2.7 Muscle Force.pdf

[00:00:01] >> This lecture is on muscle force. These are the objectives. How well muscles are able to create force depends on numerous factors. In addition to neurologic, metabolic, endocrine, and psychological factors, many other factors determine muscle strength. Some of these factors include the...

[00:00:01] >> This lecture is on muscle force. These are the objectives. How well muscles are able to create force depends on numerous factors. In addition to neurologic, metabolic, endocrine, and psychological factors, many other factors determine muscle strength. Some of these factors include the muscle size, the architecture of the muscle fibers, the passive components of the muscle, the physiological length of the muscle or the length-tension relationship of the muscle. [00:00:37] The moment arm length of the muscle, the speed of the muscle contraction, the active tension, and age, and gender. Let's look more closely at these. If muscle fibers are placed side by side, they are called parallel, and the muscles whip is greater. If muscle fibers are placed end to end they are called in series. [00:01:07] Usually parallel muscle fibers provide greater force and muscle fibers that are in series provide greater speed of motion. If you have two muscles and they're the same length, the muscle with the greater width is stronger than the one that has a smaller diameter or width. In terms of cross-section, larger muscles in normal subjects are stronger than smaller ones. [00:01:34] We also know that muscle size might increase or hypertrophy, or decrease in atrophy, with extra size or inactivity. In a fusiform or strap muscle, the muscle fascicles are parallel and long throughout the muscle. The sartorius is an example of a strap or fusiform muscle. The muscles are designed to produce greater shortening distance, but less force. [00:02:00] Pennate muscle fibers on the other hand attach at oblique angles to a central tendon. So you can see here coming at an angle to this central tendon. There are different pennate designs of muscles depending upon the number of fiber arrangements within a muscle. Unipennate muscles have one parallel fiber arrangement, whereas bipennate muscles have two groups of muscle fibers running into one central tendon. [00:02:28] Most muscles in the body are multi pennate, with more than two pennate groups attaching to more than one central tendons, so multiple tendons on this one. Pennate fascicles are shorter than fusiform fascicles. They produce greater force to the sacrifice of speed, so less speed. Since their muscle strength is proportional to the total cross- sectional area of the muscle, strength of pennate is related to the combined cross-sectional area of the pennate muscle. [00:03:03] Therefore, total strength of a pennate muscle is a sum of the cross- sectional area of each pennate. The architectural design of most muscles in the body is multipennate. Fascial layers such as fascia, endomysium, perimysium, and epimysium form a muscles passive, elastic component. The elastic components stretches when the muscle lengthens, and as those components become tot, they provide stiffness to the muscle. [00:03:36] Since that increase in the muscle stiffness occurs because of the fascia in the tendon, it's considered passive tension. The passive tension is like stretching the rubber band, as the stretch increases, the more tension is produced. When you release that tension, the greater rebound response or contraction is produced. [00:03:57] When you look at this picture here you can see that the passive elastic tension occurs after normal resting length, so beyond 100% where that muscle is being stretched. The passive tension, therefore, occurs as a result of that stretch. The resting length of a muscle is the position of the muscle where there is no tension within the muscle. [00:04:24] It cannot be precisely determined, but the resting length is defined as the length at which the maximum number of actin and myosin cross bridges is available. As the muscle either shortens or lengthens beyond that resting position, its ability to produce force decreases. So you can see that tension is lower on either side because a number of cross bridges decline when the muscle fibers move out of its resting position. [00:04:52] Active tension declines as the muscle shortens coming this direction because there are fewer cross bridges available between, actin and myosin fibers. When a sarcomere is at its shortest position, there's no remaining cross bridges available. Likewise, as the muscle fiber lengthens, the actin and myosin fibers move further apart until they don't have connection between them sufficiently to produce tension. [00:05:18] Active tension is responsible for muscle tension during shortening, whereas passive tension adds to this muscle tension during lengthening. Here's a combination of the past two slides where you have both the active length tension relationship and the passive length tension relationship shown in the same chart. Is the line of the combined forces between the active and passive tension here. [00:05:47] The passive elastic components provide tension beyond the normal resting length, whereas the active muscle tissue provides tension at less than the resting length. In a normal body, the joints don't allow extreme shortening or lengthening of the muscle which prevents muscles from entering more injurious ranges of motion. Early investigations of muscles were done in animals, and they found in the gastroc muscle of a frog that the functional range of the muscle was from about 75% to 105% of its resting length. [00:06:21] And they've found similar results as they continue to do that research in humans. Here is another depiction of the length-tension relationship. The active, Length-tension relationship, passive length- tension relationship, and total length-tension relationship curves are shown. The plateau here of the active curve signifies optimal sarcomere length at which the active tension is developed. [00:06:51] And so you can see that sarcomere length here where you have the most actin-myosin cross bridges. Isometric tension decreases as the muscle is lengthened. So the active tension decreases but the passive tension increases as that muscle is stretched. And that results in this total curve, which is a combination of those two active and passive length- tension curves. [00:07:22] You can also see here on this ascending portion as those muscle fibers get close together, that they also don't form as much force. A moment arm of a muscle is a lever arm that produces rotation around a joint. We don't have to go in too much detail on this slide. [00:07:43] But notice that as a joint moves to its range of motion, the muscles producing the movement experience a change in their moment arms both in length and the position, which is relative to the segment. So at a resting position here for the biceps, that muscle, the moment arm, and the force is going to change whether the elbow is bent, fully bent or straight. [00:08:06] This means at some points in the range of motion, a muscle generates a large torque or rotational force, and sometimes it produces less torque, depending upon when that muscle's moment arm is perpendicular to the body segment. When we think about that length tension relationship to the muscle and ultimately to the joint systems, it gets pretty complex. [00:08:25] So sarcomere length is not the same throughout the muscle, let alone between muscles with similar functions. That means for any muscle length at a particular joint position, there may be sarcomeres at many different lengths corresponding to different points on that length- tension relationship. Also when that muscle's acting at a joint like depicted here, the torque produced is not only a function of the muscle force, but also a function of the moment arm of the muscle. [00:08:52] So that any joint angle, the muscle length may be short which suggests the force will be low, but the moment arm could be long, thus maintaining a higher joint torque. From these examples, it's clear that the sarcomere length tension relationship is important, but there's a lot of other factors we have to consider as well. [00:09:13] The speed of a contraction impacts the force and muscle can develop. So you can see here velocity, this is where the speeds getting greater, and this is where the force is getting greater. As the speed becomes slower, so we're going this way, so it's getting slower, the force development increases. [00:09:37] When there is no motion right here, there you have a maximum isometric contraction. A muscles decreased ability to produce a contraction or force with increasing speed is based on the number of links between the actin and myosin that can be formed per unit of time. The maximum number of cross bridges occur at slow speeds, the more rapidly those filaments slide past each other, the smaller number of links are formed between the filaments, so less force is developed. [00:10:08] So you can see there is a loose inverse relationship between the speed of contraction and amount of force a muscle is able to produce concentrically. Active tension is a force produced by a muscle and is created by those cross bridges between the actin and myosin elements within the muscle fibers. [00:10:26] In a normal neuromotor system, the active tension is the most important factor in the production of muscle forces. The amount of active force a muscle contraction can produce is determined by the number of motor units that are recruited, and the firing rate of the active motor units. So the force of active muscle contraction is determined by the number of motor units and the firing rate of the motor units. [00:10:52] Motor units consist of the motor neuron and all the muscle fibers it innervates. Remember, the number of muscle fibers in one motor unit varies. The greater number of muscle fibers activated, the greater the active tension that's produced. And there's an inverse relationship between the size of the motor neuron and its excitability, the larger the axon, the less excitable it is. [00:11:14] The type of muscle fiber recruited within a muscle also influences the amount of tension produced by the muscle. Slower type, type one muscles are recruited before faster type two muscle fibers. And type one fibers are used more for long-lasting activities like postural muscles while type two fibers are used for quick bursts. [00:11:31] Thus, motor units are recruited in an order from the size of the motor unit, smaller ones are recruited first. The size of the muscle cells, smaller ones are recruited before larger ones, and the type and speed of contraction of the muscle fibers. Slower type one are recruited before faster type two. [00:11:48] This chart looks at age and gender, males are generally stronger than females. In both genders, however, the muscle strength increases from birth to adolescence. And then peaks between the ages of 20 to 30, before gradually declining. So this is on average, the grip strength is what's noted in this chart. [00:12:15] The grip strength of the dominant hand in males and females is about the same until puberty. After puberty, males start to exhibit a significantly greater grip strength than females, with the greatest differences occurring during middle age. So during this peak time about 30 to 50. The greater strength of males appears to be related to the greater muscle mass that they develop after puberty. [00:12:42] After puberty, the muscle mass of males can be as much as 50% greater than that on females. However, the muscle strength per cross-sectional area of a muscle is similar in both males and females, and the proportion of fast-twitch and slow-twitch fibers in a specific muscle is similar in the two groups as well. [00:13:02] So although muscle strength is related to age and gender in the population as a whole, there are many exceptions to the rule because of two factors. One, the large variation in rate at which biologic maturation occurs. And two, the large variation in individual genetics and specific conditioning levels which are acquired and maintained through diet and exercise. [00:13:25] These are the references, thank you.