*VDR*

Nine studies analysed the association between *VDR* polymorphisms and muscle phenotypes. The first, conducted in 2004 by Roth et al. [67], highlighted a significant association between the rs2228570 (*Fok1*) polymorphism and FFM. Male FF homozygotes had significantly less FFM, appendicular fat-free mass (AFFM) and skeletal muscle index (SMI) compared to f allele carriers (*p* = 0.002, *p* = 0.009, *p* = 0.001 respectively). Furthermore, when classified as sarcopenic, FF carriers were at a two-fold higher risk of being sarcopenic when compared to carriers of the f allele (*p* = 0.03). Hopkinson et al. [45] also found significant interactions between the rs2228570 polymorphism and muscle phenotypes with male FF homozygotes displaying significantly lower knee extensor (KE) strength than f allele carriers (*p* = 0.007). Similarly, Xia et al. [74] found subjects carrying one or more F alleles to have significantly lower handgrip (HG) strength, and FFM (*p* = 0.03, *p* = 0.04 respectively). Furthermore, these individuals had a significantly higher risk of sarcopenia than ff homozygotes (*p* = 0.03). In contrast, a study conducted by Gussago et al. [42] found FF homozygotes to have significantly greater HG strength than f allele carriers (*p* = 0.021).

Significant associations were also identified between the rs1544410 (*Bsm1*) polymorphism and muscle performance phenotypes although, in keeping with the above findings, results were conflicting. In a study conducted by Onder et al. [62], bb homozygotes were significantly less likely to fall than carriers of the B allele (*p* = 0.02). Similarly, in 2010, Barr et al. [27] found females who were homozygous for the b allele to have a significantly lower risk of falling than Bb/BB carriers. These individuals also performed significantly better in the rise from chair and power tests when compared to carriers of B allele (*p* = 0.03, *p* = 0.044 respectively). Contrarily to the above studies, Bahat et al. [26] found KE strength to be significantly higher in BB homozygotes compared to carriers of one or more b alleles (*p* = 0.038).

Additional *VDR* polymorphisms rs7136534 and rs7975232 (*Apa1*) were significantly associated with fall incidence and HG strength respectively (*p* = 0.002, *p* < 0.05) [28,73]. No significant associations were found for the rs731236 (*Taq1*) polymorphism.

#### *Cells* **2020**, *9*, 12

#### Other Genes

Genes encoding the androgen receptor (*AR*), thyrotropin-releasing hormone receptor (*TRHR*) and receptor activity-modifying protein 3 (*RAMP3*) were also shown to associate significantly with skeletal muscle traits (Tables 2 and 3) [57,66,72].

#### 3.5.2. Growth Factor and Cytokine Genes
