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Muscle mechanicsMuscle Length-Tension Relationship Video

Does contraction always mean shortening of the muscle?Not really; contraction simply refers to the interaction between thickand thin filament. So can you contract muscle without shortening it?Yes! Then the interaction must have changed something, if it is notthe length of the muscle then what is it? The “tone” or “tension” ofthe muscle is the other parameter that can be affected bycontraction.

When the contraction of the muscle leads to a change in the tone ofthe muscle with no change at all in its length the contraction is saidto be Isometric (Iso = same ; Metric = length).

When the contraction of the muscle leads to a change in its lengthwith no change at all in its tone the contraction is said to be“Isotonic” (Iso = same ; tonic = tone/tension).

Tension in a MyofibrilTake one muscle fibre, even a single myofibril; stretch it (manually)if there is no overlap between thick and thin filament then there is nointeraction, no contraction and no tension change recorded (occursat a sarcomere length of 3.4 μm ).

With less stretch some overlap is seen leading to interaction(between the filaments) leading to tension. Stretching with even asmaller force leads to even moreinteraction and more tension. Themaximum interaction was found tobe at the resting level (2.2 μm) andso was the highest tension (fix thesarcomere without stretching at all)when all the myosin heads areinvolved in the interaction .If you get the length shorter thanthe resting state (i.e. contraction)the tension decreases! Each thinfilament has a part of it that

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interact with the myosin heads of the other side so producing forcesin opposite direction cancelling each other and decreasing thetension.

Tension in a MuscleNow if you take the whole muscle (not just a fibril) you’ll notice asimilar pattern, but not exactly the same.

In the next curve, if you don’tstimulate the muscle and simplystretch it passive tension beginsto develop, this is simply due tothe stretching itself without theinterference of the interactionbetween the filaments (rememberwe did not stimulate the muscleyet). This passive tension is seendue to the elastic properties of themuscle as a whole (not seen insingle muscle fibres /fibrils)(stretch an elastic rubber band and there will automatically be atension due to its elastic properties). Therefore, more stretch = moretension.

Now if we stimulate the muscle the active tension enters into thegame, (the active tension is due to the interaction between thefilaments) it will be maximum at the resting length ( that is when thepassive tension = 0 ) , but it is going to be less than that if wecontract our muscle fibrils at higher or lower length than the optimalone 2.2 μm .

Now there is a total tension which resembles the sum of the activetension at a certain point and the passive tension at that same point(remember; passive tension was developed due to the elasticity ofthe muscle).

Once we have maximum overlap between the filaments at a singleside (resting length) we get the highest active tension.

Velocity of ContractionWhat happens if you are loading (putting some weight on) the

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muscle? What happens to the velocity of contraction?

When the load = 0 the velocity of contraction = max; as you increasethe load the velocity decreases and eventually you reach to a pointwhere the muscle won’t have theability to lift the load and so thevelocity of contraction will reach 0.

MeasurementsMoving on, how do we measure thecontraction of the muscle [isotoniccontraction that is; i.e. assumingthere is no load at all on the muscle =no change in tension only length ischanged (maybe some negligible change in tension)].

Fix one head of the muscleand attach the second headto a lever system thatrecords the movement of themuscle via a stylus (seeimage below), when themuscle contracts an upwardline is drawn on a paper andwhen it relaxes a downwardline is drawn. By rotating thepaper (at a certain set-speed) and allowing the muscle to contractand relax a curve is drawn. The curve we get will look like the curvebelow.

The red part refers to the contraction period and after it therelaxation period is seen. Moving backward in our force vs. timegraph we can see at which point the stimulation to our muscleoccurred. This needs us to move slowly backward (doctor said weneed to set the device at a low speed; Iguess he said that by mistake; if weslow down the paper then the curve willappear very small (the time the muscletakes to contract won’t change so theup and down lines drawn by the stylus

simple muscle twitchfigure

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will be very close to each other making things difficult); so I guesswe are supposed to actually set the device at a high speed to get anextended curve in which we can accurately determine when thechanges occurred). We notice that from the point our stimulatorstimulated the muscle to the point in which contraction began therewas some time delay, this time delay is referred to as the “LatentPeriod”. So now we have three periods in our “Simple Muscle Twitch”curve: the latent period, the contraction period and the relaxationperiod. The contraction begins when calcium concentration in themuscle begins to increase (marked by a line on the diagram atapproximately 5ms) [Calcium is released from sarcoplasmicreticulum, events before that are classified under the latent period].In the relaxation period the calcium concentration decreases bypumping of calcium from the sarcoplasm to the sarcoplasmicreticulum via calcium pumps.

Now what happens if we induceanother action potential(stimulus) before our musclecompletely relaxes? Ourcontraction gets summated, (weget a new latent period then anew contraction period, and ifthe muscle has not completelyrelaxed yet, then the new contraction will reach a level higher thanthat of the first (summation), but don’t mix the latent period with therefractory period, the latter is for action potentials while the formeris for muscle twitches, the action potential begins and ends withinthe latent period in skeletal muscles .

If you give many stimuli, then the duration between two stimuliwould be just after (or at the end of) the latent period, so still you arenot likely to meet refractory periods here, but you gave the secondstimulus before the muscle enters the relaxation period from thefirst stimulus so the summation of the twitches will be even higher,and if you give many stimuli in a similar way the muscle willcontinuously get contracted and keep reaching higher amplitudes tillreaching the “tetanus” state in which all the muscle fibres arecontracted and the muscle has reached its maximum capacity tocontract (this can occur in our bodies; contract one of your muscles

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for a long time without relaxation and then you get that muscletetanised). Some call this tetanus state “Complete tetanisation” andcalled the state in which there is some relaxation and “re-contraction”after the maximum contraction has been reached as “Incomplete-tetanisation” .

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If you stimulate the muscle wait for it to relax and then stimulateagain the amplitude will not be the same [the graph below is whenour device was set at a low speed resulting in each curve beingshown in a very small area (each line at the beginning represents acontraction and a relaxation)]. The amplitude of the secondcontraction will be higher. If we repeat that and give a thirdstimulation within a short period of the muscle relaxation from thesecond stimulus then the amplitude will be even higher and so on tilla single stimulus results in an amplitude similar to that seen in atetanised muscle; this phenomenon is called Treppe phenomenon(or the stair case effect ; the graph forms a stair-like shape). Whydoes this happen? Before answering this question, we must ask; hasall the calcium that moved out of the sarcoplasmic reticulumreturned to it when the muscle relaxed completely? No it hasn’t! Thismeans that with the next stimulus some more calcium is releasedand with the calcium remaining from the previous stimulus themuscle now has a higher concentration of calcium than the firsttime, this means that there is a higher chance for more myofibrils tocontract increasing the amplitude of the second twitch and thiscontinues with each new twitch tillall the myofibrils have contractedand then an amplitude similar tothat of tetanus is reached (Note:for each troponin C molecule to beactivated 4 calcium ions must bindto it). So, the Treppe phenomenon is not a summation (the firstphenomenon is called “Wave Summation”). This treppephenomenon is the reason why players warm up before matches (inalmost all sports); they get their muscles to have the maximumpossible amount of calcium ions and so they increase the efficiencyof their muscles and the warm-up also helps in avoiding injuries asthe blood flow to and the temperature of the muscles are increased(which lowers the chances of injuries related to wear and tear ofmuscles).

Note: The non-or-all effect applies to nerves NOT to muscles (i.e. ifyou reach the threshold you’ll get an action potential otherwise youwon’t). In muscles the amplitude of the contraction can varyaccording to the calcium concentration present in the sarcoplasm(some small stimuli initiate the contraction of only a small number

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of muscle fibres resulting in no effect being seen macroscopically,this may get one to think of the non-or-all principle so it wasimportant to mention this point here).

Motor Unitsthe whole muscle is innervated by many motor neurons . there are alot of terminals for each motor neuron and each on of these terminalend up with only one Muscle .cell , so every single motor neuroninnervates a group of M.cells ( each M.cell is innervated by only onemotor terminal ) .

So, by stimulating a single motor neuron we get just a smallamplitude and by stimulating all the nervefibres the amplitude increases, this is alsoa summation referred to as the “MotorUnit Summation”. Do all our muscles havesimilar sizes (number of muscle fibresinnervated by a single neuron) of motorunits ? Nope. In some muscles you findthat each motor unit is composed ofaround 5 muscle fibres and elsewhere it’s300! This depends on the amount ofcontrol you need over that certain muscle,the higher the control needed (e.g. fingers)the smaller the size of the motor unit (e.g.structural muscles, like those of the back, need minimal control andso have motor units of large sizes).

Sometimes, we want to lift an object and think that it is light andthen when we lift it we find that it is heavier than expected andwithin milliseconds we are able to lift it normally. Why was there adelay? According to our initial thoughts, the brain sent signals for asmaller number of motor units than actually needed and when it wasrevealed that it is not enough , more signals were sent to increasethe amplitude of contraction and this process took some time whichwas felt as the “delay”.

Note: If you have only 1 muscle fibre which you know that a certain

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nerve terminal innervates and stimulated that terminal with noaction potential generated then no contraction will be seen and onlythen we can say that this follows the all-or-none principle, but whentaking the whole muscle it becomes a different story.