Learning Objectives
Explain how the nervous system is able to regulate force generation in skeletal muscle
By the end of this section, you will be able to:
- Explain concentric, isotonic, and eccentric contractions
- Define a motor unit and explain how motor unit activation affects force generation
- Describe the length-tension relationship in a muscle fiber
- Describe the three phases of a muscle twitch
- Define wave summation, tetanus, and treppe
To move an object, referred to as a load, the muscle fibers of a skeletal muscle must shorten. The force generated by a contracting muscle is called muscle tension. Muscle tension can also be generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions (Figure 10.4.1).
In isotonic contractions, where the tension in the muscle stays relatively constant, a load is moved as the length of the muscle changes. A concentric contraction involves the muscle producing tension and shortening to move a load. An example of this is the contraction of the biceps brachii muscle when a hand weight is brought upward toward the body. An eccentric contraction occurs when the muscle tension produced is less than the load and a muscle lengthens while under tension. This type of contraction is observed when the same hand weight is lowered in a slow and controlled manner by the biceps brachii. Both concentric and eccentric contractions involve force production by the muscle and crossbridge cycling with the myosin heads pulling toward the M-line. The only difference between the two is whether the muscle length is shortening or elongating during the contraction.
An isometric contraction occurs when a muscle produces tension without a change in muscle length. Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the position of the hand weight. In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability.
Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes. These muscle activities are under the control of the nervous system. A crucial aspect of nervous system control of skeletal muscles is the role of motor units.
Motor Units
As previously discussed, the contraction of skeletal muscle fibers is triggered by signaling from a motor neuron. Each muscle fiber is innervated by only one motor neuron but a single motor neuron can innervate multiple muscle fibers. A motor unit is defined a single motor neuron and all of the muscle fibers innervated by it (