What is the definition of stress in terms of tissue properties?

The correct choice defines stress as the internal reaction that occurs in a material—or in this case, biological tissue—when subjected to an external load. Stress reflects how tissues respond to forces applied to them, and it is typically measured as the force per unit area within the tissue. This internal reaction is critical in understanding how tissues withstand loads and how they can potentially be injured under excessive strain.

Internal stress is a foundational concept in biomechanics and athletic training, as it helps practitioners assess the risk of injury and develop appropriate training programs to strengthen tissues. For example, when a muscle contracts against a weight, the internal stress generated within the muscle fibers can lead either to adaptation (and subsequent strengthening) or overuse injuries if the loads exceed the tissue's ability to cope.

The other choices relate to different aspects of the injury process or tissue response, but they do not accurately capture the essence of stress as it pertains to tissue properties. For instance, deformation refers more to strain, which describes changes in shape or length of the tissue rather than the internal response to a load. Pain is a subjective experience that might result from injury or stress but is not a definition of stress itself. Healing time relates to recovery processes, rather than how tissues respond to loads during activity. Thus