Properties of Skeletal muscles
There are the following properties of skeletal muscles shown below;
- Excitability
- Stretchability
- Contractility
- Load velocity relationship
- Length tension relationship
- Summation of contraction
- Staircase Phenomenon or treppe
- Tetanization &,
- Post-tetanic potentiation
Excitability
It is the ability of muscle fiber to respond to different types of stimulus (that change in environment) around and it is a physicochemical change.
Stretchability
It is a properties of skeletal muscles. The muscle fiber gets stretched before contraction.
Contractility
It is the response of the muscle to a stimulus. Contraction is defined as the internal events of muscle with a change in either tension or length of the muscle fibers. There are two types of contraction which are based on a change in tension or length of muscle fibers, they are; Isotonic contraction and Isometric contraction.
Isometric contraction
- Isometric (isos= equal; metron= measure) contraction means a contraction in which there is no change in length of the muscle but there is an increase in tension.
- Here contraction means mechanical activation of the muscle. It may give rise to a reduction in length or increase in tension, or both.
- In isometric contraction reduction in length is prevented so only an increase in tension occurs.
- In this, all sarcomeres don’t shorten simultaneously. The sarcomeres which shorten do so by stretching those which do not.
- For example, contraction of the muscle of the upper limb while trying to push a wall.
Isotonic contraction
- Isotonic contraction (isos=equal; tonos= tension) means a contraction in which there is a change of length at a constant tension. The tension is equal to the weight lifted during the contraction of the muscle.
- Shortening of individual sarcomeres causes shortening of the whole muscle. Since the volume of the muscle remains constant, a decrease in length is accompanied by an increase in thickness.
- It may be preloaded or after loaded. Preloaded is better at performance.
- For example, when a heavy suitcase is to be placed on a table, we tend to let it hang from our hand first before we lift it and put it on the table.
| Isometric contraction | Isotonic contraction |
| No change in length but increase in tension | No change in tension during a contraction but the shortening of length. |
| No sliding of filaments | Sliding of filaments |
| No external work done | External work done |
| Increases when load increases | Decreases when load increases. |
| The heat released is less | The heat released is more |
| E.g: attempting to lift an immoveable object, | E.g.: Simple flexion of arm. |
| Holding a weight at arm’s length | Lifting moveable object |
| Pushing a wall, etc. | Walking, etc. |
Twitch contraction
The contractile response of a skeletal muscle to a single brief stimulus is called a simple muscle twitch. or, A single action potential in a muscle fiber produces a brief, weak contraction called a twitch (too short & too weak). Duration varies from 20 ms to 200 ms depending on whether it is a fast or a slow muscle.
Load velocity relationship
The effect of load on the velocity of shortening of the muscle and their inverse relationship seen i.e. Velocity of fiber shortening is inversely proportional to the degree of load. When with the increasing load then latent period increases, distance shortened decreases, & decrease in total duration of twitch
When the load becomes so heavy that the muscle cannot lift it (Po), the velocity of contraction becomes zero. Since with Po, there is no shortening, the contraction is isometric. Thus twitch tension during isometric contraction is greater than any load that can be lifted during isotonic contraction.
Length-Tension Relationship
It is another properties of skeletal muscles. Length–tension relationship for skeletal muscle, indicates how the forcefulness of muscle contraction depends on the length of the sarcomeres before a contraction begins.
The length at which the fiber develops the greatest active tension is termed the optimal length, Io (resting length). It is about 2 to 2.2 µm.
- At a length smaller than the resting length, the actin filaments overlap each other so the number of active sites available for interacting with myosin cross-bridges is reduced.
- At length equal to or slightly greater than the resting length (corresponding to a sarcomere length of 2-2.2 microns), maximum interaction is possible between the active sites on actin and myosin cross-bridges. Hence the active tension developed is also maximum.
- At length longer than the resting length the degree of overlap between actin and myosin reduced so their interaction also reduced. Thus, the active tension developed is also less.
- At length i.e. 180% of the resting length ( a sarcomere length of 3.6 microns), there is no overlap between actin and myosin. Hence the active tension developed is zero.
- Passive tension: Tension developed in the muscle during the resting condition.
- Total tension: Tension developed in the muscle during isometric contraction.
- Active tension: Difference between the passive tension and total tension at a particular length of the muscle. It is considered as the real tension that is generated in the muscle during the contractile process and determined by the length-tension curve.
Summation of contraction
It is one of the properties of skeletal muscles. Summation means adding together individual twitch contractions. Simple means of the increasing force of muscle contraction.
Isometric tension developed in a single fiber or a muscle depends on the frequency of the stimulus applied to it.
Staircase Phenomenon (Treppe)
- Muscle stimulated rapidly but below the tetanizing frequency
- The second contraction occurs during the relaxation phase of 1st one that results in higher amplitude
- There is a progressive increase in the force of contraction for the 1st few contractions until a maximum uniform tension per contraction is reached.
- Cytoplasmic Ca++ concentration remains elevated
- Heat production decreases sarcoplasmic viscosity & facilitated enzymatic activity.
Tetanization
- Muscle when stimulated repeatedly at a very high frequency resulting in continuous contractile activity without relaxation lead to Tetanic contraction (Tetanus)
- Reason: with each stimulus, there is a fresh release of Ca2+, so that after few stimuli, Ca2+ concentration reaches its maximum in the cytoplasm.
- The strength of contraction of an intact muscle made up of many different motor units can be increased by:
- Increasing the number of motor neurons activated, thereby increasing the number of motor units contracting (Spatial recruitment)
- The frequency of action potentials of motor neuron, thereby eliciting summation or tetanus of those muscle fibers in the motor unit (Temporal recruitment)
Post-Tetanic Potentiation
- When a single stimulus is applied to a muscle immediately after tetanic contraction is over, the amplitude of contraction is higher than that of a single twitch.
- A higher cytosolic Ca++ level is responsible for this.
Applied Aspects
- Muscular Dystrophy
- Myopathies
- Myotonia
- Focal Dystonias
- Muscle Sprain
- Muscle Cramp