Serum relaxin levels affect the in vivo properties of some but not all tendons in normally menstruating young women.

Relaxin (hRLX) is a hormone reported to affect collagen synthesis. Its effects are also thought to be modulated by other sex hormones, including oestrogen, which has previously been found to be associated with alterations of in vivo tendon properties. There is thus a potential for hRLX to impact on collagen, which could result in tendon structural and mechanical properties being modified. The present study therefore aimed to determine any interaction between hRLX and tendon stiffness, in normally menstruating women (n = 12). Tendon properties were determined using a combination of dynamometry and B-mode ultrasound, whilst serum hRLX levels were established by ELISA. Serum hRLX level was seen to be negatively associated with patellar tendon stiffness (r = -0.56; P < 0.001), explaining 31% of the variance in this parameter. There was no association between hRLX and gastrocnemius tendon stiffness (P > 0.05), or with the cross-sectional area of either of the two tendons (P > 0.05). In young, normally menstruating women, hRLX appears to have a significant effect on the patellar but not the gastrocnemius tendon stiffness. Where it has an effect, this appears to be on the intrinsic properties rather than on the dimensions of said tendon. Future work to elucidate the physiological cause of this selectivity in the impact of relaxin will be key to mapping the impact of the endocrine system on the phenotype of tendinous tissue.