An excellent study showed why vitamin D helps muscle response.

The image below was photographed from a Vitamin D encyclopedia edited by Feldman and published in 2005.
The original information, as you can see on the image, was published in 2000
The image is located in the File Gallery: Sports

Summary: The muscle force increased by 50% when the person had adequate level of vitamin D

The same electrical pulse was given to a leg muscle before, during, and after the addition of vitamin D to their diet
People with low level of vitamin D initially had the least reaction to the pulse: N = 50 approx
The strength of the reaction to the pulse increased over months of adding vitamin D ( does not mention how much vitamin D)
until the strength of the reaction (the kick) was measured to be N = 75 approx.

This implies that elderly will fall less if they have enough vitamin D
The muscles will put out 50% more force when given a brain signal to react fall type situation.

This increased force is probably due to the vitamin D improving the Calcium metabolism, which is essential for muscle activity.

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See also VitaminDWiki

• all items in the Sports and D category 206 items