Locomotor System: Muscles

Muscular System

Muscles in the head

• Frontal: it moves the scalp forwards. It raises eyebrows.  It wrinkles the forehead skin.
• Occipital: it moves the scalp backwards. It is joined to the frontal muscle by the aponeurosis that covers the upper part of the cranium. 
• Nasal: there are two nasal bones. They wrinkles the nose.
• Buccinator: it inflates the cheeks.
• Orbicularis oculis: it closes the eyes.
• Orbicularis oris: it closes and presses the lips together. It can also push them forwards.
• Risorius: it pulls the lip commissure sideways. Like smiling
• Supercilii: it pulls the internal part of the eyebrows down. Like frowning.
• Zygomatic major: it moves the extremes of the lips upwards. Like laughing.
• Masseter: it closes the mouth raising the mandible.
• Temporal: it raises and retracts the mandible. If one of them relaxes and the other contracts, the mandible moves laterally.
• Levator palpebrae superioris: it raises the upper eyelid.
• Levator lavi superioris: it raises the upper lip.
• Digastric: it raises the hyoid bone and descends the mandible to open the mouth.
• Mentalis: it raises the central part of the lower lip. 

Muscles in the neck

• Sternocleidomastoid: it rotates the head. They move the head forwards (flexion) if both muscles contract at the same time.
• Splenius: their movements is the lateral flexion of the head. They are responsible for the extension of the head if both muscles contract at the same time.
• Trapezius: this muscle can pull the head back. It can also move the shoulders, raising them.
• Scalene (anterior, medius, posterior): their movements are flexion and rotation of the neck. They also move the ribs, helping during breathing.

Muscles of the anterior thorax (chest)

• Pectoralis major: their main movements are flexion, adduction and internal rotation of the arm. 
• Deltoid: their main movements are abduction, extension and internal rotation of the arm.
• Rectus abdominis: it is responsible for flexing the spine and compressing the abdomen during forced expiration.
• Oblique (external and internal): when they contract at the same time, they compress the abdomen. When the contraction is only carried out by the muscles on one side, they laterally flex the thorax. 
• Serratus: they rotate the scapula and raise the ribs if the scapula is fixed. 

Muscles of the posterior thorax (back)

• Latissimus dorsi: it is responsible for extension, adduction and internal rotation of the arm.
• Trapezius: it raises the clavicle, adduces the scapula or rotates it from down to up. It can raise or descend the scapula, depending on the part of the muscle that contracts. The uppermost part of the muscle can rotate the head.
• Rhomboid (major and minor): they adduce and rotate from up to down the scapula.
• Infraspinatus: it is responsible for the external rotation and adduction of the arm.
• Teres major: it is responsible for the extension and internal rotation of the arm.
• Teres minor: it is responsible for the external rotation, extension and adduction of the arm. 

Muscles in the arm

• Biceps brachii: it is responsible for the flexion of the elbow and the supination of the forearm. It also flexes the arm.
• Triceps brachii: it is responsible for the extension of the elbow. It also extends the arm.
• Brachialis: it flexes the forearm.
• Supinator: it is responsible for the flexion and supination of the forearm.
• Anconeus: it is responsible for the extension of the forearm.
• Radial muscles: they are responsible for extending the wrist and fingers.
• Extensor carpi radialis: it extends and adduces the wrist.
• Flexor carpi radialis: it flexes and adduces the wrist.
• Flexor digitorum: they flex the fingers.
• Extensor digitorum: they extend the fingers.
• Palmaris longus: it flexes the wrist.
• Pronatus teres: it is responsible for the pronation of the hand.
• Thenar eminence: they adduce the thumb and are responsible for the opposition of this finger.
• Hipothenar eminence: they adduce the little finger.  

Muscles in the leg

• Psoas major: it is responsible for the flexion and external rotation of the leg. It also collaborates in the flexion of the spine when the leg is fixed.
• Iliacus: it is responsible for the flexion and external rotation of the leg. It also aids in the flexion of the spine when the leg is fixed.
• Pectineus: it is responsible for the flexion and adduction of the leg. 
• Gluteus maximus: it is responsible for the extension and external rotation of the leg.
• Gluteus medius and minimus: it is responsible for the adduction and internal rotation of the leg.
• Tensor fasciae latae: it is responsible for the flexion and abduction of the leg.
• Adductor magnus: it is responsible for the adduction of the leg. It also helps in the flexion and rotation of the leg.
• Quadriceps femoris: this is a four headed muscle in the frontal part of the thigh. It is responsible for the extension of the knee joint, or the flexion of the leg if the knee joint is fixed. It is made up of four subunits:
• Rectus femoris.
• Vastus lateralis.
• Vastus medialis.
• Vastus intermedius.
• Sartorius: it is responsible for flexing and rotating the leg (in one simple contraction). It is the movement performed when the legs are crossed. 

• True hamstrings: muscular group made up of three muscles responsible for the flexion of the knee joint or the extension of the leg when the knee is fixed. It is made up of three muscles:
• Biceps femoris.
• Semitendinosus.
• Semimembranosus.

• Tibialis anterioris: it is responsible for the flexion and inversion of the foot.
• Peroneus (longus and brevis): it is responsible for the plantarflexion and eversion of the foot.
• Soleus: it is responsible for the plantarflexion of the foot.
• Gastrocnemius: it is responsible for the extension of the foot or the flexion of the knee joint if the ankle joint is fixed.
• Plantaria: it flexes the sole.
• Flexor digitorum: it flexes the toes.
• Extensor digitorum: it extends the toes.