In order to step up on a step to reach an object on a high shelf, the brain first has to send a nerve impulse from the brain. The pathway is composed of the sensory neuron that receives the message of the action that needs to be completed (Netter, 2006). The sensory neuron subsequently transmits the impulse to the spinal cord, which is connected to the motor neurons. These motor neurons are connected to specific muscles that are responsible in performing a particular action.
Stepping up on a step involves the muscles of the leg, while reaching for an object involves the muscles of the arms and hands. Once the object is reached, the arm and leg are both lowered, returning back to the original position. Muscle fiber contraction involves the sliding of myosin and actin filaments, two structural proteins that are present in a muscle fiber or myofibril. The contraction involves the expenditure of energy, which is in the form of adenosine triphosphate (ATP).
The muscles of the leg involved in stepping up to reach for an object that is positioned on a high shelf include the gastrocnemius, tibialis anterior, extensor digitorum longus and the soleus. These muscles are attached to the tibia and fibula bones of the lower leg. The gastrocnemius muscle, which is responsible for flexion of the ankle, is connected to the tibia through the Achilles tendon. The tibialis anterior and the gastrocnemius muscles are extending, while the soleus muscle is pulling.
On the other hand, the arm that is raised to reach for an object on a high shelf includes the flexor, brachialis, biceps, supraspinatus and the pronator muscles. These muscles are attached to the radio-ulnar bones of the arm. The supinator muscle is connected to the supraspinatus tendon, which in turn connects to the shoulder bone. The pulling muscles include the brachialis, biceps and the supraspinatus, while the extending muscles are the flexor and pronator.