of tonic contraction; Primary role: Agility (fine motor control of eyelids) and Protection.