What is the physiological definition of muscle strength?

From the updated list of Vitamin D indications: Vitamin D is important for bone and muscle development, function and preservation. Vitamin D is required to maintain normal blood levels of calcium and phosphate, which are in turn needed for the normal mineralisation of bone, muscle contraction, nerve conduction, and general cellular function in all cells of the body as well as bone growth and maintenance of bone density.

Janssen, H. C., M. M. Samson, et al. (2002). “Vitamin D deficiency, muscle function, and falls in elderly people.” Am J Clin Nutr 75(4): 611-615.

This review discusses results of studies where muscle strength was measured in one as leg extension power, another as handgrip strength, stair climbing ability and walking distance and presents fresh data where muscle strength was measured in terms of knee extension strength and walking distance.