People frequently write or talk about the physiological differences between muscle fibers (fast twitch vs. slow twitch, oxidative capacity, etc.), but this information is generally not very useful to the typical health and fitness enthusiast. Some understanding of how your muscles work is certainly important, but most people don’t need to know all the in-depth physiology. Instead, I believe that understanding the basic functional differences between muscles provides more practical information than you would get by learning a lot of the muscle physiology.
When looking at the functional differences between muscles you can certainly go into great depth and examine how all the various muscles function at every single joint, but in the end, muscles generally fall into two different functional categories: prime movers and stabilizers. Prime movers are the muscles that actively create movement, while stabilizers provide balance and support to your body.
Prime movers are typically the larger muscles in your body and include muscle groups such as your quads and hamstrings (upper thigh), pecs (chest), lats (back), biceps and triceps (arms), etc. They connect to your bones (by tendons) and create movement around a joint. For example, your bicep connects your upper arm to your lower arm (forearm), crossing the elbow joint, and when the bicep contracts it brings your forearm closer to your upper arm. Since the bicep contraction creates this movement, it is considered a prime mover.
Stabilizers, as their name implies, have more to do with stabilizing your body than actually creating movement. Stabilizers are smaller muscles and in many cases they are not really even visible, because they are either so small or deep under your surface muscles. These muscles help to keep your bones, joints, and muscles correctly aligned both during movement and while you are stationary.
Stabilizer muscles are also essential for maintaining good posture throughout your life. For example, the stabilizer muscles in your mid and upper back, work to keep your shoulders back and in line with the rest of your body. If those muscles become too weak or your chest and front shoulder muscles become proportionally too strong or tight, your shoulders will begin to round forward. If the stabilizer muscles are not strengthened to the point where they can reverse this change, then the shoulder rounding will progress and your posture will become worse over time, leading to additional problems.
Prime movers and stabilizers both play valuable roles in your body and any well-rounded training program will include exercises or workouts to improve both types of muscles. It is also important to note that since prime movers and stabilizers have different functions and muscular demands, they should be trained differently. Unfortunately, a lot of people try to train stabilizers as if they are prime movers and possibly an ever greater number of people don’t realize stabilizers need to be trained at all.
Really this is not surprising, because most people in the fitness industry and the media still focus on using exercise to improve how your body looks and rarely spend time explaining how training can improve the way your body functions. It is a common assumption that training will always improve the way your body functions, but this is only partially true. A well-balanced program will improve how your body functions, but many programs are imbalanced or ignore important aspects that actually lead to physical dysfunction.
Improving stabilizer muscles is as example of something that is often left out of the average training program. Since stabilizers are so small, training them usually does not cause any dramatic change to how your body looks, so they don’t get much attention and are often completely ignored. It is very tempting to only train prime movers, because they are responsible for the most calorie burning and physical change. While the majority of your training time can be spent on prime movers, at least some stabilization training should be included as well.
When resistance training (lifting weights, using exercise bands, etc.), prime movers are generally trained by performing sets of exercises where each set contains typically contains between 3 and 15 reps, depending your training goals. In general, lower reps and higher weight result in more strength gains, while higher reps and lower weight results in more local muscular endurance. However, in both cases, the muscles are trained for a certain number of reps, usually until they become fatigued, and then there is a period of rest so they can recover for the next set.
This type of training is effective, because prime movers usually only work for shorter durations (with the exception of long endurance events), but stabilizers frequently have to contract for hours every day. The difference is that stabilizers muscles are designed to produce small and sustained contractions for prolonged stabilization as opposed to the strong and brief contractions of the prime movers. As a result, stabilizer muscles do not need to be trained to produce greater amounts of force for a short period of time.
The good news is that you can actually train stabilizer and prime mover muscles at the same time, depending on the exercises you use. For example, using machines to work your leg muscles (leg press, leg extension, etc.) provides little benefit to your leg stabilizers, but exercises performed standing in a split stance (one leg forward and one leg back), on one leg, or on balance devices (Bosu, inflated discs, etc.) will challenge the stabilizer muscles along with the prime movers.
One thing to note is that when performing exercises that challenge your stabilizers, your prime movers will not be challenged as much, because you will not be able to use as much weight or perform as many reps as when your stabilizers are not used. This is because energy that would be used to contract your prime movers is spent on stabilization and control of the movement. In addition, your stabilizers may give out before your prime movers, if the stabilization demand of the exercise is high enough.
However, for most people, the benefits of including stabilization training far outweigh the negative of having a little less improvement in the prime movers. Unfortunately, there is not much incentive for people to include stabilization exercises, because they often don’t realize how important stabilizer muscles are, at least not until after they start experiencing problems associated with poorly functioning stabilizers. Also, these problems typically do not start to occur until the mid to late stages of life and they are frequently thought of as normal parts of aging, instead of preventable or reversible muscle and joint problems.
For instance, let’s go back to my earlier example of a person who has rounded shoulders. It is common for elderly individuals to have forward rounding shoulders along with excessive curvature in their upper back and spine. In most cases, this is not a normal part of aging and it is actually caused by a combination poor stabilizer muscle function, lack of flexibility, general lack of muscle use, and the postural changes that result from these issues.
When people make a point of staying active, maintaining their flexibility, and practicing good posture throughout their life, the rounding of the shoulders and other postural changes can usually be avoided. Fortunately, if you have not done everything you should have when you were younger, there is still hope. A well-rounded training program, including stabilization training that focuses on your problem areas can go a long way in reversing and preventing many of the muscle, bone, and joint problems typically associated with aging.