Acute effect of proprioceptive neuromuscular facilitation stretching on flexibility in ACTN3 R577X genotype

BACKGROUND: The purpose of this study was to examine the association between acute changes in hamstring flexibility caused by proprioceptive neuromuscular facilitation (PNF) stretching and the ACTN3 gene polymorphism. METHODS: To do so, we obtained a sample of twenty-one male students with ACTN3 polymorphism. Eleven had an RR genotype (RR group) and 10 had an XX genotype (XX group) at the ACTN3 locus. Subjects performed PNF (hold relax, HR) hamstring stretching via straight leg raises in the supine position. Specifically, each subject performed three repetitions of isometric muscle contraction (10 seconds), relaxation, and extension (raising the leg and holding) of the target muscle. This exercise was conducted solo and was designed to increase the range of motion (ROM) of the hip joint using a mechanical system. RESULTS: A two-way analysis of variance with group (i.e., RR vs. XX) and time (i.e., pre- vs. post-HR) treated as factors showed that the main effect of group was not significant (P>0.05, effect size (ES) = 0.12) but time was significant (P<0.05, ES=3.22). The interaction between group and time was also not significant (P>0.05, ES=0.16). CONCLUSIONS: We found no effect of the ACTN3 polymorphism on differences in hamstring stiffness (i.e., lengthening) as induced by PNF stretching. We therefore conclude that the mechanical changes occurring in the muscle-tendon complex that are induced by techniques such as PNF stretching do not have a detrimental effect on the flexibility of patients with one or more R allele at the ACTN3 locus.