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A mediation model of physical activity levels and cardiorespiratory fitness: The role of sleep quality and stress

This cross-sectional study of 300 young adults demonstrates that sleep quality and perceived stress significantly mediate the relationship between physical activity and cardiorespiratory fitness, both independently and through a serial pathway.

Original authors: Hemlata Vats, Richa Hirendra Rai, Saima Zaki, Ankita Dey

Published 2026-07-02

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Hemlata Vats, Richa Hirendra Rai, Saima Zaki, Ankita Dey

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: The "Fitness Chain Reaction"

Imagine your body's ability to run, climb, and breathe easily (called Cardiorespiratory Fitness) as a high-performance car engine. Usually, we think the only way to make that engine run better is to move more (Physical Activity).

However, this study suggests that moving your body isn't just about the engine; it's also about the fuel quality and the driver's mood. The researchers found that when you exercise, it doesn't just build muscle; it also fixes your sleep and calms your stress. These two things (sleep and stress) act like a "middleman" or a bridge that helps your exercise actually translate into a fitter heart and lungs.

The Cast of Characters

To understand the study, let's meet the four main players:

Physical Activity (PA): How much you move (walking, running, working out).
Cardiorespiratory Fitness (CRF): How well your heart, lungs, and muscles work together to keep you going.
Sleep Quality (SQ): How well you rest at night.
Stress: How overwhelmed or anxious you feel.

The Experiment: What They Did

The researchers gathered 300 young adults (ages 18–28) who were generally healthy. They didn't force anyone to exercise; they just asked them to report their habits and took some measurements.

The Sleep Check: They used a questionnaire (PSQI) to see if people were sleeping well or having trouble.
The Stress Check: They used a survey (PSS) to see how stressed people felt.
The Movement Check: They asked people how much they walked, worked out, or moved around during the week (IPAQ).
The Fitness Test: They had everyone do a "Step Test" (stepping up and down on a box for 3 minutes) to estimate how fit their hearts and lungs were.

The Findings: How the Pieces Fit Together

The study found that Exercise is the boss, but it works through a chain reaction. Here is how the paper describes the connections:

1. The Direct Line (The Engine)

What they found: If you move more, your fitness goes up.
The Analogy: This is like pressing the gas pedal. The more you press it, the faster the car goes. This was the strongest link.

2. The Sleep Detour (The Rest Stop)

What they found: People who moved more tended to sleep better. Better sleep, in turn, was linked to better fitness.
The Analogy: Think of exercise as a mechanic tuning your car. But if you don't park the car in a safe garage (good sleep) to let the repairs settle, the tuning doesn't stick. The study found that exercise helps you get that "good garage" rest, which then helps your engine run smoother.

3. The Stress Detour (The Driver's Mood)

What they found: People who moved more felt less stressed. Less stress was linked to better fitness.
The Analogy: Imagine driving a car while screaming and panicking (high stress). It's hard to drive efficiently. Exercise acts like a "calm-down button" for the driver. When the driver is calm, the car (your body) can perform better.

4. The "Double-Dip" Chain (The Serial Mediation)

What they found: This was the most interesting part. The study found a specific chain: Exercise → Better Sleep → Less Stress → Better Fitness.
The Analogy: Imagine a relay race.
- Runner 1 (Exercise) passes the baton to Runner 2 (Sleep).
- Runner 2 passes it to Runner 3 (Stress).
- Runner 3 finishes the race with a fitter body.
- The study showed that this specific sequence is a real, working path. Exercise helps you sleep, which helps lower your stress, which then helps your fitness.

What the Numbers Said

Sleep & Stress: They are best friends (or enemies, depending on how you look at it). If your sleep is bad, your stress goes up.
Exercise & Stress: If you exercise, your stress goes down.
Exercise & Sleep: If you exercise, your sleep gets better.
The Result: All these paths combined mean that exercise improves fitness in three ways: directly, by helping you sleep, and by calming your stress.

The Limitations (The "Fine Print")

The authors were honest about what their study couldn't prove:

It's a Snapshot, Not a Movie: They took a picture of people's lives at one specific moment. They can't say for sure that exercise caused the better sleep or that better sleep caused the lower stress. They just know they happened together.
Self-Reporting: People guessed how much they exercised and how stressed they were. Sometimes people forget or exaggerate.
The Test: They used a step test to guess fitness levels rather than a high-tech lab machine, so the numbers are estimates, not exact measurements.

The Bottom Line

The paper concludes that being fit isn't just about how hard you run. It's a team effort. Exercise is the captain, but it needs Sleep and Stress Management as its co-captains to get the best results. If you want a fitter heart and lungs, moving your body helps, but it also works by fixing your sleep and calming your mind.

Technical Summary: A Mediation Model of Physical Activity Levels and Cardiorespiratory Fitness

Problem Statement
While physical activity (PA) is widely recognized as a primary determinant of cardiorespiratory fitness (CRF), the specific behavioral and psychological mechanisms linking the two remain under-explored within a unified framework. Existing literature establishes that chronic stress and poor sleep quality are associated with reduced health-promoting behaviors and adverse physiological responses that may diminish CRF. However, few studies have simultaneously examined sleep quality and perceived stress as mediating variables to explain how PA influences CRF in young adults. This study addresses this gap by investigating whether the relationship between physical activity and cardiorespiratory fitness is mediated by sleep quality and perceived stress, both independently and through a serial pathway.

Methodology
The study employed a cross-sectional design involving 300 asymptomatic, healthy participants aged 18–28 years, recruited via convenience sampling from institutional and community settings in India. Participants with a history of sleep disorders, cardiopulmonary disease, musculoskeletal or psychiatric conditions, or use of medications affecting sleep/autonomic function were excluded.

Data collection utilized the following standardized instruments:

Physical Activity (PA): Assessed using the International Physical Activity Questionnaire (IPAQ) Long Form, quantified in Metabolic Equivalent Task minutes per week (MET-min/week).
Sleep Quality (SQ): Measured via the Pittsburgh Sleep Quality Index (PSQI), where scores >5 indicate sleep disturbances.
Perceived Stress: Evaluated using the Perceived Stress Scale 10 (PSS-10).
Cardiorespiratory Fitness (CRF): Estimated using the McArdle Step Test to predict VO₂max based on post-exercise heart rate recovery, utilizing gender-specific regression equations.

Statistical analysis was conducted using IBM SPSS Statistics (v26.0). Pearson's correlation analysis examined bivariate relationships. To test the mediation hypotheses, a serial multiple mediation analysis was performed using Hayes' PROCESS Macro (Model 6). Indirect effects were evaluated using bias-corrected bootstrapping with 5,000 resamples to generate 95% confidence intervals (CIs).

Key Results
Descriptive statistics indicated a mean age of 21.39 years, a mean BMI of 22.38 kg/m², and a mean predicted VO₂max of 45.41 mL·kg⁻¹·min⁻¹. The mean PSQI score was 5.19 (suggesting mild sleep disturbances), and the mean PSS score was 9.71.

Correlation analysis revealed significant relationships among variables:

PA was positively correlated with CRF ( $r = 0.42$ ).
PA was negatively correlated with both sleep quality scores (indicating better sleep with higher PA, $r = -0.32$ ) and stress levels ( $r = -0.41$ ).
CRF was negatively correlated with sleep quality scores ( $r = -0.43$ ) and stress levels ( $r = -0.44$ ).
Sleep quality scores were positively correlated with stress levels ( $r = 0.50$ ).

Mediation analysis confirmed that the relationship between PA and CRF operates through both direct and indirect pathways:

Direct Effect: PA had a significant positive direct effect on CRF ( $\beta = 0.0035, p < 0.001$ ).
Indirect Effects:
- PA → Sleep Quality → CRF: Sleep quality independently mediated the relationship (Indirect effect = 0.0010; 95% CI: 0.0006–0.0017).
- PA → Perceived Stress → CRF: Perceived stress independently mediated the relationship (Indirect effect = 0.0008; 95% CI: 0.0004–0.0013).
- Serial Mediation (PA → Sleep Quality → Perceived Stress → CRF): A significant sequential pathway was identified, suggesting that PA improves sleep quality, which in turn reduces perceived stress, ultimately enhancing CRF (Indirect effect = 0.0004; 95% CI: 0.0002–0.0006).

Significance and Claims
The authors claim that this study provides evidence that the association between physical activity and cardiorespiratory fitness in young adults is not solely a direct physiological adaptation but is significantly mediated by behavioral and psychological factors. Specifically, the study demonstrates that:

Higher physical activity levels are associated with improved sleep quality and reduced perceived stress.
Both sleep quality and stress levels act as independent mediators in the PA-CRF relationship.
There is a significant serial mediation effect, indicating that sleep quality and stress function as interconnected mechanisms where PA improves sleep, which subsequently lowers stress, contributing to better fitness outcomes.

The paper concludes that optimizing cardiorespiratory fitness in young adults may require a holistic approach that integrates physical activity promotion with strategies to improve sleep health and manage stress, rather than focusing on exercise alone.

Limitations
The authors acknowledge several limitations inherent to the study design:

The cross-sectional nature precludes causal inference regarding the directionality of relationships.
The use of convenience sampling limits generalizability to broader populations.
The narrow age range (18–28) restricts applicability to middle-aged and elderly populations.
Reliance on self-reported measures (IPAQ, PSQI, PSS) introduces the potential for recall bias.
The lack of objective measures (e.g., accelerometry, polysomnography, CPET) and the use of a predictive step test for VO₂max rather than direct measurement.
Unmeasured confounding variables such as caffeine intake, nutrition, and mobile usage duration.