Skeletal muscle fatigue is defined as muscle force depression following exercise or repeated contractions that mimic exercise. Oxidative stress, caused by excessive production of reactive oxygen species (ROS), has long been thought to contribute to muscle fatigue after acute exercise. However, recent studies on antioxidant treatments have suggested little impacts of oxidative stress on muscle fatigue. This review examines the chemical properties of various ROS, their source, their effects on the contractile mechanism, and their production during muscle fatigue. Based on these, this review discusses cellular mechanisms behind the limited ROS influence on muscle fatigue, with a focus on the primary ROS produced during exercise such as superoxide, hydrogen peroxide and hydroxyl radical. These ROS have been shown to exert both positive and negative effects on muscle force. Therefore, the limited effectiveness of antioxidants is thought to be due to their capacity to suppress not only the negative effects but also the positive effects of ROS. This antioxidant effect provides an important insight into the relationship between oxidative stress and muscle force. Previous studies suggest that the low levels of ROS can enhance muscle force, whereas higher levels impair it, which is referred to here as the “invert U” relationship. Developing this concept further, I proposed a hypothesis that the relationship between oxidative stress and muscle force can be divided into two phases: in phase 1, the positive and negative effects of ROS balance out, resulting in minimal impact on muscle force, while oxidative stress dramatically decreases force in phase 2 which occurs under more sever condition.

This paper explains the doubly labeled water (DLW) method and introduces research findings published in Science in 2021 and 2022. The DLW method is a non-invasive technique that uses water labeled with stable isotopes, heavy oxygen (¹⁸O) and deuterium (²H), to measure total energy expenditure (TEE) and water turnover in daily life. The main features of this method are: (1) It can measure energy expenditure in daily life in a completely unrestricted state over a 1-2 week period. (2) It has accuracy that serves as a benchmark for other simplified methods. (3) It can be measured with just spot urine collection. The 2021 Science study built a database of over 6,600 people from 29 countries worldwide and analyzed total energy expenditure throughout human life. The results revealed that total energy expenditure is highest in the late teens, does not decrease during middle age (30s-50s), and shows a 0.7% annual slowdown after the 60s. Additionally, infants showed very high metabolic rates relative to their body size. The 2022 Science study measured water turnover rates in 5,604 people from 23 countries, revealing that infants lose about 25% of their body water daily, while adults lose about 10%. The study also developed a prediction equation for water turnover that considers factors such as physical activity level, body weight, gender, humidity, whether one is an athlete, human development index, altitude, age, and temperature. These studies provide important insights that contribute to solving various challenges, including establishing dietary reference intakes, preventing dehydration, developing water and food security strategies during disasters, and creating predictive models for water shortages associated with climate change.

Many older adults who participate in exercise classes for health promotion already possess higher levels of daily physical activity and physical fitness. Integrating health promotion with non-health-related activities may be a more effective strategy for engaging individuals with lower levels of daily physical activity and physical fitness. To test this hypothesis, the present study implemented exercise (EX), disaster prevention learning (DP), and combined (EX×DP) classes. Community-dwelling individuals aged 61–87 years participated in the EX (n = 21), DP (n = 15), or EX×DP (n = 31) classes. Performance on the 10-time sit-to-stand test was lower in the EX×DP group compared to the EX group. The 5m walk at the fastest pace was faster in the EX group than in the DP group; however, no significant difference was observed between the DP and EX×DP groups. Although no significant differences were observed between the EX and EX×DP groups in habitual exercise time for walking, muscular-strength exercise, or cycling, the EX×DP group reported less time spent on other forms of exercise, such as yoga and table tennis. Additionally, self-efficacy for, and perceived health benefits of, the strength exercise were lower in the EX×DP group compared to the EX group. Habitual exercise time increased in both the EX and EX×DP groups during the six weeks following the classes relative to baseline. Notably, improvements in the 10-time sit-to-stand test were observed only in the EX×DP group. These findings suggest that integrating health promotion with non-health-related activities is a more effective strategy for engaging older adults with lower levels of physical fitness compared to the exercise-only activity.