To gain insight into the adaptive response of human tendon to paralysis, we compared the mechanical properties of the in vivo patellar tendon in six men who were spinal cord-injured (SCI) and eight agematched, able-bodied men. Measurements were taken by combining dynamometry, electrical stimulation, and ultrasonography. Tendon stiffness and Young's modulus, calculated from force-elongation and stress-strain curves, respectively, were lower by 77% (P Ͻ 0.01) and 59% (P Ͻ 0.05) in the SCI than able-bodied subjects. The cross-sectional area (CSA) of the tendon was 17% smaller (P Ͻ 0.05) in the SCI subjects, but there was no difference in tendon length between the two groups. Our results indicate that paralysis causes substantial deterioration of the structural and material properties of tendon. This needs to be taken into consideration in the design of electrical stimulation protocols for rehabilitation and experimental purposes, and when interpreting changes in the contractile speed of paralyzed muscle. Although tendons transmit contractile force to the skeleton, they do not behave as rigid bodies but extend under tension as dictated by their structural and material properties. 8,11,31,47 These properties are not static throughout the lifespan, but change according to alterations in the level of mechanical loading that the tendons habitually experience. 23,45,52 In the present study, we were concerned with the adaptations of human tendons to severe disuse caused by long-term paralysis. Most of our knowledge on the effects of disuse on tendon mechanical properties originates from animal studies and relatively short durations of experimental unloading, lasting 3-12 weeks. These studies show that even short-term disuse may reduce the stiffness and Young's modulus of tendon to between 9% and 88% of baseline values. 3,18,19,24,26,32,33,42,49,50 Identifying any biological basis behind this ϳ10-fold interstudy variation on the impact of disuse is complicated by factors such as differences in the level of mechanical unloading between experimental models, baseline differences in physiological loading between tendons in a given animal, differences in activity between animals, and differences in the duration of unloading in absolute and relative terms with respect to the animal's lifespan. These discrepancies make it difficult to assess the applicability of findings from in vitro animal work to in vivo human tendons. Recent advances in ultrasound scanning, however, have enabled the assessment in vivo of the effects of disuse on the mechanical properties of intact human tendon. 24,42 One such study showed that short-term unloading (20 days) as a result of bed-rest reduced the stiffness of human tendons by ϳ30%. 24 Longer durations of bed-rest (90 days) produced larger reductions of tendon stiffness and Young's modulus, reaching the level of ϳ60% of the baseline value. 42 This suggests that severe disuse as a result of several Abbreviations: ANOVA, analysis of variance; ASIA, American Spinal Injury Association; CSA, ...