221
Electromyogram
The result of this repulsion is that the filaments come in closer proximity to each
other, i.e., the chains on the actin filament attract the chains on the myosin filament,
pulling the actin filaments closer to the myosin filaments, resulting in a shortening
of the myofibril and therefore the entire muscle cell. After this stage, the muscle cell
subsequently goes through a repolarization phase, releasing the calcium ions. The
calcium release process takes only several milliseconds, after which the process can
start over. This depolarization and calcium release process repeats itself many times
over all sections of myofibrils within one single muscle cell, resulting in a significant
shortening of the muscle cell.
The frequency of the muscle depolarization is a function of the frequency of the
impulse trains in the stimulating nerve. The neuromuscular depolarization process
can be described as a binary mechanism, and the higher the frequency of pulses,
the stronger the contraction will be. A single muscle depolarization effect will last
between 5 and 8 ms.
There are two methods of muscle contraction: isometric and isotonic. Under
isometric contraction, the muscle is not allowed to shorten, while under isotonic
contraction, a constant force is applied by the muscle.
In general, not all muscle cells contract simultaneously; some remain idle and
take over when the contracting muscle cells relax. This operation is controlled by
distribution of the firing sequence of various motor units to provide a desirable
action by the muscle.
11.2.3 MUSCLE FORCE
The tension exerted by the muscle fibers is a function of the length of the overlap between the actin and myosin. The collection of two neighboring Z-disks, as
shown in Figure 11.3a, is called a sarcomere. The length of the sarcomere determines the maximum force that can be applied. The force diagram illustrated in
Figure 11.3b illustrates the fact that there is a maximum force that can be applied
before the force start decaying with increased contraction. In point A of this diagram, the actin and myosin are virtually separated, and the muscle is completely
extended. The force increases linearly with contraction to point B. At this point,
the actin filaments are in close proximity to each other. In point C, the actin filaments are touching and will start repelling each other. When point D is reached,
the actin filaments will overlap, and the repulsion will reduce the muscle force
dramatically. Beyond point A, the molecular chains will start forming permanent
chemical links.
The total force that a muscle can apply is a direct function of the number of muscle fibers. The force is approximately 30–40 N per cross-sectional area in square
centimeter of muscle tissue. The amount of force is a function of the frequency of
the pulse trains as well as the total duration in which the pulses appear in the stimulating neuron. This phenomenon is shown in Figure 11.4. As can be seen, after a
certain period of pulse activation, the force plateaus and exhibits a saturation effect.
Knowing how muscle contracts, extends, and produces force, we are ready to
discuss the formation and measurement of EMG.
Electromyogram
The result of this repulsion is that the filaments come in closer proximity to each
other, i.e., the chains on the actin filament attract the chains on the myosin filament,
pulling the actin filaments closer to the myosin filaments, resulting in a shortening
of the myofibril and therefore the entire muscle cell. After this stage, the muscle cell
subsequently goes through a repolarization phase, releasing the calcium ions. The
calcium release process takes only several milliseconds, after which the process can
start over. This depolarization and calcium release process repeats itself many times
over all sections of myofibrils within one single muscle cell, resulting in a significant
shortening of the muscle cell.
The frequency of the muscle depolarization is a function of the frequency of the
impulse trains in the stimulating nerve. The neuromuscular depolarization process
can be described as a binary mechanism, and the higher the frequency of pulses,
the stronger the contraction will be. A single muscle depolarization effect will last
between 5 and 8 ms.
There are two methods of muscle contraction: isometric and isotonic. Under
isometric contraction, the muscle is not allowed to shorten, while under isotonic
contraction, a constant force is applied by the muscle.
In general, not all muscle cells contract simultaneously; some remain idle and
take over when the contracting muscle cells relax. This operation is controlled by
distribution of the firing sequence of various motor units to provide a desirable
action by the muscle.
11.2.3 MUSCLE FORCE
The tension exerted by the muscle fibers is a function of the length of the overlap between the actin and myosin. The collection of two neighboring Z-disks, as
shown in Figure 11.3a, is called a sarcomere. The length of the sarcomere determines the maximum force that can be applied. The force diagram illustrated in
Figure 11.3b illustrates the fact that there is a maximum force that can be applied
before the force start decaying with increased contraction. In point A of this diagram, the actin and myosin are virtually separated, and the muscle is completely
extended. The force increases linearly with contraction to point B. At this point,
the actin filaments are in close proximity to each other. In point C, the actin filaments are touching and will start repelling each other. When point D is reached,
the actin filaments will overlap, and the repulsion will reduce the muscle force
dramatically. Beyond point A, the molecular chains will start forming permanent
chemical links.
The total force that a muscle can apply is a direct function of the number of muscle fibers. The force is approximately 30–40 N per cross-sectional area in square
centimeter of muscle tissue. The amount of force is a function of the frequency of
the pulse trains as well as the total duration in which the pulses appear in the stimulating neuron. This phenomenon is shown in Figure 11.4. As can be seen, after a
certain period of pulse activation, the force plateaus and exhibits a saturation effect.
Knowing how muscle contracts, extends, and produces force, we are ready to
discuss the formation and measurement of EMG.