Gender differences in strength and muscle fiber characteristics were studied in eight men and eight women, focusing on the biceps brachii and vastus lateralis. Women were approximately 52% and 66% as strong as men in upper and lower body strength, respectively. Men had greater strength relative to lean body mass. A significant correlation was found between strength and muscle cross-sectional area (CSA). Women had significantly smaller CSA in all muscles studied. Men had larger type I and II fiber areas and mean fiber areas in both muscles. No significant gender differences were found in strength-to-CSA ratio, biceps fiber number, or motor unit characteristics. The greater strength of men was primarily due to larger muscle fibers. Upper body strength differences may be due to women having less lean tissue in the upper body. While it is difficult to determine if larger fibers in men are due to biology or physical activity, the data suggest a biological difference. Factors affecting maximum voluntary strength include CSA, specific tension, motor unit activation, and anatomical differences. Muscle CSA is determined by fiber size and number. Studies show that untrained women have smaller fiber areas than men, but fiber numbers are contradictory. Greater specific tension in men suggests better force generation. However, inter-individual differences in specific tension exist within each gender, suggesting variability may not be gender-specific. Chronic forceful contractions increase muscle contractile proteins and fiber area. Smaller fibers in women may be due to biology, behavior, or both. If fiber numbers differ by gender, it likely represents a biological difference. The study aimed to determine if gender differences in strength are due to physical activity or biology. Subjects were matched for body mass and activity levels. Measurements included body density, lean body mass, limb lengths, CSA, fiber area, fiber number, and motor unit characteristics. The study highlights that men's greater strength is mainly due to larger fibers, suggesting a biological difference.Gender differences in strength and muscle fiber characteristics were studied in eight men and eight women, focusing on the biceps brachii and vastus lateralis. Women were approximately 52% and 66% as strong as men in upper and lower body strength, respectively. Men had greater strength relative to lean body mass. A significant correlation was found between strength and muscle cross-sectional area (CSA). Women had significantly smaller CSA in all muscles studied. Men had larger type I and II fiber areas and mean fiber areas in both muscles. No significant gender differences were found in strength-to-CSA ratio, biceps fiber number, or motor unit characteristics. The greater strength of men was primarily due to larger muscle fibers. Upper body strength differences may be due to women having less lean tissue in the upper body. While it is difficult to determine if larger fibers in men are due to biology or physical activity, the data suggest a biological difference. Factors affecting maximum voluntary strength include CSA, specific tension, motor unit activation, and anatomical differences. Muscle CSA is determined by fiber size and number. Studies show that untrained women have smaller fiber areas than men, but fiber numbers are contradictory. Greater specific tension in men suggests better force generation. However, inter-individual differences in specific tension exist within each gender, suggesting variability may not be gender-specific. Chronic forceful contractions increase muscle contractile proteins and fiber area. Smaller fibers in women may be due to biology, behavior, or both. If fiber numbers differ by gender, it likely represents a biological difference. The study aimed to determine if gender differences in strength are due to physical activity or biology. Subjects were matched for body mass and activity levels. Measurements included body density, lean body mass, limb lengths, CSA, fiber area, fiber number, and motor unit characteristics. The study highlights that men's greater strength is mainly due to larger fibers, suggesting a biological difference.