Men naturally have a higher percentage of lean muscle mass than women. Women can blame female hormones -- particularly those related to reproduction -- for this difference. Less total muscle mass doesn't give women a pass when it comes to weight training, though. Despite having less muscle mass, women can experience similar gains in strength and should train in ways similar to men to develop muscular fitness.
Studies have proven again and again that men have a greater amount of skeletal muscle than women. In one such study that examined 468 men and women and was published in a 1985 issue of the “Journal of Applied Physiology,” researchers determined that men had an average of 72.6 pounds of muscle compared to the 46.2 pounds found in women. The men had 40 percent more muscle mass in the upper body and 33 percent more in the lower body.
Men not only have more muscle, but pound for pound, their muscle is slightly stronger than a woman's -- about 5 to 10 percent, says Lou Schuler in "The New Rules of Lifting for Women." A study reported in a 1993 issue of the "European Journal of Applied Physiology and Occupational Physiology" attributed this strength difference to larger muscle fibers in men.
While men have more muscle, women tend to have 6 to 11 percent more body fat than men, notes a 2009 paper in Science Daily. How much body fat you have in relation to lean muscle mass depends on the shape you're in; but if you boil it down to essential fat, men need a minimum of about 3 percent and women about 12 percent. Essential fat is the minimum you need for proper body function; it provides insulation, padding and energy. The remaining 97 percent of body mass for men and 88 percent for women is made up of lean muscle, organs and bone. Women are better at storing fat, likely due to the hormone estrogen, which discourages a woman's body from burning calories after eating. The extra fat supports childbearing -- essentially, survival of the species.
Implications on Training
The differences in muscle mass between men and women shouldn't affect training for either gender. A woman may end up with less absolute strength than a typical man, but she can still gain muscle and strength with a progressive resistance program. The fact that women's muscle fibers tend to be smaller means she doesn't have to fear getting huge and bulky from weight training.