The masseter muscle is a facial muscle that plays a major role in the chewing of solid foods (it is one of the muscles of mastication: masseter, temporalis, medial pterygoid, lateral pterygoid). The action of the muscle is to elevate the mandible (raise the lower jaw) which results in closing of the jaws. It connects to the mandible (lower jawbone) and the maxilla/zygomatic arch. 

The levator scapulae is a long skeletal muscle of the shoulder girdle. It originates in the transverse processes of the atlas and axis as well the posterior tubercles of the 3rd and 4th cervical vertebrae (or in simple terms: it originates at the first four cervical vertebrae, and runs along the back and side of the neck). Its function is to elevate the scapula. 

The correct answer is contraction. Muscle cells shorten or contract to move body parts. These contractions can be under voluntary or involuntary control. The primary function of muscle cell contraction is movement, but contraction is responsible for many other important functions like posture, joint stability and heat production. Muscles contract to allow maintenance of postures like sitting and help stabilize joints. Heat is also generated by contraction.

The other answer choices are functions performed by epithelial cells. Epithelial cells contain neuron endings (nerves) that perceive external stimuli and function as sensory receptors. Epithelial cells form coverings over body parts, including the skin, which prevents infection from microbes. They can form taste buds, line the nose and are located in the ear and eye. Epithelial cells serve an absorptive function through active-transport systems to absorb filtered material and transport to the rest of the body. Epithelial cells also secrete fluids necessary for other functions. For example, some epithelial cells secrete mucus, which lubricates body cavities and the passageways they line.

The force velocity relationship measures the velocity of shortening of isotonic contractions when the muscle is challenged with different afterloads (the load against which the muscle must contract.) The velocity of shortening decreases as the afterload increases.

The muscle length-tension relationship measures muscle tension developed during isometric contractions (the muscle is set to fixed lengths and length is held constant). Active tension is the difference between total tension and passive tension. Active tension is the active force created when the muscle contracts. Passive tension is created by stretching the muscle to different lengths. Total tension is the tension developed when the muscle is stimulated to contract at different lengths.

An isometric contraction, as the name implies, is a contraction in which the muscle length (-metric) is kept constant (iso-). In order for this to happen, the joint angle is kept constant, but there is an increase in muscle tension during contraction. Note that during isometric contraction there is no muscle shortening.

Isotonic contractions involve constant tension in the muscle, and a change in length. The two types of isotonic contractions are concentric and eccentric. Concentric contractions involve the shortening of a muscle throughout a contraction, eccentric contractions involve the lengthening of a muscle during contraction.

All of the choices indicate muscles of the arm and hand, but they have different functions. The brachialis is involved in forearm flexion, along with muscles such as the biceps brachii and pronator teres. The triceps are involved with forearm extension, which is the opposite of flexion. The extensor carpi radialis longus is involved in extension and abduction of the hand. Lastly, the extensor carpi ulnaris is involved with extension and adduction of the hand. We can tell that the extensor carpi ulnaris and extensor carpi radialis longus are not involved in flexion because they are named "extensors," and, as stated, extension is the opposite of flexion. 

The extensor digitorum longus is not a muscle of the foot, but of the leg. It functions in extending the toes and dorsiflexing and everting the foot. The other three choices are all muscles of the foot. The extensor hallucis brevis functions in extending the big toe. The abductor digiti minimi functions in abducting the little toe. Lastly, the adductor hallucis functions in adducting the big toe.

Glucose is transported into skeletal muscles via insulin dependent facilitated diffusion. This type of diffusion requires a specific trans-membrane protein to allow for the passage of glucose. In the presence of insulin, these membrane channels allow glucose to move from outside the cell to inside the cell in an effort to lower blood glucose levels.

The signal for muscle contraction causes the release of calcium from the sarcoplasmic reticulum. This calcium floods the cell and is necessary for causing muscle contraction. Parvalbumin, a protein in the cytoplasm, binds to calcium and acts as a slow-releaser of calcium. This binding reaction of calcium with parvalbumin causes the release of heat, which is termed as 'unexplained heat.' The 'unexplained heat' is also known as 'labile heat.'