Undercarboxylated osteocalcin, muscle strength and indices of bone health in older women

•UcOC%, but not absolute ucOC concentration, was associated with increased lower-limb strength and negatively correlated with a-ACT.•The association may support a metabolic link for ucOC in muscle function.•Indices of bone quality from heel ultrasound were inversely associated with tOC, but not ucOC or ucOC%.

We investigated the association between undercarboxylated osteocalcin (ucOC) and lower-limb muscle strength in women over the age of 70 years. The study also aims to confirm the association between bone turnover markers and heel ultrasound measures. A post-hoc analysis using data collected as part of a randomized placebo-controlled trial of vitamin D supplementation. An immunoassay was used to quantify total OC (tOC), with hydroxyapatite pre-treatment for ucOC. We determined associations of absolute and relative (ucOC/tOC; ucOC%) measures of ucOC with lower-limb muscle strength, heel ultrasound measures of speed of sound (SOS) and broadband ultrasound attenuation (BUA), bone turnover markers (BTMs; P1NP and CTx) and the acute phase protein alpha-1-antichymotrypsin (α-ACT). ucOC%, but not absolute ucOC concentration, was positively associated with hip flexor, hip abductor and quadriceps muscle strength (all p < 0.05). ucOC% was negatively associated with α-ACT (β-coefficient = − 0.24, p = 0.02). tOC was positively associated with both P1NP and CTx (p < 0.001). For each per unit increase in tOC (μg/L) there was a corresponding lower BUA, SOS and SI (β-coefficient = − 0.28; − 0.23 and − 0.23, respectively; all p < 0.04). In conclusion, ucOC% is positively associated with muscle strength and negatively associated with α-ACT. These data support a role for ucOC in musculoskeletal interactions in humans. Whilst tOC is associated with bone health, ucOC% and ucOC may also be linked to falls and fracture risk by influencing muscle function.