Abstract
Context:
There has been a resurgence of vitamin D deficiency among infants, toddlers and adolescents in the United Kingdom. Myopathy is an important clinical symptom of vitamin D deficiency, yet it has not been widely studied.

Objective:
To investigate the relationship of baseline serum 25-hydroxyvitamin D concentration (25(OH)D) and parathyroid hormone (PTH) with muscle power and force.

Design:
Cross sectional study.

Setting:
Community based in a secondary school.

Participants:
Ninety-nine post-menarchal 12-14 year old females.

Main Outcome Measures:
Jumping mechanography to measure muscle power, velocity, jump height and Esslinger Fitness Index (EFI) from a two legged counter movement jump and force from multiple one-legged hops. Body height, weight and serum concentrations of 25(OH)D, PTH and calcium (Ca) were measured.

Results:
Median serum 25(OH)D concentration was 21.3nmol/L, range 2.5 – 88.5nmol/L and PTH 3.7pmol/L (range 0.47 – 26.2pmol/L). After correction for weight using a quadratic function there was a positive relationship between 25(OH)D and jump velocity (p=0.002), jump height (p=0.005), power (p=0.003), Esslinger Fitness Index (p=0.003) and force (p=0.05). There was a negative effect of PTH upon jump velocity (p=0.04).

Conclusions:
From these data we conclude that vitamin D was significantly associated with muscle power and force in adolescent girls.