Chris M, a 20-year-old male, visits his gym regularly

a. (Region of the brain where the upper motor neurons begin that initiate extension at the knee)The corticospinal tract mainly originates from the primary motor cortex (30%), and the premotor cortex (30%); the remainder is divided among several other areas, including the primary somatosensory cortex (postcentral gyrus), supplementary cortex, parietal lobe areas, and portions of the cingulate gyrus. After leaving the cortex, axons of this tract enter the posterior limb of the internal capsule and pass caudally through the brain stem to the ventral surface of the medulla, where they are contained in the medullary pyramids.

b. (Motor pathway and location of neuron-to-neuron synapse (upper motor neuron with lower motor neuron)) At the junction of the medulla and spinal cord, most of the fibers cross the midline to enter the lateral funiculus of the spinal cord and form the lateral corticospinal tract, which extends throughout the length of the cord. The fibers that do not cross continue as far as the thoracic spinal cord in the ventral corticospinal tract. These fibers eventually cross the midline and terminate in the anterior gray column of the spinal cord segments in the cervical and upper thoracic regions. The lateral corticospinal tract descends the length of the spinal cord; its fibers terminate in the anterior gray column of all the spinal cord segments.

c. (Interaction between the neurons) Most corticospinal fibers synapse with internuncial neurons, which, in turn, synapse with alpha motor neurons and some gamma motor neurons. Alpha motor neurons (lower motor neurons) innervate skeletal muscle and cause the muscle contractions that generate movement. (next step is activities in neuromuscular and muscle cell)

( Activities within neuromuscular junction and muscle cell ):

1 Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction. A nerve impulse arrives at the axon terminal; acetylcholine is released and diffuses across the synapse to the sarcolemma.

2. Acetylcholine bonds to acetylcholine receptors and makes the sarcolemma more permeable to Na+ ions, which rush into the cell.

3. The sarcolemma depolarizes, becoming negative outside and positive inside. The T tubules bring the reversal of charges to the interior of the muscle cell.

4. Depolarization stimulates the release of Ca+2 ions from the sarcoplasmic reticulum. Ca+2 ions bond to the troponin-tropomyosin complex, which shifts it away from the actin filaments. Myosin splits ATP to release its energy; bridges on the myosin attach to the actin filaments and pull them toward the center of the sarcomere, thus making the sarcomere shorter.

5 All of the sarcomeres in a muscle fiber shorten—the entire muscle fiber contracts (produces knee extension).

The muscle that contracts is the Quadriceps femoris group of muscle (namely Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) which is innervated the Femoral nerve (nerve that carries the impulse to the prime mover of knee extension).

(Structure of the knee ) Upon contraction of the muscle, the joint being moved is the Tibiofemoral joint/ knee joint , a synovial diarthrodial hinge type of joint with bone articulation of femoral condyles and tibial plateau. The knee joint is a modified type of joint with knee movements such as knee flexion, extension and minimal rotation.

( Description of lever class) The exercise being formed is a type 3 class lever wherein the effort (Quadriceps) is between the load / resistance (40 lbs) and fulcrum( knee joint). This type of lever is most commonly found in the body , mechanical advantage is reduced but more movement is possible. A small movement by effort results in a larger movement of a load and advantage is of speed and distance.