Chapter 3: Phenomena of Muscle Contraction

A single fiber is all-or-none, yet you can lift a pen or a suitcase with the same biceps, and hold either one still or move it. This chapter explains how. It covers the three factors that set the strength of contraction, the "phenomena" you will record on the kymograph and in PhysioEx (twitch, treppe, summation, tetanus and fatigue), and the two basic types of contraction, isotonic and isometric.

It follows the "Fenomena Kontraksi & Refleks Otot" session; the reflex half of that session is in Chapter 4.

Three factors that set contraction strength

The force a whole muscle develops depends mainly on:

1. The degree of stretch of the muscle before it contracts (the length–tension relationship).
2. The number of motor units activated (recruitment).
3. The frequency of stimulation (summation and tetanus).

Other factors matter too, such as fiber diameter (thicker, hypertrophied fibers have more myofibrils), fiber type, temperature and fatigue, but these three are the ones tested in the practicum.

1. Degree of stretch: the length–tension relationship

How hard a sarcomere can pull depends on how many crossbridges can form, and that depends on how much the thick and thin filaments overlap before contraction starts. The force a muscle develops is therefore a reflection of its starting length, and it is maximal when the muscle starts at an optimal length.

The classic single-sarcomere experiment (Gordon, Huxley and Julian, in the 1960s) gives four landmark points:

Optimal condition: sarcomere length 2.0–2.2 µm. In the body, the attachment of muscles to bones keeps most skeletal muscles working near this range, roughly 70% to 130% of optimal length.

At the whole-muscle level there are two components of tension:

• Active tension comes from crossbridge cycling and follows the overlap curve: highest at the optimal length and lower when the muscle is shorter or longer.
• Passive tension comes from the elastic recoil of stretched titin and connective tissue. It is zero at or below resting length and rises steeply as the muscle is stretched further, just as a rubber band resists more the more you pull it.
• Total tension = active + passive.

This relationship has everyday consequences. A long jumper crouches slightly before take-off, bringing the leg muscles close to their optimal length. In the heart, the same principle (more filling → more stretch → stronger contraction) is the Frank–Starling law you will meet in the cardiovascular block.

2. Number of motor units recruited

A motor unit is one motor neuron plus all the skeletal muscle fibers it innervates. It is the basic functional unit of contraction from the nervous system's point of view: when the neuron fires, every fiber in its unit contracts together.

Note on the slides. The slide defines a motor unit as one motor neuron and "the myofibrils it innervates". A neuron innervates whole muscle fibers (cells), not myofibrils, which are structures inside a fiber.

Key facts:

• Each muscle contains many motor units, and the fibers of one unit are scattered among fibers of other units rather than clumped together.
• Motor units vary in size. Muscles for fine control have small units (an extraocular muscle may have about 10–20 fibers per neuron); large postural and power muscles have large units (the gastrocnemius has over a thousand fibers per neuron).
• The strength of contraction depends on how many motor units are active. Adding units is called recruitment, and it is the main way force is graded. The session put it simply: the more motor units, the stronger the contraction, and vice versa.
• Units are recruited in order of size (Henneman's size principle): small, fatigue-resistant units are activated first for light tasks, and large, powerful, fast-fatiguing units are added only when more force is needed.
• During sustained contraction, the nervous system rotates activity among units (asynchronous recruitment), letting some rest while others work. This delays fatigue.

In the practicum, raising the stimulus voltage mimics recruitment. A weak stimulus reaches threshold in only a few fibers; a stronger one excites more, until every fiber responds (the maximal stimulus). Beyond that, raising the voltage adds nothing (see Chapter 5).

3. Frequency of stimulation

Stimulating a muscle repeatedly can increase its force of contraction. The key is timing. A muscle action potential lasts only 1–2 ms and the refractory period is equally short, but the twitch it triggers lasts 10 to more than 100 ms. So a second action potential can arrive while the fiber is still contracting from the first. More Ca²⁺ is released before the previous Ca²⁺ has been pumped away, more crossbridges stay attached, and the tension builds on itself.

Frequency and recruitment work together: to lift a heavier weight, the nervous system both recruits more motor units and makes each fire faster.

The muscle twitch

A twitch is the response to a single stimulus: one quick cycle of contraction and relaxation. It can be produced by a sudden electrical excitation of the nerve supplying a muscle, or by a brief electrical stimulus applied to the muscle itself. The result is a sudden contraction that lasts a fraction of a second.

A twitch recorded on a myogram has three phases:

1. Latent period. The few milliseconds between the stimulus and the start of tension. The fiber looks inactive, but a lot is happening inside: the action potential spreads, Ca²⁺ is released from the SR, and crossbridges begin to attach and take up the slack in the elastic elements.
2. Contraction phase. From the start of tension to peak tension, while crossbridges are cycling.
3. Relaxation phase. From peak tension back to zero, as Ca²⁺ is pumped back into the SR. It is usually longer than the contraction phase.

Twitch duration varies with fiber type: about 10 ms for the fast fibers of the extraocular muscles, and 100 ms or more for slow postural fibers such as those of the soleus. When stimuli are spaced far enough apart for complete relaxation, each stimulus gives an identical, separate twitch.

Treppe: the staircase effect

Treppe (German for "staircase") is a progressive increase in the strength of contraction when a muscle is stimulated repeatedly, at short intervals, with the same stimulus intensity, but with each stimulus arriving only after the muscle has completely relaxed. The first few twitches each rise a little higher than the one before, then they level off. It was first described in heart muscle by Bowditch, so it is also called the staircase effect of Bowditch.

The explanation usually given is that the muscle "warms up":

• Ca²⁺ is not completely cleared between twitches, so each stimulus releases Ca²⁺ onto a slightly higher starting level, and more troponin sites are occupied;
• the heat of contraction raises muscle temperature, speeding up enzymes, including myosin ATPase;
• the elastic elements are already slightly stretched.

Treppe is why athletes warm up before competition.

Summation

Summation (sumasi, also called wave summation or temporal summation) is the appearance of a new contraction before the previous one has ended. Part of the second contraction is added on top of the first, so the total tension rises. As the frequency increases, the summed tension keeps climbing.

Treppe and summation are easy to confuse:

Tetanus: unfused and fused

If the frequency of stimulation keeps increasing, summation turns into tetanus, a sustained contraction.

• Incomplete (unfused) tetanus. The stimuli come fast enough that the muscle cannot fully relax, but slowly enough that it relaxes partially between them. The record rises to a wavy plateau. Valleys (dips) between the peaks show that there is still a short gap between stimuli.
• Complete (fused) tetanus. The stimuli come so fast that no relaxation at all is possible between them. The peaks and valleys merge into a smooth, straight line at the maximum tension the muscle can develop. There are no valleys, because stimulation is continuous with no gap. The tension of a complete tetanus is typically three to four times that of a single twitch.
• Maximal tetanic tension is reached when a further increase in frequency no longer increases the tension.

Note on the slides. One slide in the practicum deck swaps the definitions, describing fused tetanus as stimulation "before the wave reaches relaxation" and unfused tetanus as the one where "peaks and valleys merge into a straight line". The correct pairing is the one above: unfused = partial relaxation, wavy; fused = no relaxation, smooth.

Normal voluntary movements are brief, smooth tetanic contractions. The smoothness comes from asynchronous firing of many motor units; any single unit is usually in unfused tetanus.

Two meanings of "tetanus". Physiological tetanus is the normal sustained contraction described here. The disease tetanus is caused by the toxin of Clostridium tetani, which blocks inhibitory neurons in the spinal cord. Motor neurons then fire uncontrollably, causing painful spasms, lockjaw (trismus) and arching of the back.

Fatigue

Fatigue (kelelahan) is a decline in the force a muscle can produce even though it is still being stimulated. On a record of fused tetanus, fatigue appears as the plateau sagging and then falling toward zero despite continuing stimuli. The capacity for work declines roughly in step with the depletion of muscle glycogen.

Fatigue is divided by where it arises:

Two points are worth remembering:

• Lactic acid used to be blamed for fatigue. Current evidence suggests lactate itself is not a major cause. Accumulation of inorganic phosphate (which impairs Ca²⁺ release and crossbridge force) and Ca²⁺ leak from the SR are more likely culprits.
• Fatigue is protective. It stops a muscle before its ATP falls so low that crossbridges lock into rigor. ATP levels in a fatigued muscle actually stay surprisingly close to normal.

In the practicum, fatigue follows complete tetanus. It is attributed to accumulated lactic acid, ADP and Pi after high-intensity activity, and the tension recovers partially after a rest.

Isotonic and isometric contraction

Muscle contractions are classified by what happens to length and tension:

• Isotonic contraction (iso = same, tonos = tension): the muscle changes length while the tension stays about the same, because the force it develops is enough to move the load.
• Isometric contraction (metron = measure, length): the muscle develops tension but does not change length, because the load is too heavy to move (or the task is to hold a position).

In the classic experiment, a muscle is hung from a support with a weight attached. With a 20 kg weight, the muscle develops 20 kg of tension, shortens and lifts the weight: an isotonic contraction. With a 30 kg weight that needs more than the muscle's maximal force (25 kg in the example), tension rises to its maximum but the weight never leaves the ground: an isometric contraction.

Isotonic: concentric and eccentric

• Concentric contraction: the muscle shortens while generating force, for example the biceps when you lift a cup to your mouth.
• Eccentric contraction: the muscle lengthens while still generating force, because the load is greater than the force it exerts, for example the biceps as you lower the cup slowly, or the quadriceps when you walk downstairs. Eccentric work can produce the highest forces and causes most delayed-onset muscle soreness.

Isometric

Examples are holding a heavy bag at your side, pushing against a wall, or the postural muscles keeping the head upright. Every isotonic contraction actually begins with a short isometric phase: tension must first rise until it equals the load before any shortening can happen. This is why, in PhysioEx, heavier loads lengthen the latent period before the muscle starts to shorten.

Load and velocity

The heavier the load, the longer the latent period, the slower the shortening and the shorter the distance moved. When the load equals the muscle's maximum force, shortening velocity falls to zero and the contraction becomes isometric. With no load at all, velocity is at its maximum. This load–velocity relationship is tested in PhysioEx Activity 7 (Chapter 5).

Key points

• Whole-muscle force depends on starting length, the number of motor units recruited and the frequency of stimulation.
• Active tension is maximal at the optimal length (sarcomere 2.0–2.2 µm); passive tension from titin rises with stretch; total = active + passive.
• A motor unit is one motor neuron and all the fibers it supplies. More units = more force; recruitment follows the size principle.
• Twitch = response to one stimulus: latent period, contraction, relaxation.
• Treppe: stimuli after full relaxation, gradually rising twitches. Summation: stimuli before relaxation, peaks add up. Unfused tetanus: wavy plateau. Fused tetanus: smooth plateau at maximal tension.
• Fatigue is a fall in force despite continued stimulation; it can be central or peripheral, and it rises as glycogen falls.
• Isotonic = length changes (concentric shortening, eccentric lengthening); isometric = tension without length change. Heavier loads mean a longer latent period and slower shortening.