One of the most influential factors is muscle fiber type. We have two basic types of muscle fibers, often referred to as "slow twitch" and "fast twitch." Slow twitch muscle fibers are best used for cardiovascular (aerobic) activities. They produce small levels of force for long periods of time and thus are better suited for endurance activities. Fast twitch fibers are best used for anaerobic activities. They produce high levels of force for short periods of time and are best suited for power activities such as weightlifting.

Most men and women have an equal combination of both slow twitch and fast twitch fibers. However, some people inherit a high percentage of slow twitch fibers that enhance their performance in endurance activities, such as long distance runners. Most world class marathon runners have a very high amount of slow twitch fibers. World class sprinters or football players, for example, have relatively more fast twitch muscle fibers. Although both fiber types respond positively to strength training exercises, the fast twitch types experience greater increases in muscle size and strength, and thus may obtain greater and/or faster results from a strength training program.

Age

Another factor over which we have little control is age. Studies show that people of all ages can increase their muscle size and strength as a result of a safe and effective strength training program. However, the rate of strength and muscle gain appears to be greater from age 10-20, the years of rapid growth and development. After reaching normal physical maturity, muscular improvements usually don't come as quickly.

Gender

Gender does not affect the quality of our muscle, but does influence the quantity. Although men's and women's muscle tissue are characteristically the same, men generally have more muscle tissue than women do because muscle size is increased by the presence of testosterone, the male sex hormone. The larger the muscles, the stronger the person; this is why most men are stronger than most women.

Limb and Muscle Length

Another strength factor that is naturally determined is limb length. Persons with short limbs tend to be able to lift more weight because of advantageous leverage factors (arms and legs). Similarly, differences in strength development may come about because of variation in muscle length. Some people have long muscles, and some people have short muscles. Persons with relatively long muscles have greater potential for developing size and strength than persons with relatively short muscles.

Point of Tendon Insertion

Muscle strength is also influenced by the point of tendon insertion. For example, let's say Jim and John both have the same arm and muscle length. However, Jim's biceps tendon attaches to his forearm farther from his elbow joint than John's does. This gives Jim a biomechanical advantage: he is able to lift more weight than John in biceps exercises such as the Biceps Curl.