*IGF1* and *IGFBP3*

The interaction between the Insulin-like Growth Factor 1 (*IGF1*) gene and muscle phenotypes was particularly evident in the intervention studies (Table 3). Both Kostek et al. [50] and Hand et al. [43] demonstrated that carriers of one or more 192 alleles achieved significantly greater KE strength improvements in response to resistance training (RT), compared to non-carriers (*p* = 0.02, *p* < 0.01). However, in a cross-sectional study conducted by Mora et al. [61], no significant differences were observed in muscle strength between carriers and non-carriers of the 192 allele (*p* = 0.024).

Significant associations were also noted for polymorphisms rs35767 of the *IGF1* gene and rs2854744 of the Insulin-like Growth Factor Binding Protein 3 (*IGFBP3*) gene. Kostek et al. [51] observed black females carrying the CC genotype of the rs35767 polymorphism to have significantly less total FFM and muscle cross sectional area (CSA) than TT carriers (both *p* < 0.05). Furthermore, male CC homozygotes performed significantly worse in the single leg chair stand test than carriers of the T allele (*p* < 0.05). In a study conducted by Yang et al. [75], CC carriers of the rs2854744 polymorphism had a 4.3 times higher risk of having low SMI compared to AA carriers (*p* < 0.05).
