Does exercise produce hypertrophy and an increase in the stiffness of hamstring tendons?

Background: Anterior Cruciate ligament (ACL) tears are the most common ligamentous injury in the knee in athletes. To return to active sporting activity, ACL reconstructions are often necessary. Hamstring tendon autografts are most commonly used in Anterior Cruciate Ligament reconstruction [3] . An important consideration during ACL reconstruction is the size of the hamstring graft. Graft sizes of less that 8mm are reported to be a significant risk factor for graft failure [4] . Therefore, one should maximize the size of the tendon graft to avoid complications of graft failure. We hypothesize that 12 weeks of resistance training can produce hamstring tendons (gracilis, semitendinosus) hypertrophy and increase their stiffness. Methods: We conducted an interventional cohort study where subjects underwent 12 weeks of hamstring resistance training. A standardised ultrasound technique was used to measure the cross sectional tendon area of the gracilis and semitendinosus tendons before and after the intervention. Tendon elastography was used to measure tendon modulus. The primary outcome measure was the tendons’ cross-sectional area and secondary outcome measure was the tendon modulus. Results: 9 participants completed the study. The median increase in crosssectional area for left Semitendinosus, right Semitendinosus, and left Gracilis was 0.02mm2 (IQR = 0.07) (p-value = 0.76), 0.01mm2 (IQR =0.03) (p-value = 0.73) and 0.01mm2 (IQR = 0.02) (p-value = 0.44) respectively. There was a mean decrease in cross-sectional area of the right gracilis of 0.01mm2 . Utilising tendon elastography with the ultrasound, the tendon modulus was measured and had increase in tendon modulus in the left and right Semitendinosus and right gracilis (-4.58 (p=0.03), -3.32 (p=0.13) and -0.03 (p=0.66) respectively). The tendon modulus had a mean decrease in the left gracilis of 0.7 (p=1). Conclusion: 12 weeks of resistance training resulted in potentially clinically significant increase in semitendinosus cross-sectional area. These findings suggest that it is possible to increase the cross-sectional area of hamstring tendons through resistance training, thus increasing their mechanical strength for better ACL reconstruction outcomes. Larger patient numbers are required to determine if statistically significant tendon hypertrophy can be achieved.