It’s probably been years since you sat through an anatomy or physiology class, and chances are you didn’t save your textbooks. But a basic understanding of how your musculature works, grows and thrives can make your time in the gym more intentional and productive. Here, then, is a comprehensive overview of the need-to-know facts about building muscle, preventing injury and living healthier.
There are three kinds of muscle tissue in the human body:
- Skeletal muscle gets the lion’s share of attention and is your main focus when it comes to exercise. This tissue is attached to your bones and is responsible for maintaining your posture and moving your body around in space.
- Cardiac muscle is controlled by the heart’s sinoatrial node and produces rhythmic contractions that cause your heart to pump.
- Smooth muscle tissue is found in your skin, blood vessels and organs and along the tracts of the urinary, respiratory and reproductive systems. It allows for functions such as digestion, peristalsis and blood circulation.
Both cardiac and smooth muscles are involuntary, meaning you do not have conscious control over their actions. Skeletal muscle is voluntary, meaning you can contract and extend it at will to run, jump, sit upright or type.
Skeletal Muscle Fibers: Type I vs. Type II
Skeletal muscles contain a mix of different fibers, and the exact ratio depends on the specific muscle in question as well as the individual person. There are two basic kinds of fibers — Type I and Type II — which function in different ways. Type I fibers are often called “slow-twitch” because they are slow to produce maximal tension and generate less force than Type II fibers. But what they lack in speed and power, they make up for in endurance. They are more resistant to fatigue than Type II fibers and are crucial when doing an endurance exercise such as distance running or similar sustained steady-state cardio.
Type II muscle fibers are also called “fast-twitch” because they contract quickly and produce bursts of power. These fibers are further categorized into Type IIa and Type IIb (also called Type IIx). Type IIb fibers produce the most force and fatigue the quickest. They are primarily engaged in actions such as a 50-yard dash or a single, max-effort barbell snatch. Type IIa fibers are a mix of Type I and Type IIb, and they are slightly less powerful than Type IIb but boast greater endurance. These are engaged in longer duration yet still quick activities such as a fast 200-meter run.
Training For Type
Just as you can voluntarily contract your skeletal muscle, you also can control its shape and growth in order to achieve a certain look or level of performance. For instance, Type II fibers are larger than Type I, so if a bigger booty or badass biceps are among your fitness goals, program your workouts to target those fibers. Endurance-based athletes such as long-distance cyclists should ostensibly target their Type I muscle fibers, and power-based athletes such as CrossFitters who do endless box jumps and short sprints should focus on Type II.
Regardless of your goals, however, you ultimately need to develop both slow- and fast-twitch fibers to live an active, well-rounded life. “Type II fibers specifically decrease as you age, which leads to muscle wasting (sarcopenia) that has implications on overall strength and mortality,” explains Helen Kollias, Ph.D., director of science at Precision Nutrition. And besides giving you staying power on race day, “Type I fibers are important contributors to metabolism and postural support,” she adds.
Contraction at the Cellular Level
All muscle cells — skeletal, cardiac and smooth — contain structures called myofibrils, which are composed of actin and myosin, the interlocking contractile components of your muscle tissue. For a muscle to move, the central nervous system sends an electrical impulse to the muscle fibers and causes the actin and myosin to move toward or away from each other. A concentric contraction happens when a muscle produces more force than an outside resistance, such as when lifting a weight. Here, actin and myosin slide past one another and cause the muscle to shorten. An eccentric contraction occurs when a muscle produces less force than an outside stimulus, such as when lowering a weight. Here, actin and myosin are pulled apart and the muscle lengthens.
The How's of Hypertrophy
A progressively challenging strength-training program leads to hypertrophy — e.g., muscle growth — but how exactly does this happen? Lifting weights causes microscopic tears in your muscle tissue, which initiates a repair response in your body to rebuild the damaged cell fibers. It also stimulates the growth of new myofibrils within that cell. More myofibrils means larger cells and in turn larger muscles over time.
If hypertrophy is your ultimate goal, the National Academy of Sports Medicine recommends lifting at 75 to 85 percent of your one-rep max for three to five sets of six to 12 reps. “Building muscle is best achieved with heavier weights and lower reps because a muscle grows when it is strained beyond the maximum force it can sustain,” says Myles Spar, M.D., MPH, author of Optimal Men’s Health (Oxford University Press, February 2020). “If you can do 12 reps of an exercise, the weight isn’t heavy enough.”
However, recent research also suggests that metabolic fatigue — achieved by working until your muscles can no longer generate force (failure) — also may contribute to hypertrophy. “Mechanical and metabolic stress occur in tandem, making it difficult to tease out the effects of one from the other,” says Brad Schoenfeld, Ph.D., CSCS, in his research paper published in Sports Medicine. In other words, there is more to hypertrophy than just tearing and repairing.
Regardless, a few things are for certain: You should never skip meals or you could become catabolic, which is when the body breaks down muscle tissue to use as fuel. And since the real build happens during recovery, allow at least 48 to 72 hours before training the same muscle group again, Spar recommends.
Stay consistent and you’ll likely see noticeable results within three months. Then it’s time to switch things up so you keep progressing. Play with the variables — increase your weight, decrease your rest, use new exercises — anything new and different is a challenge for your body and prevents complacency.
The G Factor
When it comes to muscles — their size, shape or how quickly they grow — there’s one important wild card: genetics. “Muscle size and even how many muscles you have is dependent on genetics,” Kollias says. “For example, some people have five muscles in their quadriceps, not four. Also, how you respond to exercise has a genetic component.” The specifics and how they compare to lifestyle are yet to be fully understood but the take-away remains that yes, you may indeed be able to blame your parents for something concrete.
Dynamic or Static — The Great Debate
At some point in recent history, stretching became a controversial topic — when to do it, what kind to do and what good it does the body. Some evidence indicates that static stretching — holding a stretch for a duration of time — before resistance training impairs your ability to lift heavy weights, so if you’re concerned with your one-rep max, save the static stuff for postworkout. Instead, warm up with dynamic stretches such as leg swings, arm circles or Cat/Cow Pose. These raise your body temperature, improve muscle extensibility, and prepare your mind and body for action.
Muscular injuries are categorized as either acute or overuse. “An acute injury occurs through a trauma or macro event, and you know when the damage occurred,” says Corey Phelps, NASM-CPT. “An overuse injury happens over a prolonged period due to overtraining, compensatory movement patterns or micro-traumas.” For example, if you trip on the treadmill and land in the splits, the resulting hamstring tear is an acute injury. But the shinsplits that popped up around week nine of your marathon training is an overuse injury.
Strains and tears are among the most common acute injuries and occur when the muscle fibers are overstretched or torn. This usually results in pain, swelling, weakness, decreased mobility and, in severe cases, surgical intervention. The solution: time and rest.
How Sore is Too Sore?
You’ve for sure had those days after an extra grueling workout when you feel invincible — until the next morning when you pull the towel rack off the wall trying to stand up from the john. This delayed onset muscle soreness (DOMS) is caused by those thousands of small muscle tears you provoked in your quest for growth and is a normal result of hard training. Stretching, gentle movement, heat/ice application and over-the-counter pain medication offer some relief, but time is the only real solution for DOMS, and recovery can sometimes take up to a week.
Rhabdomyolysis (rhabdo), on the other hand, is also characterized by muscle pain and weakness, but it is a very serious condition that requires immediate medical attention. Here, the muscle fibers break down and their contents are released into your bloodstream. Rhabdo can be caused by extreme exercise as well as blunt force trauma, infection and other medical conditions. “One way to tell the difference [between DOMS and rhabdo] is that rhabdo hurts when you’re not moving; DOMS does not hurt until you move,” says Joe Cannon, MS, exercise physiologist and IDEA personal trainer. Some symptoms include dark urine, muscle swelling, stiffness, weakness, fever, nausea and vomiting, and if left untreated, it could result in kidney failure.
Muscle and the March of Time
Sarcopenia is the term for the age-related decrease in muscle tissue and therefore in mobility and strength as we age. It is likely caused by a variety of factors, including a natural drop in testosterone, a decline in physical activity and/or malnutrition associated with reduced food intake. But you don’t have to be a retiree to be affected. “After the age of 30, we start to lose muscle if we are not actively training to keep it,” Phelps says. “Inactive adults lose up to 3 percent every year.”
The good news is that it’s never too late to get — or stay — fit. “The phenomenal thing about muscles is that even as we age, they are still capable of growing and responding to training,” says Cathy Richards, MA, exercise physiologist specializing in strength training for older adults. “Multiple studies have shown that with lifelong training, the muscles of older adults can retain significant levels of strength and endurance. Furthermore, if you don’t start strength training until your older years, the muscles still respond and get stronger.”
There is no shortage of marketing surrounding those mail-in DNA spit kits that promise to reveal the mysteries of your muscle composition. While none of them can guarantee you a spot on the Olympic track team, they can detect a genetic variant in the ACTN3 gene — aka “the gene for speed” — which has been associated with a higher proportion of fast-twitch muscle fibers.