As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers.
This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smallest motor units in the muscle.
This allows a feather to be picked up using the biceps brachii arm muscle with minimal force, and a heavy weight to be lifted by the same muscle by recruiting the largest motor units. When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction.
To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize a skeletal muscle.
When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction.
The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship. Figure 2. The Ideal Length of a Sarcomere. Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80 percent to percent. The ideal length of a sarcomere to produce maximal tension occurs at 80 percent to percent of its resting length, with percent being the state where the medial edges of the thin filaments are just at the most-medial myosin heads of the thick filaments Figure 2.
This length maximizes the overlap of actin-binding sites and myosin heads. If a sarcomere is stretched past this ideal length beyond percent , thick and thin filaments do not overlap sufficiently, which results in less tension produced. If a sarcomere is shortened beyond 80 percent, the zone of overlap is reduced with the thin filaments jutting beyond the last of the myosin heads and shrinks the H zone, which is normally composed of myosin tails.
Eventually, there is nowhere else for the thin filaments to go and the amount of tension is diminished. If the muscle is stretched to the point where thick and thin filaments do not overlap at all, no cross-bridges can be formed, and no tension is produced in that sarcomere.
This amount of stretching does not usually occur, as accessory proteins and connective tissue oppose extreme stretching. A single action potential from a motor neuron will produce a single contraction in the muscle fibers of its motor unit.
This isolated contraction is called a twitch. A twitch can last for a few milliseconds or milliseconds, depending on the muscle type. The tension produced by a single twitch can be measured by a myogram , an instrument that measures the amount of tension produced over time Figure 3.
Each twitch undergoes three phases. Figure 3. A Myogram of a Muscle Twitch. A single muscle twitch has a latent period, a contraction phase when tension increases, and a relaxation phase when tension decreases. During the latent period, the action potential is being propagated along the sarcolemma.
A series of action potentials to the muscle fibers is necessary to produce a muscle contraction that can produce work. Normal muscle contraction is more sustained, and it can be modified by input from the nervous system to produce varying amounts of force; this is called a graded muscle response.
The frequency of action potentials nerve impulses from a motor neuron and the number of motor neurons transmitting action potentials both affect the tension produced in skeletal muscle. The rate at which a motor neuron fires action potentials affects the tension produced in the skeletal muscle.
If the fibers are stimulated while a previous twitch is still occurring, the second twitch will be stronger. You can lower it under total control using eccentric contractions but when you try to return it to the shelf using concentric contractions you cannot generate enough force to lift it back up.
Strength training, involving both concentric and eccentric contractions, appears to increase muscle strength more than just concentric contractions alone. However, eccentric contractions cause more damage tearing to the muscle resulting in greater muscle soreness.
If you have ever run downhill in a long race and then experienced the soreness in your quadriceps muscles the next day, you know what we are talking about. Muscle size is determined by the number and size of the myofibrils, which in turn is determined by the amount of myofilament proteins. Thus, resistance training will induce a cascade of events that result in the production of more proteins.
Often this is initiated by small, micro-tears in and around the the muscle fibers. If the tearing occurs at the myofibril level the muscle will respond by increasing the amount of proteins, thus strengthening and enlarging the muscle, a phenomenon called hypertrophy.
This tearing is thought to account for the muscle soreness we experience after a workout. As mentioned above, the repair of these small tears results in enlargement of the muscle fibers but it also results in an increase in the amount of connective tissue in the muscle.
When a person "bulks up" from weight training, a significant percent of the increase in size of the muscle is due to increases in the amount of connective tissue.
It should be pointed out that endurance training does not result in a significant increase in muscle size but increases its ability to produce ATP aerobically. Obviously our muscles are capable of generating differing levels of force during whole muscle contraction. Some actions require much more force generation than others; think of picking up a pencil compared to picking up a bucket of water.
The question becomes, how can different levels of force be generated? Multiple-motor unit summation or recruitment : It was mentioned earlier that all of the motor units in a muscle usually don't fire at the same time. One way to increase the amount of force generated is to increase the number of motor units that are firing at a given time. We say that more motor units are being recruited. The greater the load we are trying to move the more motor units that are activated.
Normally they will fire asynchronously in an effort to generate maximum force and prevent the muscles from becoming fatigued. As fibers begin to fatigue they are replaced by others in order to maintain the force.
There are times, however, when under extreme circumstances we are able to recruit even more motor units. You have heard stories of mothers lifting cars off of their children, this may not be totally fiction. Watch the following clip to see how amazing the human body can be. Muscle recruitment. Video Transcription Available. Wave summation: Recall that a muscle twitch can last up to ms and that an action potential lasts only ms.
Also, with the muscle twitch, there is not refractory period so it can be re-stimulated at any time. If you were to stimulate a single motor unit with progressively higher frequencies of action potentials you would observe a gradual increase in the force generated by that muscle. This phenomenon is called wave summation. Eventually the frequency of action potentials would be so high that there would be no time for the muscle to relax between the successive stimuli and it would remain totally contracted, a condition called tetanus.
Essentially, with the high frequency of action potentials there isn't time to remove calcium from the cytosol. Maximal force, then, is generated with maximum recruitment and an action potential frequency sufficient to result in tetanus.
Initial Sarcomere Length: It has been demonstrated experimentally that the starting length of the sarcomere influences the amount of force the muscle can generate. This observation has to do with the overlap of the thick and thin filaments. If the starting sarcomere length is very short, the thick filaments will already be pushing up against the Z-disc and there is no possibility for further sarcomere shortening, and the muscle will be unable to generate as much force.
On the other hand, if the muscle is stretched to the point where myosin heads can no longer contact the actin, then again, less force will be generated. Maximum force is generated when the muscle is stretched to the point that allows every myosin head to contact the actin and the sarcomere has the maximum distance to shorten. In other words, the thick filaments are at the very ends of the thin filaments. These data were generated experimentally using frog muscles that were dissected out and stretched between two rods.
Intact muscles in our bodies are not normally stretched very far beyond their optimal length due to the arrangement of muscle attachments and joints. However, you can do a little experiment that will help you see how force is lost when a muscle is in a very short or a very stretched position. This experiment will use the muscles that help you pinch the pad of your thumb to the pads of your fingers. These muscles are near maximal stretch when you extend your arm and also extend your wrist. As your wrist is cocked back into maximal extension, try to pinch your thumb to your fingers.
See how weak it feels? Now, gradually flex your wrist back to a straight or neutral position. You should feel your pinch get stronger. Now, flex your elbow and your wrist. With your wrist in maximal flexion, the muscles you use to pinch with are near their most shortened position. Try pinching again. After contraction the muscle relaxes back to a resting level of tension.
Together these three periods form a single muscle twitch,. If an additional action potential were to stimulate a muscle contraction before a previous muscle twitch had completely relaxed then it would sum onto this previous twitch increasing the total amount of tension produced in the muscle.
This addition is termed summation. Within a muscle summation can occur across motor units to recruit more muscle fibers, and also within motor units by increasing the frequency of contraction. When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more and larger motor units are excited. The largest motor units have as much as 50 times the contractile strength as the smaller ones; thus, as more and larger motor units are activated, the force of muscle contraction becomes progressively stronger.
A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which become progressively larger as greater amounts of force are required.
For skeletal muscles, the force exerted by the muscle can be controlled by varying the frequency at which action potentials are sent to muscle fibers.
Action potentials do not arrive at muscles synchronously, and, during a contraction, only a certain percentage of the fibers in the muscle will be contracting at any given time.
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