| Course | HLTH 4340 Fit and Well: Core Concepts in Exercise Science |
|---|---|
| Week | Week 2 |
| Paper type | Energy systems brief |
| Length | About 710 words, 5 pages |
| Format | APA 7 student paper |
| School | Walden University |
| Program | BS in Public Health |
| Updated | October 2026 |
Free sample paper for HLTH 4340 Week 2
Three Energy Systems in Three Sections: Substrate, Rate and Capacity for Each, Duration Ranges Labeled as Conventions and a 400-Meter Run Mapped Onto All Three
Student Name
Walden University
HLTH 4340: Fit and Well: Core Concepts in Exercise Science
Instructor Name
Month Day, Year
Three Energy Systems in Three Sections: Substrate, Rate and Capacity for Each, Duration Ranges Labeled as Conventions and a 400-Meter Run Mapped Onto All Three
Purpose
Muscle contraction runs on adenosine triphosphate (ATP), and the muscle holds only enough to sustain hard work for a few seconds. During intense exercise the demand for ATP can rise to as much as 1,000 times the resting rate (Baker et al., 2010). Three energy systems regenerate it: the phosphagen system, glycolysis and mitochondrial respiration, also called the oxidative system. This brief describes each by the substrate it draws on, the rate at which it can supply ATP and the total amount it can supply before it fades. It then explains how the three work together and maps one event, the 400-meter run, onto them.
The Phosphagen System
The phosphagen system regenerates ATP from phosphocreatine stored in the muscle, in a single reaction catalyzed by creatine kinase. Its rate is the highest of the three systems, because one enzymatic step stands between the store and the ATP. Its capacity is the smallest, because phosphocreatine stores are limited and fall quickly during maximal effort. Textbook tables usually describe this system as dominant for roughly the first 10 seconds of all-out work. That figure is a teaching convention for the period of highest power, not a switching point measured in muscle.
Glycolysis
Glycolysis breaks down muscle glycogen and blood glucose to pyruvate without needing oxygen in that step, producing ATP at a high rate, though below the phosphagen rate. Its capacity is larger than the phosphagen system's but still limited during maximal work. When glycolytic flux is high, much of the pyruvate is converted to lactate. The traditional explanation that lactic acid causes the fall in muscle pH has been challenged: Robergs et al. (2004) reviewed the biochemistry and concluded that the protons come chiefly from ATP breakdown when demand exceeds mitochondrial supply, and that lactate production actually slows acidosis. The brief therefore names hydrogen ion accumulation, not lactate, as the byproduct associated with fatigue in this system.
Mitochondrial Respiration
The oxidative system uses carbohydrate and fat, and to a small extent protein, in the presence of oxygen. Its capacity is very large, because fat stores alone could supply energy for many hours. Its rate is the lowest of the three, limited by how fast oxygen can be delivered and used. Gastin (2001) notes that this system is often wrongly described as slow to respond. It is active from the first seconds of exercise and rises quickly.
The Systems Work Together
All three systems contribute during almost all exercise, and what changes with time and intensity is the share each provides. Gastin (2001) traces the common picture of the systems switching on one after another to diagrams from the 1960s and 1970s that rested on questionable estimates of anaerobic energy release. His review places the point at which aerobic and anaerobic contributions are equal during maximal exercise between 1 and 2 minutes, most probably around 75 seconds, earlier than traditional teaching suggested. Dominance shifts gradually; it does not pass from one system to the next.
One Event Mapped: The 400-Meter Run
Spencer and Gastin (2001) measured energy contributions in 20 highly trained Australian runners during treadmill runs simulating track events, using the accumulated oxygen deficit method. In the 400-meter event, which takes highly trained runners roughly 50 seconds, the aerobic system supplied 43% of the energy. The figure was 29% for 200 meters, 66% for 800 meters and 84% for 1,500 meters. The crossover to mainly aerobic supply came between 15 and 30 seconds in the 400-, 800- and 1,500-meter events. A 400-meter run, often described as an anaerobic event, therefore draws heavily on all three systems: phosphocreatine in the opening seconds, glycolysis throughout and oxidative supply that becomes the largest single source well before the finish.
What Moves the Boundaries
The shares depend on intensity first. A slower pace shifts supply toward the oxidative system at every time point. Training status matters as well: in Spencer and Gastin's sample, the middle-distance specialists had higher peak oxygen uptakes than the sprinters, and endurance training raises oxidative capacity. Duration ranges in any text should therefore be read as approximations for maximal effort by a typical trained person, not as fixed times.
References
Baker, J. S., McCormick, M. C., & Robergs, R. A. (2010). Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism, 2010, Article 905612. https://doi.org/10.1155/2010/905612
Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine, 31(10), 725-741. https://doi.org/10.2165/00007256-200131100-00003
Robergs, R. A., Ghiasvand, F., & Parker, D. (2004). Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 287(3), R502-R516. https://doi.org/10.1152/ajpregu.00114.2004
Spencer, M. R., & Gastin, P. B. (2001). Energy system contribution during 200- to 1500-m running in highly trained athletes. Medicine & Science in Sports & Exercise, 33(1), 157-162. https://doi.org/10.1097/00005768-200101000-00024
What the HLTH 4340 Week 2 instructions ask for
Week 2 sets a short written explanation of how muscles regenerate ATP, and the prompt names three systems that must each be covered. Graders look for the same descriptors applied to every system, so substrate, rate of supply and total capacity should appear in each section in one order. The prompt also asks how the systems relate, and the answer it wants is concurrent contribution rather than a relay. Duration ranges are allowed, but they should be framed as approximations for maximal effort with a source attached. Many versions of the task add a practical example, an event or activity mapped onto the systems. Sources should be physiology reviews or primary measurements in journals; a trainer's website will not support a claim about acidosis. A brief of a few pages with headings fits the task well. It does not ask for training recommendations, and adding them takes space away from the physiology.
Inside the HLTH 4340 Week 2 example
After a purpose section that states the three descriptors, the brief takes the phosphagen system first: phosphocreatine as substrate, one enzymatic step via creatine kinase, the highest rate and the smallest store. Glycolysis follows, with glycogen and glucose as fuel and a rate below the phosphagen system's. That section reports the dispute over lactic acid directly: Robergs et al. (2004) place the protons in ATP breakdown and find that lactate production slows acidosis. Mitochondrial respiration comes third, with fat, carbohydrate and a little protein, a huge capacity and the lowest rate, and with Gastin's point that it responds within seconds. A separate interaction section traces the switch-on picture to diagrams from the 1960s and 1970s. The 400-meter mapping gives Spencer and Gastin's aerobic shares across four distances, from 29% at 200 meters to 84% at 1,500. The brief closes on pace and training status as the factors that shift each share.
Reading the HLTH 4340 Week 2 rubric
Expect the grading to start with physiological accuracy, where a pathway paired with the wrong fuel or a wrong rate ranking is a serious loss. Next comes the handling of time ranges. Graders accept slightly different numbers across texts, but a range stated as a fixed switching point, with no source, tends to cost credit. The interaction passage is often the analytical row: a brief that lets one system stop before the next begins misses it. Folk claims, lactic acid and next-day soreness above all, are marked down wherever they appear. Credit for evidence goes to reviews and measured studies, and this brief carries four journal sources for that reason. Organization counts because parallel sections let a grader compare systems line by line. APA mechanics close out the score, including correct entries for review articles in physiology journals. A brief that maps a real event using measured shares usually sits in the highest band.
HLTH 4340 Week 2 help: mistakes that cost marks
Students lose marks on this brief most often by drawing the three systems as a timeline. Phrases such as "after 10 seconds the body switches to glycolysis" will read as a misunderstanding, however tidy they look. A second frequent fault is blaming lactic acid for the burn and the fall in pH without acknowledging the biochemistry dispute. Third, the oxidative system gets called slow to start when the evidence shows it rising from the first seconds. Before submitting, check that every section answers substrate, rate and capacity in the same order. Attach a source and the word "approximately" to each duration figure. For the worked example, use measured contributions from a study rather than percentages from a poster. Finally, keep claims about soreness and recovery out unless the prompt asks for them. A custom brief can map a different event, such as a 1,500-meter race or a rowing sprint, onto the same three sections.
Get a HLTH 4340 Week 2 example written to your instructions
Forward the week's prompt and its rubric; a written brief modeling all three pathways is back inside 24-48h, the first custom sample free. Nobody at this desk signs into a classroom. The document that earns a grade is composed by whoever is enrolled, working from the model rather than from a copy.
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HLTH 4340 Week 2 questions, answered
Where can I find a free HLTH 4340 Week 2 sample paper?
This page holds a full Week 2 brief on the three energy systems, including the 400-meter mapping and four journal references, with margin notes explaining each section's job.
Does the HLTH 4340 Week 2 example say lactic acid causes fatigue?
No. Following Robergs and colleagues (2004), it names hydrogen ion accumulation from ATP breakdown as the source of acidosis and notes that lactate production actually slows the fall in pH.
When do aerobic and anaerobic systems contribute equally?
Gastin's 2001 review places the crossover during maximal exercise between one and two minutes, most probably around 75 seconds, which is earlier than older textbook diagrams implied.
Is the 400-meter run an anaerobic event?
Not purely. Spencer and Gastin measured trained runners drawing 43% of their energy aerobically over 400 meters, with the aerobic share becoming dominant well before the finish line.
How long does the phosphagen system last?
Texts usually give about 10 seconds of all-out work, but the brief labels that number a teaching convention for peak power rather than a measured point where the system stops.