Why is it that one hotshot at the gym can lift 100 pounds with ease while another struggles to lift 75 pounds? Muscle strength is defined as the amount of force a muscle can generate with a single effort. Not surprisingly, individuals vary a great deal in their muscle strength before training and in their capacity to build greater muscle strength through training. But have you ever wondered why? Here are five factors that determine how strong a person is.
You’re probably already familiar with fast-twitch and slow-twitch muscle fibers. Slow-twitch muscle fibers have a slow rate of shortening, but they’re also highly resistant to fatigue. They’re primarily called into play during aerobic exercise and endurance training. In contrast, fast-twitch muscle fibers can generate force 3 to 5 times faster than slow-twitch fibers, but they fatigue more rapidly. Fast-twitch muscles are the ones called into play when you need to generate large amounts of force to lift a large load such as a heavy weight.
The majority of people have roughly equal quantities of fast and slow-twitch muscle fibers, but people who excel at sports that involve strength may have a higher ratio of fast-twitch to slow-twitch muscle fibers. Having more fast-twitch muscle fibers is one factor that contributes to muscle strength.
Another factor that determines strength is muscle mass. People with larger muscle cross-sectional areas can generate greater amounts of force. This should come as no surprise. If you have more muscle mass, you have a larger number of muscle fibers that can produce force.
Other Muscle Composition Factors That Affect Strength
If you have two people and one has long arms and the other short arms, the person with the shorter arms will be able to generate more force assuming both have equal amounts of muscle mass, bony tissue, and connective tissue. The person with shorter arms has an advantage because the weight is closer to their body. So leverage is a factor in muscle strength. Muscle strength is also partially determined by where the tendon of the muscle attaches to the bone. These are all factors that are genetically determined and ones you have little control over.
Muscle Strength and Gender
Men have greater absolute muscle strength than women. In fact, men have 50% greater muscle strength in the upper body and 30% more in the lower body relative to women. But when you take into account muscle mass, the strength differential between men and women becomes almost non-existent. When you measure bench press strength in men and women, the strength differential between the two sexes drops to only 2.5% when you divide strength by body mass. So men are stronger primarily because they have more muscle mass.
Age is another factor that affects muscle strength. Muscle strength declines with age, but this is primarily due to a decrease in muscle cross-sectional area and a decline in the amount of contractile tissue within the muscle fibers. The good news? Regular strength training limits loss of muscle strength with aging. Even elderly people can increase muscle size and strength through training.
Muscle Strength and Neural Efficiency
The nervous system also plays a role in muscle strength. Your brain and nervous system have the power to activate more motor units when they need to generate larger amounts of force. Through strength training, your body learns to recruit more motor units and increase how often these units fire. This is one of the ways you gain additional strength through resistance training.
The Bottom Line?
These are the primary factors that determine how strong you are from a muscle standpoint. You don’t have control over all of these factors, but a regular strength training program that progressively overloads the muscles builds strength no matter where you start from. Everyone can benefit from resistance training regardless of their age and sex.
Exercise Physiology. Fifth Edition. McArdle, Katch, and Katch. 2001.
Physical Therapy. Vol. 82. No. 1. pages 62-68.