Palmer, TB. Acute effects of constant-angle and constant-torque static stretching on passive stiffness of the posterior hip and thigh muscles in healthy, young and old men. J Strength Cond Res 33(11): 2991-2999, 2019-The purpose of this study was to examine the acute effects of constant-angle (CA) and constant-torque (CT) static stretching on passive stiffness of the posterior hip and thigh muscles in healthy, young and old men. Fifteen young (25 ± 3 years) and 15 old (71 ± 4 years) men underwent 2 passive straight-leg raise (SLR) assessments before and after 8 minutes of CA and CT stretching using an isokinetic dynamometer. Passive stiffness was calculated during each SLR as the slope of the final 10% of the angle-torque curve. The results indicated that passive stiffness decreased from pre- to post-stretching for both treatments (p ≤ 0.001-0.002) and age groups (p ≤ 0.001-0.046); however, greater decreases were observed for the CT than the CA stretching (p = 0.045) and for the old than the young men (p < 0.001). In addition, baseline stiffness was greater for the old than the younger men (p = 0.010) and was also negatively related (r = -0.721; p < 0.001) to the changes in stiffness from pre- to post-stretching. These findings suggest that holding stretches at a constant tension may be a more effective strategy for altering passive stiffness of the posterior hip and thigh muscles. The greater stretch-induced stiffness decreases observed for the older men provide support that acute static stretching may be particularly effective for reducing stiffness in the elderly. As a result, it may be advantageous to prescribe static stretching before exercise for older adults, as this may be used to elicit substantial declines in passive stiffness, which could help reduce the risk of subsequent injury events in this population.