BACKGROUND: Achilles tendinopathy is a considerable problem for active people. The degenerative processes associated with tendinopathy may be associated with changes in the inherent mechanical properties of the musculotendinous unit. PURPOSE: The purpose of this study was to compare Achilles tendon-aponeurosis strain between male athletes with and without Achilles tendinopathy. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Fifteen healthy men (age, 35 +/- 9 years; height, 1.78 +/- 0.05 m; mass, 79 +/- 11 kg) and 14 men with midportion Achilles tendinopathy (age, 40 +/- 8 years; height, 1.77 +/- 0.06 m; mass, 80 +/- 9 kg) who were all running over 20 km per week participated in the study. Each participant was tested in a single session that involved maximal isometric plantar flexion efforts being performed on a calf-raise apparatus while synchronous real-time ultrasonography of the triceps surae aponeurosis was recorded. Achilles tendon-aponeurosis strain (%) was calculated by dividing tendon displacement during plantar flexion by resting tendon length (intrarater reliability: intraclass correlation coefficient = .92). RESULTS: Participants in the Achilles tendinopathy group (5.2% +/- 2.6%) had significantly (P = .039) higher Achilles tendon-aponeurosis strain compared with the control group (3.4% +/- 1.8%). In contrast, there were no significant between-group differences for maximal isometric plantar flexion force. CONCLUSION: Achilles tendon-aponeurosis strain is higher in male athletes with tendinopathy than those without. The results of this study provide a rationale for current clinical approaches to management of Achilles tendinopathy, whereby repetitive mechanical loading may impart a positive benefit through reduced compliance of the musculotendinous unit.