text: Chapter 4
  cells called osteoblasts lay down new bone tissue to strengthen the bone, while other cells called osteoclasts
  reabsorb bone tissue. So long as these two processes are in balance, bone continues to strengthen and remain
  healthy.
  4-16. Poor nutrition and excessive physical training can overwhelm the remodeling process and lead to bone
  stress injury. Although bone stress injury is less disabling than a traumatic fracture, bone stress injury is an
  avoidable misuse injury often characterized by pain. Doctors treat these injuries with modified physical
  activity, rest, and adequate and appropriate nutrition. They occur most often in the metatarsal, tibia, femur
  and ilium (pelvic bones). Occurring more often in Soldiers in the initial phase of training, bone stress injury
  resolves as the remodeling process catches up with bone absorption. In some cases, stress injury can progress
  to a fracture such as a tibial stress fracture. Certain types of stress fractures, particularly to the neck of the
  femur, can be medical emergencies with disabling consequences if not immediately stabilized with surgery.
  MUSCLE
  4-17. When muscles contract, they pull on bones and cause movement around joints. For example, a biceps
  muscle contraction repositions the forearm toward the shoulder as the forearm hinges around the elbow joint
  as a bicep curl. The resulting force is called torque. When the whole body moves across a marching surface,
  torque is created around the point where the foot meets the ground. So long as the body keeps being
  repositioned successfully, it will continue to move. If muscles fail to reposition the body’s segments, the
  body will lose balance and fall. The body will not move efficiently in the desired direction and it will
  eventually fatigue or become injured—either by misuse or by traumatic contact with the ground or another
  object.
  4-18. Purposeful movement is a function of repeated coordination of forces generated within the body with
  forces outside the body—intrinsic and extrinsic forces. Practice and coaching builds perception of how to
  better perform basic movements, and subsequently assists with progression to more advanced movements.
  MUSCLE FIBER
  4-19. In contrast to the spherical shape of other cells in the body, a muscle cell or fiber is a relatively long,
  cylindrical microscopic structure. Muscle fibers are packed together in progressively larger groups called
  muscle fascicles that form the whole muscle. Muscles have different roles. Some muscles stabilize body
  segments, others create large limb movements, or and some create small movements of the hands and fingers.
  These small movements are referred to as fine motor control. The function of the muscle will be driven by
  the fiber type. There are three muscle fiber types. They are described in paragraphs 4-20 through 4-22 and in
  figure 4-1.