The patellar tendon is one of the most common donor sites for anterior cruciate ligament (ACL) surgery (Kapoor et al., 2004). This surgical procedure involves removal of the central portion from the patellar tendon for insertion as the new ACL graft. In recent years a number of studies have illustrated the adaptability of tendons to changes in loading conditions (e.g., Reeves 2006, Arampatzis et al., 2007; Kongsgaard et al., 2007; Couppe et al., 2008). Intuitively, after ACL surgery, the stiffness of the donor patellar tendon should decrease, however, it remains unknown in humans whether this tendon ever recovers its mechanical properties and further how this relates to recovery of in-series muscle function. We investigated the mechanical properties of the human patellar tendon and the morphology and function of the knee extensors in 12 males (mean ± SD age: 37 ± 14 years) who had undergone surgical reconstruction of the ACL using a patellar-tendon bone graft between 1 and 10 years before the study (OP), with the uninjured contralateral leg serving as a control (CTRL). All subjects gave written, informed consent. Patellar tendon mechanical properties were assessed in vivo by combining dynamometry with ultrasound imaging. Patellar tendon stiffness was calculated from the gradient of the tendon’s force-elongation curve. Tendon stiffness was normalised to the tendon’s dimensions to obtain the tendon’s Young’s modulus. Knee extensor muscle cross-sectional area (CSA) was assessed using magnetic resonance imaging. Voluntary activation level of the knee extensors was assessed using electrical muscle stimulation. CSA of the OP patellar tendons was larger by 21% compared to CTRL tendons (OP: 141 ± 35 mm2; CTRL: 112 ± 21 mm2; P<0.01, Paired Student’s t-test). Patellar tendon stiffness was not significantly different between the OP and CTRL tendons (OP; 4386 ± 1514 N.mm-1; CTRL: 4405 ± 1282 N.mm-1), but the Young’s modulus was lower by 24% in OP tendons (OP; 1.7 ± 0.6 GPa; CTRL: 2.1 ± 0.6 GPa; P<0.01). This recovery of tendon stiffness in OP tendons was achieved through a compensatory enlargement of the patellar tendon CSA, presumably due to scar tissue formation. The newly formed tendon tissue had inferior properties as indicated by the reduced Young’s modulus of the whole tendon, but it increased to such a level that enabled recovery of tendon stiffness. Consistent with the recovery of tendon stiffness, knee extensor torque (OP: 228 ± 70 Nm; CTRL: 229 ± 66 Nm), muscle CSA (OP: 44 ± 9 cm2; CTRL: 46 ± 9 cm2) and muscle activation level (OP: 95 ± 2%; CTRL: 96 ± 2%) were not significantly different between OP and CTRL legs. This study highlights the remarkable adaptability of the patellar tendon in recovering its stiffness following surgical intervention. The normalisation of tendon stiffness may be integral to permitting recovery of in-series muscle function.