What the Colorado Experiment Proved About Muscle Growth

The Colorado Experiment was a 28-day strength training demonstration conducted in May 1973 at Colorado State University, in which bodybuilder Casey Viator reportedly gained about 63 pounds of muscle while losing roughly 18 pounds of fat. Orchestrated by Arthur Jones, the inventor of Nautilus exercise machines, the experiment was designed to prove that brief, extremely intense workouts could produce dramatic results. More than fifty years later, it remains one of the most cited and most disputed events in the history of strength training, and unpacking why tells you a lot about how muscle actually grows.

What Happened During the 28 Days

Arthur Jones brought Casey Viator to Colorado State University, where exercise physiologist Elliott Plese supervised the measurements. Viator trained on Nautilus machines roughly three times per week, performing just one set of each exercise taken to the point of complete muscular failure. Workouts were short, sometimes under 30 minutes. Jones trained alongside Viator under the same protocol and reported gaining about 15 pounds of muscle himself during the same period.

The claimed numbers for Viator were staggering: starting at around 168 pounds and finishing near 213 pounds, with body-fat percentage dropping substantially. If taken at face value, Viator would have added muscle at a rate that no controlled study has ever replicated. Jones published the results through his own channels and used them relentlessly to market both the Nautilus machines and his high-intensity training philosophy. The experiment was never submitted to or published in a peer-reviewed journal.

Why the Results Were Almost Certainly Misleading

The single most important detail that Jones downplayed is that Casey Viator was not a novice. He had won the AAU Mr. America title in 1971 at just 19 years old. In the months before the Colorado Experiment, Viator suffered a serious industrial accident involving an allergic reaction to a tetanus injection that hospitalized him and caused significant weight loss. He also reportedly scaled back or stopped training during that period. By the time he arrived in Colorado, he was substantially below his previous muscular peak.

This matters because what looks like explosive new muscle growth was almost certainly regrowth of muscle Viator had already built. The body reacquires previously held muscle far more easily than it builds new tissue from scratch, a phenomenon researchers now call muscle memory. Viator was also known to have used anabolic steroids during his competitive career. Even though he may not have used them during the 28-day experiment itself, the prior exposure creates lasting biological advantages that make rapid regain plausible in a way that has nothing to do with the training protocol.

What Science Knows About Muscle Memory

The concept that a previously trained muscle bounces back faster after a layoff is well established, though the mechanism behind it is still debated. For years, the leading theory was that extra nuclei added to muscle fibers during training stick around permanently, even after the muscle shrinks from disuse. These retained nuclei, the thinking went, let the muscle rebuild faster when training resumes because the cellular machinery is already in place.

A systematic review and meta-analysis of human and animal studies challenged that idea directly, finding that myonuclei are not permanent but are actually lost during periods of atrophy and with aging. The authors concluded that the permanence of these extra nuclei does not appear to explain muscle memory and suggested that epigenetic changes, essentially chemical modifications to DNA that alter how genes are expressed, likely play a more important role in the phenomenon.

1PubMed Central. Myonuclear permanence in skeletal muscle memory: a systematic review and meta-analysis of human and animal studies

Whatever the underlying mechanism, the practical reality is clear: someone regaining lost muscle is in a fundamentally different biological situation than someone building new muscle. Applying Viator’s results to a person who has never been heavily muscled is like measuring how fast a fluent speaker relearns a language after years away and concluding that anyone can achieve fluency in a month.

High-Intensity Training and What It Actually Produces

Jones used the Colorado Experiment to argue that high-intensity training, performing fewer sets at maximum effort, was superior to the higher-volume programs most bodybuilders followed. The HIT philosophy calls for training to momentary muscular failure, keeping workouts brief, and allowing extended recovery between sessions. The question is whether the approach works better than traditional multi-set training when you strip away the confounding factors of the Viator demonstration.

A controlled study comparing low-volume high-intensity training to a traditional three-set protocol in trained men found that both approaches produced significant improvements in muscular performance with large effect sizes. The high-intensity group actually had significantly greater gains on three of nine exercises tested and larger effect sizes on eight of nine. Body composition did not change significantly in either group, though the trends in muscle mass and fat mass slightly favored the high-intensity group.

2PubMed Central. A comparison of low volume ‘high-intensity-training’ and high volume traditional resistance training methods on muscular performance, body composition, and subjective assessments of training

These findings suggest that HIT is a legitimate and time-efficient training method, but they also reveal something Jones would have preferred to gloss over: the differences between HIT and higher-volume training are relatively modest. Both work. The Colorado Experiment’s dramatic results did not come from the training protocol alone, no matter how forcefully Jones argued otherwise.

Tempo, Tension, and the Nautilus Factor

Part of Jones’s marketing pitch was that Nautilus machines, with their specially shaped cams, provided variable resistance that matched how strong a muscle is at different points in a movement. A standard barbell or cable stack applies roughly constant resistance, but human muscles produce different amounts of force depending on joint angle. Jones argued this mismatch limited results and that Nautilus machines solved the problem.

Research on variable-cam systems has confirmed the basic engineering premise: a well-designed cam can produce resistance patterns that broadly track the natural fluctuations in human torque across joint angles, with the best agreement typically near the point of peak torque and more variability at extreme positions of flexion and extension.

3Applied Sciences. Validation of a Novel Variable-Cam System: Electromyographic and Kinetic Analysis

Jones also emphasized slow, controlled repetitions, and HIT practitioners today often use deliberate tempos of several seconds per phase. Research on time under tension has shown that when total time under load is equalized between protocols, the number of repetitions matters less than you might expect. Two groups training with different rep speeds but identical total time under tension produced similar gains in both muscle size and strength, indicating that volume cannot be evaluated separately from how long the muscle is actually working.

4The Journal of Strength & Conditioning Research. Equalization of Training Protocols by Time Under Tension Determines the Magnitude of Changes in Strength and Muscular Hypertrophy

So the Nautilus machines and the emphasis on controlled tempo were not snake oil. They reflect genuine training principles. But they do not explain gains of 63 pounds in 28 days, and the fact that the principles are sound does not validate the extraordinary claims Jones built on top of them.

How Fast Muscle Actually Grows

To understand why the Colorado Experiment’s numbers strain credulity, it helps to know what happens at the cellular level after a hard workout. Muscle protein synthesis, the process by which damaged muscle fibers are repaired and enlarged, ramps up quickly after resistance training. Research has shown it is elevated by about 50 percent at four hours post-exercise, more than doubled at 24 hours, and then drops back toward baseline by around 36 hours.

5PubMed. The time course for elevated muscle protein synthesis following heavy resistance exercise

Separate work confirmed that a single bout of heavy resistance exercise can increase biceps muscle protein synthesis for up to 24 hours, after which the rate begins to decline.

6PubMed. Changes in human muscle protein synthesis after resistance exercise

This window of elevated protein synthesis is the engine behind hypertrophy, and it has hard biological limits. Even under ideal conditions with optimized nutrition and training, most research suggests that a natural trainee can add somewhere in the range of one to two pounds of actual muscle tissue per month. Over 28 days, even an aggressive estimate for a well-fed beginner lands nowhere near the figures Jones reported. The only plausible explanation for Viator’s gains involves regaining previously built muscle, the effects of prior steroid use, and water and glycogen replenishment in depleted muscles, all of which can happen far faster than genuine new tissue growth.

The Eccentric Emphasis

One aspect of Nautilus training and HIT that deserves separate attention is the emphasis on the lowering phase of each repetition, known as the eccentric or negative portion. Jones designed Nautilus machines partly to load the eccentric phase more effectively, and HIT practitioners generally stress controlled negatives as a key driver of muscle growth.

There is real science behind this. A study comparing eccentric-only to concentric-only training, matched for total work, found that eccentric training was more effective at producing hypertrophy. The researchers also observed that significant increases in muscle size could appear as early as four weeks of training. Most of the strength gains in both groups came from neural adaptations rather than increased muscle size, at least in the early weeks.

7PubMed Central. Neuromuscular Adaptations to Work-matched Maximal Eccentric versus Concentric Training

The eccentric advantage is real but modest. It does not multiply muscle growth to the degree Jones implied, and it certainly cannot account for the Colorado Experiment’s extraordinary claims. Still, Jones was ahead of his time in recognizing that the lowering phase of a lift matters, and most modern training programs now incorporate eccentric loading intentionally.

Hormonal Responses and the Intensity Argument

Jones and his followers also argued that training to failure with high intensity triggered a larger hormonal response, particularly in testosterone and growth hormone, and that this hormonal surge was responsible for superior muscle growth. The hormonal piece of this claim has partial support: high-intensity resistance exercise does produce greater acute spikes in testosterone and cortisol compared to moderate-intensity protocols.

8PubMed. Hormonal responses to high- and moderate-intensity strength exercise

Similarly, a pilot study examining women’s growth hormone responses to exercise at different intensities found that a single bout of high-intensity exercise significantly increased pulsatile growth hormone secretion over a 12-plus-hour window compared to a resting control condition.

9PubMed Central. Pilot study: an acute bout of high intensity interval exercise increases 12.5 h GH secretion

Where the argument falls apart is in the assumption that these transient hormonal spikes meaningfully drive long-term muscle growth. Research over the past two decades has increasingly shown that acute post-exercise hormone elevations are poor predictors of who actually gains the most muscle. The hormonal story was compelling marketing in the 1970s, but it is far less persuasive today. Mechanical tension and progressive overload, not fleeting hormone spikes, appear to be the primary drivers of hypertrophy.

Why Some People Respond More Than Others

One often-overlooked element of the Colorado Experiment is that Casey Viator was genetically exceptional. He had won Mr. America at 19 and possessed an unusual combination of muscle fiber composition, bone structure, and hormonal profile that made him an extreme outlier. Even if the experiment had been perfectly controlled, his results would not have been generalizable.

Research into the genetics of hypertrophic response has found that people cluster into distinct response categories. An analysis of gene expression in skeletal muscle identified differences among hypertrophic response groups even before training began, suggesting that extreme responders were essentially “primed” to grow at the genetic level. The researchers identified several genes and signaling pathways that appear to promote or inhibit muscle growth and thus shape where a person falls on the response spectrum.

10International Journal of Exercise Science: Conference Proceedings. Genetic Regulation of Myofiber Hypertrophy?

This genetic variability means that taking any single individual’s results, especially results from someone like Viator who sat at the extreme right tail of the response curve, and presenting them as evidence for a training system is fundamentally misleading. It would be like studying Usain Bolt’s sprint workouts and telling everyone they can run a sub-10-second 100 meters if they just follow his program.

The Nutrition Question Jones Never Addressed

The Colorado Experiment’s write-ups focused almost exclusively on the training protocol, with little attention to what Viator was eating. In any muscle-building context, nutrition is half the equation. A review of the evidence on energy surplus and muscle growth noted that while a caloric surplus does provide a clear anabolic stimulus that can promote muscle gain even independent of resistance training, the specific surplus needed to optimize muscle growth relative to fat gain has never been validated in a training population.

11PubMed Central. Is an Energy Surplus Required to Maximize Skeletal Muscle Hypertrophy Associated With Resistance Training

In Viator’s case, he was almost certainly eating at a large surplus while regaining lost weight, and a body that is both calorically replete and returning to a previously established set point partitions nutrients very differently than one trying to build entirely new tissue. The refeeding dynamic, well documented in starvation recovery research, involves feedback loops between body composition changes and appetite regulation that accelerate the recovery of both fat mass and lean mass.

12Wiley Online Library. Physiology of weight regain: Lessons from the classic Minnesota Starvation Experiment on human body composition regulation

Viator was not building from nothing. He was refilling a biological container that already existed, fueled by abundant calories and aided by physiological mechanisms that prioritize restoration of previously held body composition. Without accounting for any of this, the Colorado Experiment’s numbers are essentially meaningless as evidence for what a training protocol can do.

Cardiovascular and Metabolic Effects of High-Intensity Protocols

One area where the HIT philosophy has found unexpected vindication is in cardiovascular fitness. Jones primarily promoted HIT for building muscle, but the circuit-style, minimal-rest format of many HIT programs turns out to have genuine cardiorespiratory benefits. A study of overweight women performing high-intensity functional circuit training found improvements in peak oxygen uptake, reductions in body mass, fat mass, and waist-to-hip ratio, along with gains in fat-free mass and multiple measures of physical function.

13PubMed Central. Functional High-Intensity Circuit Training Improves Body Composition, Peak Oxygen Uptake, Strength, and Alters Certain Dimensions of Quality of Life in Overweight Women

Research in heart failure patients comparing high-intensity interval training to progressive circuit resistance training found that both modalities improved peak oxygen consumption and metabolic equivalents, though each format had slightly different secondary advantages.

14PLoS ONE. High-intensity interval training versus progressive high-intensity circuit resistance training on endothelial function and cardiorespiratory fitness in heart failure: A preliminary randomized controlled trial

The metabolic stress generated by intense resistance training with short rest periods also creates a distinct physiological environment that contributes to adaptation. The accumulation of metabolites in working muscles during high-effort sets influences hormonal release, oxygen availability within muscle cells, and cellular swelling, all of which provide additional growth signals beyond pure mechanical tension.

15PubMed Central. Role of metabolic stress for enhancing muscle adaptations: Practical applications

Jones would have loved this research. The irony is that the strongest modern case for HIT-style training has less to do with building record-breaking muscle mass and more to do with time efficiency and metabolic health, practical benefits he never emphasized because they were far less dramatic than “63 pounds of muscle in 28 days.”

What the Colorado Experiment Actually Proved

If you strip away the marketing, the steroid history, the muscle memory, and the genetic outlier, the Colorado Experiment demonstrated something genuinely useful that got buried under its own hype: that brief, intense resistance training produces meaningful results. You do not need to spend two hours in a gym. You do not need dozens of sets per muscle group. A focused, hard effort taken to or near failure can stimulate significant adaptation. Modern research has repeatedly confirmed this, even if the effect sizes are not as dramatic as Jones wanted people to believe.

The experiment also accidentally demonstrated how powerful muscle memory and refeeding effects are, even though Jones had no interest in that interpretation. A previously muscular, previously enhanced athlete recovering from illness and detraining can regain mass at rates that look supernatural to anyone who does not know the backstory. That lesson is arguably more valuable than anything Jones intended to teach, because it explains a pattern millions of people experience on a smaller scale every time they return to the gym after a break and find their strength comes back faster than expected.