BIO 2110 Module 4: sample paper, in real form

Skeletal Muscle Structure and Contraction: Tracing Four Flights of Stairs from Sarcomere to Soreness

[Student Name]

Thomas Edison State University

BIO-2110: Anatomy and Physiology I

Module 4 Written Assignment

Mentor: [Mentor Name]

June 14, 2026

From Whole Muscle to Sarcomere

A skeletal muscle is a nested structure, and the nesting is what allows one organ to pull in a single direction with graded force. The whole muscle is wrapped in epimysium. Inside it, bundles called fascicles are wrapped in perimysium, and inside each fascicle every muscle fiber is wrapped in endomysium. These three sheets are continuous with one another and with the tendon at each end, so tension generated anywhere inside the muscle is transmitted to bone rather than lost in soft tissue. A gastrocnemius in a composite 44 year old adult holds on the order of a million fibers arranged this way (Marieb & Hoehn, 2019).

A muscle fiber is itself an unusual cell. It forms by the fusion of many precursor cells, so a single fiber several centimeters long carries hundreds of nuclei pressed just under its membrane, each governing protein production in the region around it. Filling the interior are myofibrils, cylinders roughly 1 to 2 micrometers across that run the length of the fiber. Because the myofibrils lie parallel and their internal banding lines up across the whole fiber, skeletal muscle appears striped under a microscope, and that striping is a visible record of an internal order rather than a decoration (Betts et al., 2022).

The repeating segment of a myofibril is the sarcomere, bounded by two Z discs and about 2 micrometers long at rest. Thin filaments of actin anchor to the Z discs and project inward. Thick filaments of myosin sit in the middle, each carrying hundreds of heads angled toward the thin filaments. Two membrane systems complete the picture. Transverse tubules are inward folds of the cell membrane that carry an electrical signal into the depth of the fiber, and the sarcoplasmic reticulum wrapped around each myofibril stores calcium. The tubules meet the reticulum at regular positions, so the signal reaches every sarcomere at nearly the same instant.

The Contraction Cycle and the Fuel Behind It

Contraction begins outside the muscle. A motor neuron reaching the fiber releases acetylcholine into the synaptic cleft, the fiber membrane depolarizes, and an action potential spreads across the surface and down the transverse tubules. Voltage sensors in the tubule membrane are mechanically coupled to calcium release channels in the sarcoplasmic reticulum, so the electrical signal opens those channels directly rather than through a slow chemical relay. Calcium floods the space around the filaments within milliseconds, binds troponin, and troponin drags tropomyosin off the actin filament, exposing the sites the myosin heads need (Hall & Hall, 2021).

What follows is a cycle that repeats for as long as calcium and fuel are present. A myosin head, already cocked, binds actin and pivots, dragging the thin filament toward the center of the sarcomere. A fresh molecule of ATP then binds the head, which releases actin, and splitting that ATP recocks the head for another stroke. Neither filament changes length. The Z discs are simply pulled closer together, which is why the model is called sliding filament rather than shortening filament. The role of ATP in detachment also explains rigor mortis, since with no ATP left after death the heads stay bound and the body stiffens.

Fuel arrives in a predictable order, and the order matters for anything the muscle is asked to do. The ATP already dissolved in the fiber powers only the first two or three seconds of hard work. Creatine phosphate donates a phosphate to rebuild ATP for roughly the next 10 to 15 seconds. Anaerobic glycolysis then carries the load for perhaps another 30 to 60 seconds, producing ATP quickly and lactate as a by-product. Beyond that point aerobic respiration in the mitochondria supplies nearly all of it, which is why sustained activity depends on delivering oxygen rather than on stores held inside the fiber (Betts et al., 2022).

Length, Recruitment, and the Control of Force

Force is not fixed, and the first reason is geometry. Classic experiments on single fibers showed that tension varies with sarcomere length along a predictable curve, peaking where thick and thin filaments overlap enough for the largest number of heads to reach binding sites, and falling away when the sarcomere is stretched too far for contact or compressed until the filaments collide (Gordon et al., 1966). A muscle held near the middle of its range therefore produces more force than the same muscle at either extreme, which is the physiological reason a joint has a strong angle and a weak one.

The second control is how many fibers are asked to work. A motor neuron branches to supply a group of fibers scattered through the muscle, and every fiber in that group contracts whenever the neuron fires. The ratio is set by the precision the muscle needs. A neuron serving the eye muscles may supply fewer than ten fibers, while a neuron in the calf may supply a thousand or more. Lifting a coffee cup calls on a few small groups, and pushing off a stair calls on many large ones. Force is adjusted by adding groups, not by making a single fiber pull harder.

The third control is timing. A single stimulus produces a brief twitch, but stimuli arriving before the fiber has fully relaxed add together, and at a high enough frequency the separate twitches fuse into one smooth sustained contraction. Fiber composition then sets the character of the result. Slow oxidative fibers are rich in mitochondria and myoglobin, resist fatigue, and dominate postural muscles, while fast glycolytic fibers hold more glycogen, generate force quickly, and tire within a minute of maximal effort. Most human muscles hold a mixture, which is why the same calf can both hold a stance and drive a jump (Marieb & Hoehn, 2019).

Application: Four Flights of Stairs and the Soreness Two Days Later

Consider a composite adult of 44 who has been largely inactive for two years and climbs four flights, 60 steps in all, in 38 seconds. The first two or three seconds run on ATP already sitting in the fibers. Creatine phosphate covers the next stretch, and anaerobic glycolysis carries most of the remaining climb, which is why the thighs burn near the top and breathing stays heavy for a minute after arriving. Each step is a shortening contraction of the quadriceps and gluteal muscles, with Z discs pulled together inside millions of sarcomeres, and more fiber groups are called on as the ones already working begin to fatigue.

The descent is the more interesting half. Going down, the same quadriceps contract while lengthening, resisting gravity instead of overcoming it, which is a lengthening or eccentric contraction. Fewer fiber groups are active during that work, so the load carried by each active sarcomere is higher, and some sarcomeres are pulled past the overlap that lets their heads reach binding sites. The result is microscopic disruption of Z discs and of the membrane around them, followed by an inflammatory response over the following day or two. That sequence, not lactate, is what produces soreness peaking 24 to 48 hours later.

Read together, the structures explain the experience. The burning during the climb comes from the fuel pathway a 38 second effort has to use. The trembling at the top comes from recruitment reaching its ceiling. The soreness two days later comes from lengthening contractions loading sarcomeres at their weakest lengths, and the repair that follows is also the signal that drives adaptation, which is why the same climb feels easier a month later. Federal activity guidance describes that adaptation at the level of habit rather than cells, recommending regular moderate activity plus muscle strengthening on at least two days out of every seven (U.S. Department of Health and Human Services, 2018). This paper explains physiology and offers no advice to any individual reader.

References

Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). Anatomy and physiology 2e. OpenStax, Rice University. https://openstax.org/details/books/anatomy-and-physiology-2e

Gordon, A. M., Huxley, A. F., & Julian, F. J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibres. The Journal of Physiology, 184(1), 170-192.

Hall, J. E., & Hall, M. E. (2021). Guyton and Hall textbook of medical physiology (14th ed.). Elsevier.

Marieb, E. N., & Hoehn, K. (2019). Human anatomy and physiology (11th ed.). Pearson.

U.S. Department of Health and Human Services. (2018). Physical activity guidelines for Americans (2nd ed.). https://health.gov/paguidelines