Interactions of Skeletal Muscles in the Body (p. 325)

Muscles only pull; they are not capable of pushing.

The muscle that provides the major force for the specific movement is called the prime mover or the agonist.

Muscles that oppose or reverse a particular movement are called the antagonists.

Synergists help the prime movers by adding extra force to the same movement, or by reducing undesirable or unnecessary movements.

Naming Skeletal Muscles (pp. 325-326)

1. Bone or body region with which the muscle is associated.
2. Shape
3. Relative size (maximus, minimus, longus, and brevis)
4. Direction in which fibers run in relation to an imaginary line of the body, such as the midline.
5. Number of origins (biceps, triceps, or quadriceps)
6. Location of origin and insertion.
7. Action (flexor or extensor)

Muscle Mechanics: Importance of Fascicle Arrangement and Leverage (pp. 326-330; Figs. 10.1-10.3)

In skeletal muscles the common arrangement of the fascicles varies, resulting in muscles with different shapes and functional capabilities. (p. 326; Fig. 10.1)

1. Circular - fascicles are arranged in concentric rings
2. Convergent - a broad origin and fascicles converge toward a single tendon of insertion
3. Parallel - the long axis of the fascicles runs parallel to the long axis of the muscle
4. Fusiform - a spindle-shaped parallel arrangement of fascicles
5. Pennate - fascicles are short and attach obliquely to a central tendon that runs the length of the muscle

The operation of most skeletal muscles involves the use of leverage and lever systems, partnerships between the muscular and skeletal systems. (pp. 326-330; Figs. 10.2, 10.3)

A lever is a rigid bar that moves on a fixed point, or a fulcrum, when a force is applied to it.

The applied force, or effort is used to move a resistance or load.

The distance between the point force (effort) is applied and the fulcrum is the effort arm.

The distance between the load and the fulcrum is the load arm.

The way levers work is a lot like the way a see-saw works - if two people sit on a see-saw at the exact same distance from the center point (the fulcrum) they will balance if they weigh the same. If one person is heavier the see-saw can still be balanced, the heavier person just has to move closer to the fulcrum.

That can be expressed this way: [weight of person #1] X [distance person #1 is from center point (fulcrum)] = [weight of person #2] X [distance person #2 is from center point (fulcrum)]

If person #1 is really heavy (a load) and person #2 wants to move them up in the air it can be done with little effort by adjusting the distance between the load and the center point, or fulcrum (the load arm) and the distance between the place effort will be exerted and the fulcrum (the effort arm).

Since [load] X [length of load arm] = [effort] X [length of effort arm] you could move almost any load with little effort just by making the load arm short and the effort arm long.

That's why Archimedes, the Greek mathematician, physicist, and engineer said "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.”

We use levers everyday for a number of purposes besides moving bones. Some levers allow us to move great loads with little effort, while others require more effort but allow us to move loads very quickly over longer distances.

Power levers operate at a mechanical advantage:

- The effort arm is longer than the load arm, so it takes less force to move a heavier load

- Speed and range of motion are sacrificed

Speed levers operate at a mechanical disadvantage:

- The effort arm is shorter than the load arm, so it takes more force to move a lighter load

- Speed and range of motion are increased

In your body, your joints act as the fulcrums, the bones as the levers, and the muscle contraction as the effort. 

There are three types of levers: first-class, second-class, and third-class.

• 1st class levers have the fulcrum between the load and the effort
• May be power levers or speed levers, depending on placement of the fulcrum

• 2nd class levers the load is between the fulcrum and the effort
• The effort arm is always longer than the load arm, these levers are always power levers
• Uncommon in the body

• 3rd class levers - effort is between the load and the fulcrum, effort arm is always shorter than load arm, these levers are always speed levers.
• Most skeletal muscles act as 3rd class lever systems

Major Skeletal Muscles of the Body (pp. 330-333; Figs. 10.4-10.25; Tables 10.1-10.17)

Muscles of the Head, Part I: Facial Expression (pp. 335-338; Fig. 10.6; Table 10.1)

• Muscles of the scalp include the epicranius consisting of the frontal belly and the occipital belly.
• Muscles of the face include corrugator supercilii, orbicularis oculi, zygomaticus, risorius, levator labii superioris, depressor labii inferioris, depressor anguli oris, orbicularis oris, mentalis, buccinator, and platysma.

Muscles of the Head, Part II: Mastication and Tongue Movement (pp. 338-339; Fig. 10.7; Table 10.2)

• Muscles of mastication include the masseter, temporalis, medial pterygoid, lateral pterygoid, and the buccinator. 
• Muscles promoting tongue movements are the genioglossus, hyoglossus, and the styloglossus.

Muscles of the Anterior Neck and Throat: Swallowing (pp. 340-341; Fig. 10.8; Table 10.3)

• Suprahyoid muscles include digastric, stylohyoid, mylohyoid, and geniohyoid.
• Infrahyoid muscles include sternohyoid, sternothyroid, omohyoid, thyrohyoid, and the pharyngeal constrictor muscles (superior, middle, and inferior).

Muscles of the Neck and Vertebral Column: Head and Trunk Movements (pp. 342-345; Fig. 10.9; Table 10.4)

• Anterolateral neck muscles include the sternocleidomastoid, and scalenes (anterior, middle, and posterior).
• Intrinsic muscles of the back include splenius capitis, splenius cervicis, erector spinae (sacrospinalis), iliocostalis, longissimus, spinalis, semispinalis, and the quadratus lumborum.

Muscles of the Thorax: Breathing (pp. 346-347; Fig. 10.10; Table 10.5)

• Muscles of the thorax include the external intercostals, internal intercostals, and the diaphragm.

Muscles of the Abdominal Wall: Trunk Movements and Compression of Abdominal Viscera (pp. 348-349; Fig. 10.11; Table 10.6)

• Muscles of the anterolateral abdominal wall include the rectus abdominis, external oblique, internal oblique, and the transversus abdominis.

Muscles of the Pelvic Floor and Perineum: Support of Abdominopelvic Organs (pp. 350-351; Fig. 10.12; Table 10.7)

• Muscles of the pelvic diaphragm include the levator ani and the coccygeus.
• Muscles of the urogenital diaphragm include the deep transverse perineal muscle and the external urinary sphincter.
• Muscles of the superficial space include the ischiocavernosus, bulbospongiosus, and the superficial transverse perineal muscle.

Superficial Muscles of the Anterior and Posterior Thorax: Movements of the Scapula (pp. 352-353; Fig. 10.13; Table 10.8)

• Muscles of the anterior thorax include the pectoralis minor, serratus anterior, and the subclavius.
• Muscles of the posterior thorax include the trapezius, levator scapulae, and the rhomboids (major and minor).

Muscles Crossing the Shoulder Joint: Movements of the Arm (pp. 354-356; Fig. 10.14; Table 10.9)

• Muscles moving the arm include the pectoralis major, latissimus dorsi, deltoid, subscapularis, supraspinatus, infraspinatus, teres minor, teres major, and the coracobrachialis.

Muscles Crossing the Elbow Joint: Flexion and Extension of the Forearm (p. 357; Fig. 10.14; Table 10.10)

• Posterior muscles include the triceps brachii and the anconeus.
• Anterior muscles include the biceps brachii, brachialis, and the brachioradialis.

Muscles of the Forearm: Movements of the Wrist, Hand, and Fingers (pp. 358-361; Figs. 10.15, 10.16; Table 10.11)

• Anterior superficial muscles include the pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and the flexor digitorum superficialis.
• Anterior deep muscles include the flexor pollicis longus, flexor digitorum profundus, and the pronator quadratus.
• Posterior superficial muscles include the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, and the extensor carpi ulnaris.
• Posterior deep muscles include the supinator, abductor pollicis longus, extensor pollicis brevis and longus, and the extensor pollicis indicis.

Summary of Actions of Muscles Acting on the Arm, Forehead, and Hand (pp. 362-363; Fig. 10.17; Table 10.12)

Intrinsic Muscles of the Hand: Fine Movements of the Fingers (pp. 364-366; Fig. 10.18; Table 10.13)

• Thenar muscles in ball of thumb include the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and the adductor pollicis.
• Hypothenar muscles in ball of little finger include the abductor digiti minimi, flexor digiti minimi brevis, and the opponens digiti minimi.
• Midpalmar muscles include the lumbricals, palmar interossei, and the dorsal interossei.

Muscles Crossing the Hip and Knee Joints: Movements of the Thigh and Leg (pp. 367-372; Figs. 10.19-10.20; Table 10.14)

• Anteromedial muscles include the iliopsoas, which is composed of the iliacus and the psoas major, and the sartorius.
• Muscles of the medial compartment of the thigh include the adductor group, which is made up of the adductor magnus, adductor longus and the adductor brevis, the pectineus, and the gracilis.
• Muscles of the anterior compartment of the thigh include the quadriceps femoris group, which is made up of the rectus femoris, vastus lateralis, vastus medialis and vastus intermedius, and the tensor fasciae latae. 
• Posterior muscles are the gluteal muscles, which include the gluteus maximus, gluteus medius, and the gluteus minimus.
• Lateral rotators include the piriformis, obturator externus, obturator internus, gemellus, and the quadratus femoris.
• Muscles of the posterior compartment of the thigh include the hamstrings, which consists of the biceps femoris, semitendinosus, and the semimembranosus.

Muscles of the Leg: Movements of the Ankle and Toes (pp. 373-378; Figs. 10.21-10.23; Table 10.15)

• Muscles of the anterior compartment include the tibialis anterior, extensor digitorum longus, fibularis (peroneus) tertius, and the extensor hallucis longus.
• Muscles of the lateral compartment include the fibularis (peroneus) longus and the fibularis (peroneus) brevis.
• Superficial muscles of the posterior compartment include the triceps surae, which is composed of the gastrocnemius and the soleus, and the plantaris.
• Deep muscles of the posterior compartment include the popliteus, flexor digitorum longus, flexor hallucis longus, and the tibialis posterior.

Summary of Actions of Muscles Acting on the Thigh, Leg, and Foot (pp. 379-380;  Fig. 10.24; Table 10.16)

Intrinsic Muscles of the Foot: Toe Movement and Arch Support (pp. 381-383; Fig. 10.25; Table 10.17)

• The muscle found on the dorsum of the foot is the extensor digitorum brevis.
• Muscles on the sole of the foot found in the first layer are the flexor digitorum brevis, abductor hallucis, and the abductor digiti minimi.
• Muscles on the sole of the foot found in the second layer are the flexor accessorius (quadratus plantae) and the lumbricals.
• Muscles of the sole of the foot found in the third layer include the flexor hallucis brevis, adductor hallucis, and the flexor digiti minimi brevis.
• Muscles of the sole of the foot found in the fourth layer include the plantar and the dorsal interossei.