Patellar tendon adaptation in relation to load-intensity and contraction type

Background: Loading leads to tendon adaptation but the influence of load-intensity and contraction type is unclear. Clinicians need to be aware of the type and intensity of loading required for tendon adaptation when prescribing exercise. The aim of this study was to investigate the influence of contraction type and load-intensity on patellar tendon mechanical properties. Method: Load intensity was determined using the 1 repetition maximum (RM) on a resistance exercise device at baseline and fortnightly intervals in four randomly allocated groups of healthy, young males: (1) control (no training); (2) concentric (80% of concentric-eccentric 1RM, 4 x 7-8); (3) standard load eccentric only (80% of concentric-eccentric 1RM, 4 x 12-15 repetitions) and (4) high load eccentric (80% of eccentric 1RM, 4 x 7-8 repetitions). Participants exercised three times a week for 12 weeks on a leg extension machine. Knee extension maximum torque, patellar tendon CSA and length were measured with dynamometry and ultrasound imaging. Patellar tendon force, stress and strain were calculated at 25%, 50%, 75% and 100% of maximum torque during isometric knee extension contractions, and stiffness and modulus at torque intervals of 50-75% and 75-100%. Within group and between group differences in CSA, force, elongation, stress, strain, stiffness and modulus were investigated. The same day reliability of patellar tendon measures was established with a subset of eight participants. Results: Patellar tendon modulus increased in all exercise groups compared with the control group (p < 0.05) at 50-75% of maximal voluntary isometric contraction (MVIC), but only in the high eccentric group compared with the control group at 75-100% of MVIC (p < 0.05). The only other group difference in tendon properties was a significantly greater increase in maximum force in the high eccentric compared with the control group (p < 0.05). Five repetition maximum increased in all groups but the increase was significantly greater in the high load eccentric compared with the other exercise groups (p < 0.05). Conclusion: Load at different intensity levels and contraction types increased patellar tendon modulus whereas muscle strength seems to respond more to load-intensity. High load eccentric was, however, the only group to have significantly greater increase in force, stiffness and modulus (at the highest torque levels) compared with the control group. The effects and clinical applicability of high load interventions needs to be investigated further. (C) 2013 Elsevier Ltd. All rights reserved.