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.

Whether decreases in lean tissue (LT) and adipose tissue (AT) during body weight (BW) reduction differ between males and females is unclear. To investigate sex differences in changes in body composition and energy deficits during BW reduction, sexually mature 15-week-old rats were divided into a group euthanized before BW reduction (n = 6 males, n = 7 females) and a group euthanized after three days of fasting (n = 6 males, n = 6 females). Energy expenditure and BW were measured during the study period. Losses in lean tissue (ΔLT) and adipose tissue (ΔAT) were calculated using simultaneous equations based on ΔBW, which is the sum of ΔLT and ΔAT, and the energy lost from the body is the sum of the energy lost from ΔLT and ΔAT. BW reduction was significantly greater in males than females, total energy expenditure was significantly greater in males than females, and the energy required to reduce BW by 1 kg was significantly less in males (3304.8 kcal/kg [SD 327.5]) than females (3893.0 kcal/kg [SD 356.5]). Both ΔLT and ΔAT were significantly greater in males than in females. The ΔLT:ΔAT ratio was significantly different between males (67:33) and females (57:43). Sex differences exist in the metabolic responses to BW reduction, and females lose more energy-dense adipose tissue during BW reduction than males, demonstrating that a greater energy deficit is required to reduce BW in female rats than male rats.

Excess accumulation of visceral fat induces insulin resistance in skeletal muscle and impairs glucose tolerance. Although weight loss improves glucose tolerance by reducing visceral fat mass, the effects of the speed of weight loss on glucose metabolism are unclear. This study compared the effects of long-term energy restriction–induced weight loss and short-term fasting–induced weight loss on glucose tolerance in rats fed a high-fat diet. After 2 weeks of high-fat diet feeding, male Wistar rats were subjected to either 30% calorie restriction for 2 weeks (CR) or 3-day fasting (FAST). After the intervention period, body weight and intra-abdominal fat mass decreased to a similar extent in both weight-loss groups. The maximum insulin-stimulated glucose uptake and GLUT-4 content in the epitrochlearis muscle were significantly higher in the FAST group than in both control rats fed ad libitum (CON) and the CR group, with no significant differences between the CON and CR groups. Blood glucose levels during the oral glucose tolerance test (OGTT) were similar in the CON and CR groups, but insulin levels were significantly lower in the CR group than in the CON group, indicating improved insulin resistance in the CR group. However, compared to the CON and CR groups, plasma glucose and insulin levels were significantly higher and lower, respectively, in the FAST group during the OGTT. These findings suggest that rapid weight loss through short-term fasting, in contrast to long-term energy restriction, may impair glucose tolerance by reducing insulin secretion capacity.