Muscle Mechanics. 대박자료

[문형철]

근육은 신경과 함께 인체 기능의 핵심이다. 

그 핵심적 기능을 이해할때 근육의 문제로 발생하는 통증, imbalance, optimal movement, weight distribution(체중분산)을 완성할 수 있다.

와우 완전 짱자료네 ㅎㅎ

Muscle Mechanics.pdf

근육의 기본 특성

근육의 기본적인 5가지 특성

1. 흥분성(excitability)

2. 전도성(conductivity)

3. 수축성(contractility)

4. 신장성(extensibility)

5. 탄력성(elasticity)

You can investigate muscle mechanics by eval‎uating the performance of a human subject but because you are working with a complex machine it can be difficult to interpret your findings. It is much simpler to work with isolated muscles that can be tested analytically in the laboratory. The traditional preparations shown in figure 1 usually use frog gastrocnemius

muscles so they do not need to be kept warm and the muscle is tested in two modes: isometric where the length of the muscle is not allowed to alter, and isotonic where the load is kept constant. Isometric loading can be achieved in human subjects using a dynamometer – grip strength dynamometers being the commonest device. Isotonic loading is achieved by lifting weights slowly. 

- 근육은 straightforward linear tension generators가 아니다. 하지만 완전히 unexpected 방식으로 움직임.

- 본 강의는 근육이 어떻게 기능하는가에 대해 살펴봄. 

- 근육의 mechanical properties와 근육이 tension생성을 어떻게 하는가에 대해서 알아봄. 

- isometric은 근육의 길이가 고정되고, tension은 달라질 수 있음.

- isotonic은 근육의 tension이 고정되고, 근육의 길이는 달라질 수 있음. 구심성 등장성 수축과 원심성 등장성 수축으로 나뉨. 

- 근수축하는 동안 일어나는 일

1) action potential(10ms)

2) Ca2+ level in cytoplasm(50ms)

3) delayed tension production(3ms~80ms)

- single twitches

- summation of two twitches

- steppe twitch

- tetanus

In an isolated muscle experiment you have precise control over the stimulation of the nerve. If the nerve is given a single, short pulse as a stimulus then the muscle produces a characteristic response known as a twitch. This is a short spasm of contraction that rapidly generates a small amount of force and then declines to zero over a longer period. If a second

stimulus is applied before the twitch has decayed to zero then you get a summation effect with a peak force that is higher than that of a single twitch: this is known as mechanical summation. If a third stimulus has a similar additive effect. If a slow train of stimuli are given then the force trace is like the one shown in figure 2 with the force reaching a bumpy plateau. With a faster train of stimuli a higher plateau is reached and the force curve is now smooth. This is known as tetanus. The frequency of stimulus required for a smooth response is the fusion frequency.

- muscle twitch는 한번 자극이 주어지고, 근수축이 zero로 되돌아가기 전에 다시 자극이 주어지면 더 높은 twitch가 일어남. 이를 mechanical summation이라고 함. 만약 세번째 자극이 추가로 주어지면 그림과 같이 근수축이 배가됨. 

- 자극으로 인한 slow train이 주어지면 force trace는 그림과 같이 근수축이 가중되어 bumpy plateau에 도달함. 

- 자극으로 인한 faster train이 발생하고 그림과 같은 커브가 나타나는데 이를 강축 tetanus라고 함. 

- 근전도 검사를 해보면 자극이 주어지면 smallest motor unit가 처음 recruited되고, 두번째 세번째 자극이 연속되면 가중되어 그림과 같이 muscle recruited됨. 이러한 motor action potential은 주어진 electrode에 에서 characteristic shape를 가지는데, 이는 motor unit의 size와 motor unit의 fiber에 electrode의 거리에 의존함. 

The stimulus frequency is not the only control over the tension generated. The size of the stimulus in an in vitro experiment controls the number of nerve fibres that get activated and hence the number of motor units that contract. This effect can be seen in vivo (figure 3).  Electromyography (EMG) records the electrical activity of a muscle. If a muscle is

contracting very weakly only a single motor unit may be activated. As tension is increased additional motor units get recruited.

- 자극 주파수는 tension 생성을 조절하지 않음. 

- 자극 size는 그림 3과 같이 nerve fiber의 수를 조절하여 근육수축을 위한 motor unit 숫자를 활성화하고 조절함. 

- muscle이 매우 약하게 수축하면 single motor unit가 활성화 될 수 있음. tension이 증가함에 따라 추가적인 motor unit가 동원됨. 

size principle

1. 하나의 motor neuron은 몇개의 muscle fiber에 neuromuscular junction을 가짐

2. 어떤 motor unit가 동원될것인가에 대한 질서는 random이 아님. smaller motor unit가 먼저 동원되고 그림 4와 같이 large motor unit가 뒤따름. 

3. 작은 운동단위는 미세한 움직임을 조절하는데 사용하고 low force에서 사용됨. higher force에서 작은변화는 필요하지 않음. 만약 큰 운동단위가 힘 발화를 꺼버린다면 당신은 작은 힘에도 적용할수 없을 것임.

The order in which motor units get recruited is not random. Smaller motor units get recruited first followed by larger ones (see figure 4). This makes sense because small motor units are used for fine control which is required at low forces. At higher forces small changes in force are not necessary. If big motor units fired off force you would not be able to apply very small forces at all.

- 어떤 motor unit가 동원될것인가에 대한 질서는 random이 아님. 

- smaller motor unit가 먼저 동원되고 그림 4와 같이 large motor unit가 뒤따름. 

- 작은 운동단위는 미세한 움직임을 조절하는데 사용하고 low force에서 사용됨. 

- higher force에서 작은변화는 필요하지 않음. 만약 큰 운동단위가 힘 발화를 꺼버린다면 당신은 작은 힘에도 적용할수 없을 것임.

When a muscle is stimulated it does not instantly produce force. As can be seen from figure 5 there is a delay in tension production after electrical activity is detected. The force builds up to its maximum fairly slowly. Similarly there is a lag in tension reduction after electrical activity ceases.

- 근육이 자극될때, 즉시 힘을 생성하지는 않음. 그림 5에서 보는 바와같이 tension 생성을 하는데 약간 지연됨. maximum 힘의 생성은 천천히 발생함. electrical activity가 멈춘 후 tension감소는 약간 지연됨(lag in tension).

- 반응시간(reaction time)은 인체 부상방지에 매우 중요한 역할을 함. 

As we have seen previously the force that can be produced by a given muscle depends on the length of the muscle. This can be seen in figure 6 along with a diagram of the overlapping actin and myosin filaments that explain why this occurs. This diagram is actually a little misleading since it reports the force generated by active contraction of the muscle. This is not the only way that force is generated in the body.

- 앞에서 살펴본 바와 같이 근육길이에 의존하여 근육의 힘이 생성될 수 있음. 

- 그림 6에서 보는 바와같이 액틴과 미오신 필라멘트 overlaping 그림은 근육의 길이와 tension관계를 설명함. 

- 근절의 길이가 2.0um~2.25um일때 근육의 최대힘이 발생함.

A muscle consists of an active force generating component and a parallel connective tissue component. This connective tissue does not actively generate force but if it is stretched beyond its resting length it acts just like a rubber band and produces a passive, elastic force. Figure 7 shows the effect of both of these force generating elements on the actual force output of a muscle. As you can see forcibly stretching a muscle well beyond its resting length will generate a force higher than that produced by active contraction.

- 근육은  active force generating component 와 a parallel connective tissue component로 구성됨. 결합조직은 능동적으로 힘을 만들어내지는 않음. 하지만 resting length를 넘어서 스트레치 된다면 rubber band와 같이 수동적인 elsatic force를 생성함. 

- 그림 7은 근육의 actual force output에 있어서 근육과 결합조직의 힘을 생성하는 효과를 보여줌. 근육이  resting length를 넘어서 억지로 스트레칭됨에 따라 근육이 active contraction에 의해 생성하는 힘보다 더 강한 힘이 생성될 수 있음. 

Figure 8 shows how the effect of muscle length can be demonstrated on the hamstrings. The hamstrings (except for the short head of biceps femoris) act over both the hip joint and the knee joint: they extend the hip and flex the knee. Thus when the hip is extended the muscle is already shorter than its resting length and unable to produce its maximum amount of force. If the hip is flexed then the longer hamstring are able to apply a higher force to flex the knee.

- 그림 8은 근육길이의 효과를 보여줌.  그림a는 근육길이가 매우 짧은 상태에서 tension을 생성하는데 한계가 있음을 설명. 그림 b는 근육길이가 길어져 좀더 강한 tension이 생성되는 것을 보여줌. 

Figure 9. Force/velocity curve

As well as the length of a muscle having an effect on the maximum force it can generate, so does the contraction velocity. That means that if a muscle is contracting rapidly it cannot generate as much force as when it is stationary, and an even greater force is required to stretch a maximally active muscle (see figure 9).

- 근육 수축속도가 최대힘을 생성할 수 있는 만큼, 근육 길이 또한 최대힘을 생성할 수 있는 효과를 가짐.

- 이 말의 의미는 만약 근육이 빠르게 수축한다면 그것이 정적일때 만큼 많은 힘을 생성하기 힘듬. 심지어 강한 힘은 최대로 active muscle 스트레치가 필요하다. 

참고) force-velocity property

1) 구심성수축에서 속도가 빠를때 150파운드 이겨냄

2) 구심성수축에서 속도가 느릴때 200파운드를 이겨냄

3) 원심성수축에서 등척성 수축할때 250파운드를 이겨냄

4) 원심성수축에서 속도가 느릴때 300파운드를 이겨냄. 

Figure 10 tries to make this clear. Imagine that you are bench pressing a heavy weight. The maximum weight you can lift off your chest rapidly is quite low. The maximum weight you can lift slowly is somewhat higher, and the maximum weight you can maintain the height of is higher still. An even higher weight will force you to lower it slowly. This relates exactly to graph in figure 9. A muscle applying force without shortening is known as an isometric contraction. A muscle applying force and shortening is a concentric contraction. A muscle applying force but being extended anyway is performing an eccentric contraction.

- 그림 10은  이 관계를 명확하게 보여줌. 당신이 bench pressing 을 할 때를 상상해 보라. 당신이 빠르게 벤치를 들때는 더 가벼운 벤치를 들 수 밖에 없다. 그러나 천천히 들때는 더 무거운 벤치를 들 수 있고, 들어올린 채로 유지할때는 더 무거운 것을 들 수 있다. 심지어 더욱 무거운 벤치는 당신이 벤치를 들어올리는 것을 더욱더 느리게 만들 것이다. 이것은 그림 9와 연결이 되어있다. 

Concentric muscle activity is what we normally think about muscles doing. We do not think much about isometric activity but we use it all the time to maintain posture. Eccentric muscle activity is also common and is often used at the ends of activities to slow down movements and is obviously used in situations when energy is being lost such as walking down stairs or landing from a jump. Figure 11 shows some examples.

- 구심성 근수축은 우리가 근육에 대하여 생각하는 방향으로 움직이는 것. 우리는 isometric activity에 대해서 많이 생각하지 않지만 우리는 자세를 유지하기 위해 isometric activity를 많이 이용함. 원심성 수축은  slow down 움직임의 끝에서 사용됨. 

Just to make life more complicated it turns out that the contraction speed of muscle fibres is also not constant.  Experiments on isolated muscles have revealed two distinct populations of muscle fibres that have different time courses for the basic twitch response: so called fast twitch and slow twitch fibres.

- 인체가 복잡하게 움직일때 근수축이 항상 일정한 것은 아니다. 근육을 isolated시켜 실험을 해보면 근육수축의 속도에 따라 두가지 특징이 나타난다. fast twitch, slow twitch

참고) There are two broad types of voluntary muscle fibers: slow twitch and fast twitch. Slow twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly.

- 근섬유는 slow twitch and fast twitch 두가지 타입이 있음

- slow twitch 섬유는 오랜기간 일정하게 힘을 내지만 큰힘을 내지 못하고, fast twitch는 빠르고 강한 힘을 내지만 쉽게 근피로에 빠짐. 

- slow twitch fiber는 산소를 에너지로 사용하고, type 1 fiber(스산원 이렇게 외워야겠다)

- fast twitch fiber는 포도당을 에너지로 사용하고 type 2 fiber

These different fibre types can also be identified by using stains that are specific for ATPase activity. It turns out that the fibres differ in their metabolism as well as their twitch response. 

- 두가지 근섬유는 ATPase 활성을 위한 특이한 염색반응을 이용하여 확인할 수 있음. 이는 대사방식의 차이임. 

Some muscle fibres are primarily used for rapid bursts of activity. These are (unsurprisingly) the fast twitch fibres and they obtain their energy using the anaerobic, glycolytic pathway. However this type of muscle fibre fatigues very quickly. It relies on locally stored glycogen for its energy source and this is rapidly depleted. It also produces lactic acid which is toxic and needs to be oxidised after the activity has finished – this is the oxygen debt that sprinters build up. 

- 포도당을 에너지로 이용하는 무산소 운동은 fast twitch fiber. 하지만 쉽게 근피로에 빠지고 젖산생성으로 toxic...

For slower, sustained activity slow twitch fibres operate aerobically. They can continue to produce force for long periods without significant fatigue. There is also a third type of fibre that has intermediate properties and can metabolise either aerobically or anaerobically. As you can see from figure 13 these fibre types have a variety of names. And as you should suspect from the fact that they are numbered there are some other fibre type subgroupings.

- 산소를 에너지로 이용하는 유산소운동은 slow twitch fiber. type 1 fiber

Fibre types seem to depend on the muscle usage. Sprinters develop more fast twitch fibres and fewer slow twitch ones. The converse is true for long distance runners. Muscle biopsy is sometimes used to assess the fibre type composition of an athlete’s muscles to check on the progress of training or rehabilitation. However human muscles are always fairly mixed in terms of fibre type: they always appear the same approximate colour overall. 

- 단거리 달리기 선수는 fast twitch 섬유가 발달하고, 느린 움직임(마라톤 선수는) slow twitch가 발달함. 

However some birds and fish have extreme fibre type compositions. Thus the dark meat of chicken legs is almost entirely slow twitch and the white meat in the breast is fast twitch. This corresponds to the fact that chickens are mostly terrestrial and they use their legs for sustained locomotion whereas their wings are for escape flights and need to contract rapidly but only for short periods of time. Birds such as pigeons that fly over longer distances have dark breast meat. In figure 14 you can see the two types of muscle in the body of a fish. The central dark area is slow, aerobic muscle that powers sustained swimming and the paler areas are fast, anaerobic muscle used for short bursts of speed (prey capture and escape).

- 새와 물고기는 하나의 섬유만 존재함. 그래서  닭다리의 dark meat는 slow twitch가 대부문. 

The arrangement of the muscle fibres also has an important role to play. As you can see from figure 15 the muscle fibre direction is not always in the same direction as the line of pull of the muscle. You can also see just how many individual muscles we have – be glad this is not an anatomy course!

- 근육의 배열은 근육이 움직이는데 중요한 역할

- 그림에서 보는 바와같이 fiber 방향이 항상 근육이 당기는 line(근육의 line of pull)과 같지는 않음. 

설명추가)

- muscle force는 angle of pennation의 각때문에 만들어짐. 

- 아래의 그림 A의 경우 pennation각이 거의 없이 병렬배열되어 있을때는 힘을 만들기보다는 자세유지근으로서의 역할을 주로 함. 즉 myofibril의 길이가 짧을수록 힘은 더 세다. 즉 muscle fiber length와 muscle length는 다름. 

- 근섬유의 모양들

- parallel and pennate 두가지 카테고리로 구분함. 

- 가자미근은 parallel배열

- 반막양근은 penniform muscle

- 대외사두근은 bipenniform 배열

- 상완삼두근은 multipenniform근육

- parallel 근육은 근섬유의 직접적인 짧아짐에 따라서 근육이 짧아짐. 

- pennate 근육은 다름. 

- pennation은 수축의 형태를 다르게 보여줌. 

When the line of action of the muscle does not match the line of action of the fibres then the muscle is known as pennate. There are a number of sub-classifications illustrated in figure 16 but the important property of these pennate muscles is the angle of pennation: the angle between the two lines of action.

- 근육의 움직임 라인이 근섬유 움직임 라인과 일치하지 않을때 이를 pennate라고 함. 

- 중요한 특징은 pennation angle. 

The maximum force a muscle can generate depends on its physiological cross-section area. In fact the maximum force can be calculated by multiplying the PCA by constant (approximately 20 to 100 N.cm-2). In a non-pennate muscle this is simply the area of a slice taken in the middle of a muscle perpendicular to the line of pull. However as can be seen

from figure 17 this would miss some of the muscle fibres. In this case the cross-sectional area would need to be taken perpendicular (at right angles) to the average fibre direction so as to include all the fibres in the muscle.

- 근육의 최대힘은 생리적 cross-section area에 달려있음. 사실상 최대힘은 PCA에 의해서 계산될 수 있음. 

- non-pennate muscle에서 ???

Figure 18 shows how the cross-sectional area is effected by the pennation angle but there is another factor that needs to be taken into account. The angle of pennation has increased the cross-sectional area but the line of pull is no longer the same as the line of pull of the muscle. To compensate for this we need to multiply by the cosine of the pennation angle (this will be 1.0 when the angle is 0° and 0.0 when the angle is 90°).

- 

We can express these relationships as a series of equations:

Equation 1.

Fmax = PCA × K

Where Fmax is the maximum force the muscle can generate, PCA is the physiological cross

sectional area and K is a constant (20 to 100 N.cm-2).

For non-pennate muscles

Equation 2.

PCA = m / (ρ L)

Where m is the mass of the muscle, ρ is its density (1.056 g.cm-2) and L is the length of the

muscle fibres.

For pennate muscles

Equation 3.

PCA = m cos θ / (ρ L)

Where θ is the angle of pennation.

The length of the muscle fibres depends on the exact geometry of the muscle but tends to be much smaller in pinnate muscles so although cos θ is ≤ 1.0, the PCA of a pinnate muscle of given mass will be larger than the same sized non-pennate muscle.

Figure 19 shows how the pennation angle varies among some of the muscles in the hind limb. The high pennation angle of the calf muscles (Soleus and Gastrocnemius) allows them to produce more force for their size.

In case you think that pennation gives you something for nothing it is worth remembering that the amount a muscle can contract depends on its fibre length. Muscles fibres can contract to about 60% of their resting length. Since the muscle fibres in pennate muscles are shorter than the non-pennate equivalent the amount of contraction is similarly reduced. In addition since the line of contraction is not the same as the line of action you need to put in the cos θ factor too. All in all pennation is a non-ideal mechanism. The most efficient option is to have the line of action parallel to the muscle fibres – any angular difference means that energy (as we shall see later) is wasted producing tension in directions where it cannot be used. However the physical layout of the skeleton is such that we often need high absolute forces rather than contraction distance and pennation is a mechanism that allows this.

[문형철]

근육의 기본적인 5가지 특성
1. 흥분성(excitability)
2. 전도성(conductivity)
3. 수축성(contractility)
4. 신장성(extensibility)
5. 탄력성(elasticity)

[문형철]

산소를 에너지로 이용하는 유산소운동은 slow twitch fiber. type 1 fiber

[문형철]

근수축하는 동안 일어나는 일
1) action potential(10ms)
2) Ca2+ level in cytoplasm(50ms)
3) delayed tension production(3ms~80ms)

[문형철]

연어 근육은 빨간색, 닭은 흰색, 참새는 빨간색 ... 음.. 사람은 흰색과 빨간색이 석여있고...