The skeleton is broken down into the axial skeleton, the appendicular skeleton, and the joints of the body.

-The Axial Skeleton is made up of the Skull (Cranium), the Vertebral Column (C1 to the Coccyx), the ribs, and the sternum. The Vertebral column is made up of 5 groups. The Cervical spine has 7 vertebrae, the Thoracic spine has 12 vertebrae, the Lumbar spine has 5 vertebrae, the Sacral spine has 4 fused vertebrae, and the coccygeal group has 3 – 5 vertebrae.

-The Appendicular Skeleton includes the shoulder/pectoral girdle, the pelvic (right and left coxal bones), and the bones of the extremities of the body.

–Joints can be discussed in two different groups. The first group involves the amount of movement the joint allows. The second group discusses the axis or axes of movement for the joint.

1st group

• Fibrous Joints = They allow basically zero movement. Like what we see with the sutures of the skull.
• Cartilaginous Joints = These joints allow limited levels of movement. Like what we see with the intervertebral disks.
• Synovial Joints = Joints allowing a considerable amount of movement, and they make up most of the next grouping of joints that discusses the axes of movement. An example for this joint would be elbows or knees.

2nd group

• Uniaxial Joints rotate over just one axis and are essentially hinges. The elbow is a prime example of this joint.
• Biaxial Joints rotate over two perpendicular axes and the best example is the ankle or wrist.
• Multiaxial joints allow movement in all three axes of space. They include joints like the shoulder, or the hip.

There are three layers of fibrous connective tissue in the body. The first is Epimysium. Epimysium is the outermost layer of connective tissue. This layer is contiguous with the tendons in the body and the muscle as a whole. Tendons attach to bone periosteum and pull from these locations. The epimysium covers the muscle. Beneath our epimysium we have our muscle fibers that are grouped into bundles. These bundles are covered by the Perimysium. This is our second layer of connective tissue. The third and the smallest fibrous connective tissue is the Endomysium. This is inside of the perimysium, and it is used to cover the muscle fibers individually. It is also contiguous with the Sarcolemma.

There are two types of myofilaments: Myosin and Actin. These filaments are contained in the myofibrils and are used for the contraction of muscle cells. Myosin filaments have about 200 molecules of myosin. Myosin filaments have a hinge point, a fibrous tail, and a globular head. This head is used for the interaction it has with actin. Actin strands have two strands in a double helix shape. These two together make up sarcomeres and are organized longitudinally.

For the Sliding filament theory, it is said that the actin filaments on the end of the sarcomeres slide into the myosin, and this shortens the fiber. There are 5 phases of the theory.

• Resting Phase: There is not a lot of calcium in the myofibril, and not many of the myosin cross bridges are binding to actin.
• Excitation Coupling Phase: The sarcoplasmic reticulum releases calcium due to stimulation. Calcium then bonds with troponin. Tropomyosin along the actin shifts, and the cross bridge will now attach quicker to actin.

• Contraction Phase: Pulling action energy results from hydrolysis of ATP.
• Recharge Phase: This happens when calcium is available in the myofibril.
• Relaxation Phase: This happens when the motor nerve ceases to be stimulated. The calcium that was used, is put back to the sarcoplasmic reticulum.

The three types of muscle fibers are Type I, Type IIa, and Type IIx.

• Type I muscle fibers are known as the slow twitch ones. These muscle fibers are very efficient with energy use and resistant to fatigue. They have a high capacity for supply of aerobic energy, but a limit for their potential for rapid force development. This last part is shown in their low activity of ATPase and low anaerobic power.
• Type II muscle fibers (both IIa and IIx) are basically the opposite of Type
  I. They are both known as Fast Twitch fibers. These fibers have lower aerobic power ability. They can develop rapid force, have high ATPase activity and high anaerobic power. The differences in Type IIa and IIx resides in their aerobic oxidative energy supply capacity. The Type IIa fibers have more aerobic metabolism capacity along with increased number of capillaries compared to Type IIx.

For sprinters, we would see more Type II fibers, as it is a fast movement that doesn’t last long. If you look at the body types of sprinters, you will see that they have larger muscles than marathon runners. The athletes running marathons would have more Type I fibers. So, you can see Type II fibers are typically larger compared to Type I.

The All-or-none principle is a principle that deals with the activation of muscles. The principle states that all of the muscle fibers within a muscle unit contract at once and develop their force at the same time. So, one fiber is unable to be contracted without the other fibers that are innervated in the group contracting. In other words, we could say that a stronger action potential is unable to produce a stronger contraction. There either is a contraction, or there isn’t. We can’t have half contractions.

There are three main ways that athletes can use to increase their force production:

• By incorporating training phases with heavier loads. By doing this we are able to optimize our neural recruitment.
• By increasing our muscles’ cross sectional area that is used for the activity we want to improve in. This essentially makes us more efficient with the desired activity.
• By performing multijoint, multiple muscle compound exercise. Doing these explosively will optimize the recruitment of fast twitch muscles.

Proprioception is the information our body takes in regarding the kinesthetic sense of the position of body parts with respect to gravity. All of this information is processed subconsciously. Proprioceptors are the receptors we have in our tendons, muscles and joints. These receptors give us information for proprioception. There are two main proprioceptors are Muscle Spindles and Golgi Tendon Organs.

• Muscle Spindles are special muscle fibers inside a connective tissue sheath. They run parallel to the normal fibers. These fibers give back information regarding the length of the muscle and the rate of change in length. All impulses go to the spinal cord. Since muscle spindles tell the degree of activation, they also tell us the force needed to overcome resistance.
• Golgi tendon organs are proprioceptors located inside of tendons. These are essentially the same as muscle spindles, just with a different location. They give feedback based on stretch and tension within the muscles.
  Neural input from the GTOs inhibits muscle activation. This process is used to protect from too much tension.

The heart is a muscular organ made up of two separate connected pumps. The pumps are known as ventricles. The right ventricle takes care of pumping blood to the lungs. The left ventricle oversees pumping blood into the rest of the body. There are four valves that make up the heart. The tricuspid and the mitral valve prevent the blood flow from the ventricles back to the atria during contraction. The aortic and the pulmonary valve prevent backflow from the ventricles during relaxation. Valves are passively controlled due to the pressure gradient pushing blood against it to close and pushing forward to open. The conduction system is made up of a few different things. The sinoatrial is where the rhythmic electric impulses start. Next is the atrioventricular node where the impulse gets delayed before passing to the ventricles. We also have the atrioventricular bundle which conducts impulses to the ventricles. The left and right bundle branches divide into purkinje fibers that are used to conduct impulses throughout the ventricles.

The skeletal muscle pump is the act of the skeletal muscles assisting with the return of blood back into the heart. It is essential for blood to be pumped back, as the blood is required to fight gravity going back to the heart. Contracting muscles put pressure in specific ways on the veins along with one way valves that prevent backflow. Because of this, it is important to keep moving following exercise.