Study analysis · PLoS ONE · 2019
Changing your workouts randomly doesn’t make you bigger—but it *does* make you *want* to keep working out.
If you switch up your exercises every time you lift, you’ll feel more excited to train, even if you grow just as much as someone doing the same routine.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study is like a fair test where people were randomly split into two workout groups — one with the same exercises every time, and one with random exercises. It shows that switching exercises might help people feel more motivated to keep working out, and that both groups got stronger and built similar amounts of muscle.
What’s the bottom line?
Some people do the same workout every time, others change it up. This study checked if changing exercises randomly makes people more excited to work out and if it helps build muscle and strength.
How strong is this study?
The study was well-organized with a clear plan, but it had a small number of people and didn’t hide which group they were in, which could affect results. Because of this, we can trust the findings somewhat, but should be careful about saying it proves something for everyone.
0 / 100
- COI disclosureconflicts of interest not disclosed
- Data availabilitydata not shared
- Code availabilitycode not shared
59 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=21)+2.0/20
- Follow-up+10/10
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Blinding status is 'Unknown', which downgrades confidence in causal inference; small sample size (n=21) increases risk of bias and limits generalizability; short duration (8 weeks) may not reflect long-term effects.
Key takeaways
- 01
Both groups gained about 0.12–0.18 cm in leg muscle size and 10–12 kg in squat strength.
- 02
The group that changed exercises felt much more motivated, while the fixed group felt slightly less motivated.
- 03
Changing workouts didn’t make muscles grow more, but it did make people want to keep training more.
Surprising findings
- Randomizing exercises didn’t improve muscle growth—but *did* dramatically boost motivation.Many fitness programs promote 'muscle confusion' as a way to grow faster, but this study shows hypertrophy was identical—yet motivation skyrocketed.
- The fixed routine group actually became *less* motivated over time—even though they gained strength and muscle.People assume progress = motivation, but this shows you can get stronger and still feel less excited to train if the routine feels stale.
Practical takeaways
Use an app or randomizer to pick your exercises each session if you're feeling bored or unmotivated.
This only applies to resistance-trained men; results may differ for beginners, women, or longer programs.
medium confidenceStick to a fixed routine if you prefer structure and tracking progress—but add novelty occasionally to stay engaged.
Motivation declined slightly in the fixed group, suggesting even small changes might help maintain enthusiasm.
medium confidenceTrain 4x per week with moderate-to-heavy loads to build strength and muscle effectively.
All sessions were supervised—real-world adherence may vary.
high confidenceWhy this study matters
Motivation Boost from Variety
The group that had exercises and rep ranges randomized each session saw a significant increase in intrinsic motivation (effect size ES = 1.28), while the fixed routine group showed a slight decline. This difference had a moderate effect size (Cohen’s d = 0.58).
Staying motivated is one of the hardest parts of fitness—this shows a simple tweak (changing exercises) could help people stick with training long-term.
Same Gains, Different Approach
Both groups gained about 0.12–0.18 cm in thigh muscle thickness and 10–12 kg in squat strength over 8 weeks, with no meaningful differences between fixed and varied routines (ES < 0.2).
You don’t have to change your routine to build muscle or strength—consistency works. But if you’re bored, switching things up won’t hurt your progress.
High-Frequency Training Works
Participants trained 4 times per week for 8 weeks, completing at least 95% of sessions under supervision. Both groups made large strength gains, showing this frequency is effective for trained lifters.
Four-day splits are popular—but this proves they work when done consistently, even in already trained individuals.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Some people do the same workout every time, others change it up. This study checked if changing exercises randomly makes people more excited to work out and if it helps build muscle and strength.
Research results
Both groups gained about 0.12–0.18 cm in leg muscle size and 10–12 kg in squat strength. The group that changed exercises felt much more motivated, while the fixed group felt slightly less motivated.
What this means - more context
Changing workouts didn’t make muscles grow more, but it did make people want to keep training more.
The study aimed to compare the effects of a fixed versus randomly varied resistance training program on muscle thickness, maximal strength, and intrinsic motivation in resistance-trained men.
An 8-week resistance training program performed 4 times per week led to significant gains in muscle thickness and strength in both fixed and randomly varied exercise groups, with no significant differences between groups. However, intrinsic motivation significantly increased in the varied group (moderate effect size) while showing non-significant declines in the fixed group.
Methods Used
Twenty-one resistance-trained men (19 completed) were randomized into fixed (CON) or exercise-randomized (EXP) groups. Both performed 3 sets of 6 exercises, 4 times/week for 8 weeks. EXP used a mobile app to randomize exercises and rep ranges per session. Outcomes included muscle thickness, 1RM strength, and intrinsic motivation via questionnaire.
Main Finding
Randomly varying resistance exercises and rep ranges significantly improved intrinsic motivation (Cohen’s d = 0.58, ES = 1.28) compared to a fixed routine, while producing equivalent gains in lower-body muscle thickness (0.12–0.18 cm increases) and maximal strength (10–12 kg squat, 4–7 kg bench gains), with trivial between-group differences (ES < 0.2).
Confidence Level
Moderate confidence due to small sample size (n=19 completers), but strengths include randomization, supervised training, and objective measures for muscle and strength. Lack of blinding and unreported confidence intervals limit precision.
Study Flags
Red Flags
- •Small sample size (n=21, 19 completers)
- •Lack of blinding of participants or assessors
- •Limited generalizability to women, untrained individuals, or longer durations
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Randomizing exercises didn’t improve muscle growth—but *did* dramatically boost motivation.
Many fitness programs promote 'muscle confusion' as a way to grow faster, but this study shows hypertrophy was identical—yet motivation skyrocketed.
Practical Takeaways
Use an app or randomizer to pick your exercises each session if you're feeling bored or unmotivated.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study is like a fair test where people were randomly split into two workout groups — one with the same exercises every time, and one with random exercises. It shows that switching exercises might help people feel more motivated to keep working out, and that both groups got stronger and built similar amounts of muscle.
Strengths
- Randomization confirmed and described
- Control group with matched training volume and frequency
- Objective outcome measures (1-RM, ultrasound, validated questionnaire)
Weaknesses
- Blinding status is 'Unknown' — likely unblinded, introducing performance and detection bias
- Small sample size (n=21, final n=19) reduces statistical power
- Only 15 of 19 completed motivation questionnaire, introducing potential attrition bias
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Some people do the same workout every time, others change it up. This study checked if changing exercises randomly makes people more excited to work out and if it helps build muscle and strength.
Research results
Both groups gained about 0.12–0.18 cm in leg muscle size and 10–12 kg in squat strength. The group that changed exercises felt much more motivated, while the fixed group felt slightly less motivated.
What this means - more context
Changing workouts didn’t make muscles grow more, but it did make people want to keep training more.
The study aimed to compare the effects of a fixed versus randomly varied resistance training program on muscle thickness, maximal strength, and intrinsic motivation in resistance-trained men.
An 8-week resistance training program performed 4 times per week led to significant gains in muscle thickness and strength in both fixed and randomly varied exercise groups, with no significant differences between groups. However, intrinsic motivation significantly increased in the varied group (moderate effect size) while showing non-significant declines in the fixed group.
Methods Used
Twenty-one resistance-trained men (19 completed) were randomized into fixed (CON) or exercise-randomized (EXP) groups. Both performed 3 sets of 6 exercises, 4 times/week for 8 weeks. EXP used a mobile app to randomize exercises and rep ranges per session. Outcomes included muscle thickness, 1RM strength, and intrinsic motivation via questionnaire.
Main Finding
Randomly varying resistance exercises and rep ranges significantly improved intrinsic motivation (Cohen’s d = 0.58, ES = 1.28) compared to a fixed routine, while producing equivalent gains in lower-body muscle thickness (0.12–0.18 cm increases) and maximal strength (10–12 kg squat, 4–7 kg bench gains), with trivial between-group differences (ES < 0.2).
Confidence Level
Moderate confidence due to small sample size (n=19 completers), but strengths include randomization, supervised training, and objective measures for muscle and strength. Lack of blinding and unreported confidence intervals limit precision.
Study Flags
Red Flags
- •Small sample size (n=21, 19 completers)
- •Lack of blinding of participants or assessors
- •Limited generalizability to women, untrained individuals, or longer durations
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Randomizing exercises didn’t improve muscle growth—but *did* dramatically boost motivation.
Many fitness programs promote 'muscle confusion' as a way to grow faster, but this study shows hypertrophy was identical—yet motivation skyrocketed.
Practical Takeaways
Use an app or randomizer to pick your exercises each session if you're feeling bored or unmotivated.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study is like a fair test where people were randomly split into two workout groups — one with the same exercises every time, and one with random exercises. It shows that switching exercises might help people feel more motivated to keep working out, and that both groups got stronger and built similar amounts of muscle.
Strengths
- Randomization confirmed and described
- Control group with matched training volume and frequency
- Objective outcome measures (1-RM, ultrasound, validated questionnaire)
Weaknesses
- Blinding status is 'Unknown' — likely unblinded, introducing performance and detection bias
- Small sample size (n=21, final n=19) reduces statistical power
- Only 15 of 19 completed motivation questionnaire, introducing potential attrition bias
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study was well-organized with a clear plan, but it had a small number of people and didn’t hide which group they were in, which could affect results. Because of this, we can trust the findings somewhat, but should be careful about saying it proves something for everyone.
0 / 100
- COI disclosureconflicts of interest not disclosed
- Data availabilitydata not shared
- Code availabilitycode not shared
59 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=21)+2.0/20
- Follow-up+10/10
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Blinding status is 'Unknown', which downgrades confidence in causal inference; small sample size (n=21) increases risk of bias and limits generalizability; short duration (8 weeks) may not reflect long-term effects.
Standing
Who’s using this study?
The videos and claims on this site that lean on this study, and the researchers who wrote it.
1 video from House of Hypertrophy cite this study, drawing 1 claim from it.
- Contradicted
Evidence contradicts this claim.
Evidence
Authored by
5 researchersIf this is your work, this is how we attribute it on Fit Body Science. Eneko Baz-Valle is listed as the lead author.