Muscle strain is often caused by over stretching, during which actin and myosin heads are pulled to the extent that they are no longer overlapping, possibly to the point of causing tears in muscle tissue. Tears in ligaments are sprains, rather than strains.

The only type of muscle that is not striated is smooth muscle. Smooth muscle does not use sarcomeres for contraction - rather, each muscle cell is a spindle that is covered in a mesh of contractile fibrils. These fibrils contract in unison when calcium enters the cell.

The smallest contractile unit in muscle tissue is the sarcomere. Myofibrils are made up of many sarcomeres attached end-to-end at a series of dark lines (hence the term "striated") called Z lines. Each sarcomere contains actin and myosin filaments, which pull together during contraction to shorten the sarcomere.

The sliding filament theory of muscle contraction involves the myosin head attaching to actin and then pulling during the power stroke. Troponin is a protein attached to tropomyosin, a thin strand wrapping around the actin filament. When calcium enters the cell, troponin moves toward it, pulling the tropomyosin strand away from actin binding sites and allowing the myosin head to bind. 

During a muscle cell action potential, calcium enters the cell via t-tubules, which are specialized invaginations of the sarcoplasmic reticulum. Calcium binds with troponin, which pulls the tropomyosin strand away from actin binding sites and allows myosin heads to bind. Neither sodium nor potassium bind to troponin, and ATP and ADP both bind to myosin, rather than troponin.

Muscle cells have a specialized endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic reticulum regulates the calcium ion concentration in the cytoplasm of striated muscle cells, and so plays a significant role in muscle contraction and relaxation. The T-tubule is a specialized invagination of the sarcoplasmic reticulum, and the sarcomere is the single contractile unit of a muscle fibril. There is no muscle structure called the mycoplasmic reticulum.

The intervertebral disc articulate with the vertebrae via cartilaginous symphysis joints. These joints are amphiarthrotic, meaning that they allow for slight mobility. 

Synovial joints are all diarthrotic and characterized by a flexible joint capsule filled with synovial fluid. Synarthrotic joints have no movement - examples include the skull bones and the joints of the teeth to the jaw.

The head of the femur articulates with the acetabulum, a concave surface on the pelvis formed by the union of three bones: the ilium, the ischium, and the pubis. 

The obturator foramen is a large opening in the pelvis formed by the pubis and the ischium bilaterally. It does not form any joints but rather allows the passage of the obturator artery, nerve, and vein.

The glenoid fossa is the surface onto which the head of the humerus articulates in the shoulder, and the patella is a part of the knee.

Bursae are synovial fluid sac between bones and overlying tissues. They provide a cushion between the bone and tendons or musculature around them, allowing for reduced friction and painless movement.

A tendon is a connective tissue point of attachment of muscle to bone, while a ligament is a connective tissue attachment from bone to bone. A capsule is a fibrous, fluid filled structure surrounding a synovial joint.  

Bruxism, or involuntary tooth grinding, can cause inflammation and damage in the temporomandibular joint, which is the articulation between the condyle of the mandible and the temporal bone. 

The xiphisternal joint is between the xiphoid process and the body of the sternum. The intermetacarpal joints are formed between the metacarpal bones of the hand, and the pisotriquetral joint is between the pisiform and triquetrum of the wrist.