INTRODUCTION: The human patellar tendon is highly adaptive to changes in habitual loading, but little is known about its acute mechanical response to exercise. This research evaluated the immediate transverse strain response of the patellar tendon to a bout of resistive quadriceps exercise. METHODS: Twelve healthy adult males (mean age, 34.0 ± 12.1 yr; height, 1.75 ± 0.09 m; and weight, 76.7 ± 12.3 kg) free of knee pain participated in the research. A 10- to 5-MHz linear array transducer was used to acquire standardized sagittal sonograms of the right patellar tendon immediately before and after 90 repetitions of a double-leg parallel-squat exercise performed against a resistance of 175% bodyweight. Tendon thickness was determined 20-mm distal to the pole of the patellar, and transverse Hencky strain was calculated as the natural log of the ratio of post- to preexercise tendon thickness and expressed as a percentage. Measures of tendon echotexture (echogenicity and entropy) were also calculated from subsequent grayscale profiles. RESULTS: Quadriceps exercise resulted in an immediate decrease in patellar tendon thickness (P < 0.05), equating to a transverse strain of -22.5% ± 3.4% and was accompanied by increased tendon echogenicity (P < 0.05) and decreased entropy (P < 0.05). The transverse strain response of the patellar tendon was significantly correlated with both tendon echogenicity (r = -0.58, P < 0.05) and entropy after exercise (r = 0.73, P < 0.05), whereas older age was associated with greater entropy of the patellar tendon before exercise (r = 0.79, P < 0.05) and a reduced transverse strain response (r = 0.61, P < 0.05) after exercise. CONCLUSIONS: This study is the first to show that quadriceps exercise invokes structural alignment and fluid movement within the matrix that are manifested by changes in echotexture and transverse strain in the patellar tendon.