It is best to have a good basic knowledge of exercise physiology when designing and implementing a rehabilitation and/or conditioning
program. Exercise physiology is a discipline that looks at how exercise affects the body. This same science has application
in the field of physical therapy.
At any given time the body performs at a given metabolic level. The systems of the body as a whole are conditioned to maintain
homeostasis at this level. If the demands placed upon the body increase over time, the body adapts and conditions itself to
maintain homeostasis at this new level. An understanding of the physiological changes involved in this process helps us to
develop programs that allow us to regulate the conditioning or reconditioning of the body. They can be used to condition the
body as a whole or to focus on various body segments. In physical therapy, rehabilitative programs can be developed to treat
injuries, to stimulate healing of the injuries, and enhance reconditioning of the injured segment. Due to the abnormal biomechanical
forces acting upon the body during the reparative phases of healing, certain segments become stronger than other segments.
Exercise programs should then be implemented to remedy any conditioning imbalances of the body as a whole.
Muscle is the tissue most affected by exercise activities or disuse associated with injury. Understanding muscle function
and the molecular events of muscle contraction provides a basis for the concepts of physical therapy and exercise physiology.
There are three types of muscle in the body: skeletal, smooth, and cardiac. This chapter will focus on skeletal muscle. Skeletal
muscles in general connect one bone to another bone. Each muscle, itself, is made up of thousands of individual muscle cells.
A muscle fiber, or muscle cell, is long, round, and surrounded by a membrane called the sarcolemma. Inside these cells are
myofibrils, which are made up of filaments composed of protein. These contractile proteins are arranged in units called sarcomeres.
Actin and myosin are the two types of protein chains in the sarcomere. They interact as a result of enzymatic and chemical
reactions to produce muscle cell contraction. Enzymes are protein molecules that specifically interact with the actin and
myosin substrates to allow the chemical reactions to occur. Calcium and phosphate are the chemical components of contraction.
Phosphate is in the form of adenosine triphosphate (ATP). ATP is located at the end of the myosin leverage arm. A calcium
ion opens a receptor site on the actin protein chain. Energy is created when ATP releases a phosphate (P) ion producing adenosine
diphosphate (ADP). The resultant energy allows the ADP to create a bond between the open actin receptor site and the myosin
leverage arm. This bond changes the myosin structure providing a leverage action to produce a muscle contraction between the
two fibers. The ADP is released and the lever arm is freed to reattach. Energy is then required to add a P to the ADP recreating
ATP which is then used for further contractions.
Muscle structure and function
The accumulated contractions of the muscle fibers create contraction of the muscle tissue. Muscle contraction is controlled
by its innervation. One nerve diverges to innervate many muscle fibers. The resultant combination of this nerve and the fibers
it innervates is called a Motor Unit. Muscle contraction as a whole is a result of an accumulation of motor units. The muscle
fibers are grouped together and organized with other fibers by a sheath of connective tissue which is named according to its
level of organization. Endomysium covers each of the muscle fibers themselves, and perimysium separates discrete bundles of
fibers. Epimysium is the connective tissue layer that surrounds the grouped bundles. The fascia is the sheath that covers
the epimysium and serves to protect each muscle from movement over hard structures or movement from adjacent muscles. The
arrangement of these fibers plays a role in the function of each muscle. It is a combination of motor unit group and fiber
arrangement that dictates the resultant type of muscular contraction.
In general, muscle contraction and work is transferred through the tendon. Tendons are dense, parallel-fibered connective
tissue made of collagen. The musculotendinous junction is a layered transition between muscle fibers and the collagen of the
tendons. Frequently there is tendons of origin and insertion running throughout the length of the length of the structure.
Musculotendinous structure is closely tied to functional requirements. Structural shapes of muscle are fusiform and pennate.
Pennate muscles are divided into unipennate, bipennate, and multipennate. Pennate structures allow a muscle to lift great
loads but through a small range of motion as in the vertebae. Fusiform structures have the ability to lift a small load at
a great velocity through a large rang of motion i.e. antebrachium. These two types can work together if both strength and
speed of movement are needed at a particular joint, such as the shoulder or hip joint.