Acute changes in passive muscle stiffness after resistance exercise: a narrative review of effects of program variables

Abstract

Resistance exercise may empirically be believed to cause acute increases in passive muscle stiffness in sports and rehabilitation. An acute increase in muscle stiffness limits the joint range of motion (ROM) and may indirectly increase the risk of musculoskeletal injuries and impair athletic performance in some sports events. Thus, a comprehensive understanding of resistance exercise-induced acute changes in passive muscle stiffness is essential in sports and clinical settings. Many studies have investigated acute changes in passive muscle stiffness after resistance exercise. However, no clear consensus has been reached, possibly because of differences in program variables (e.g., contraction mode, exercise ROM, and load) among studies. The present review aimed to provide an overview of the types of resistance exercises with different combinations of program variables that induce acute or insignificant changes in passive muscle stiffness (shear modulus assessed by ultrasound shear wave elastography). This review suggests that 1) muscle stiffness is acutely increased by eccentric-only resistance exercise with a combination of a wide ROM, high load, and high volume; 2) muscle stiffness is acutely decreased by eccentric-only resistance exercise with a combination of a wide ROM, long muscle length, and long duration when exercise is performed with a low to moderate load and/or volume; 3) muscle stiffness does not acutely change after concentric-only resistance exercise; and 4) acute changes in stiffness after resistance exercise depend on measured muscles, joint positions, and time points.