I often hear athletes talk about the “dumb” muscles. According to them, muscles do not have a mind of their own and only respond to the amount of weight that they must overcome in each exercise. The muscles are considered as objects to be worked so that they become bigger and stronger.
To a limited extent, thinking of the muscles as dumb can be considered accurate. When a resistance must be overcome, the muscles will respond by giving you the strength to move the weight. However, I consider the muscles very intelligent.
The reason for this is that muscles do not work on their own, but instead function in close coordination with the nervous system. The nervous system as we know is the center of our intelligence, mental thinking and motor commands. It controls when a muscle will contract, with what intensity it will contract, how many muscle fibers will be recruited, what the frequency of their contraction will be and the simultaneous integration of other muscles in a coordinated manner.
It is important to understand that there is a high degree of intramuscular coordination between the various muscles that have the same action. For example, in the biceps curl, the brachialis does most of the work in the initial movement (when starting with straight arms) and the biceps takes over in the middle range. The brachioradialis comes into play only when needed, i.e., when the resistance is sufficiently great and if there is additional supination or pronation.
There is also intermuscular coordination with other muscles that are involved in a complex exercise. For example, when rising up from a squat there is coordination between the quadriceps, hamstrings and gluteus maximus in their concentric contractions and the erector spinae of the back and the side hip muscles in their isometric contractions.
It is usually impossible to have only one muscle involved in an activity. Most often there are a multitude of muscles working in a very complex, but integrated manner.
Even more complex are the two-jointed muscles. These muscles cross two joints (as for example, the hamstrings which cross the knee and hip) and have an action at each of the joints.
The hamstring muscle is a good example of the typical two-jointed muscle. At the lower insertion end the hamstrings are involved in the leg curl (knee flexion) which develops mainly the mid to lower hamstring. At the upper end (which is attached to the pelvis) the upper hamstrings are involved in hip extension, which is seen in straight leg pulldowns on a pulley or with Active Cords in the pawback. The hamstring muscles have a powerful action at both joints.
Other two-joint muscles such as the biceps muscle of the arm and gastrocnemius muscle of the shin have a very strong action at one end, and a less powerful action at the other end. For example, the biceps is most powerful in elbow flexion as in the bicep curl exercise and is an assistant in shoulder joint flexion. The gastrocnemius is most powerful in ankle extension and a strong assistor in the leg curl exercise.
Because of their dual actions many of these muscles are not as powerful as most single-joint muscles, i.e., where the muscle crosses only one joint. Examples of this are the vastus muscles of the thigh quadriceps and the brachialis muscle of the upper arm. These single joint muscles can be considered the true workhorses of most joints.
For more information on the muscles and muscle actions see Biomechanics and Kinesiology of Exercise.