Jinan study 2019–2025: Stable BMI but slipping strength and endurance among 16–18-year-olds — what schools should monitor and do

Table of Contents

1. Key Highlights:
2. Introduction
3. What the Jinan data show: body size, BMI and short‑term fluctuations
4. Muscular fitness trends: lower‑limb power down, abdominal endurance up, upper‑body strength down for boys
5. Speed, cardiorespiratory endurance and flexibility: nuanced signals
6. Why standing long jump and endurance run make effective sentinel indicators
7. Translating findings into school practice: concrete program examples
8. Measurement quality, statistical approach and what the numbers mean
9. Comparisons with regional and international evidence
10. Policy and curriculum recommendations
11. Limitations that affect interpretation and next research steps
12. What these findings mean for parents, coaches and students
13. FAQ

Key Highlights:

• Large school-based dataset (37,067 test records) from three Jinan senior high schools shows small gains in height and stable BMI from 2019–2025, but declines in explosive lower‑limb power (standing long jump) and boys’ upper‑body strength (pull‑ups), while girls’ abdominal endurance (sit‑ups) improved substantially.
• Standing long jump and the 1,000‑/800‑m endurance run emerged as practical sentinel indicators for routine monitoring within the National Student Physical Health Standard (NSPHS) battery.
• Findings point to the need for sex‑sensitive, school‑level programs emphasizing progressive lower‑limb power training, structured endurance development, and targeted strategies to arrest declines in upper‑body strength among boys.

Introduction

Between November 2019 and November 2025, three senior high schools in Jinan administered the National Student Physical Health Standard (NSPHS) battery to students aged 16–18. The resulting dataset — 37,067 annually collected test records — offers a detailed, annual snapshot of late‑adolescent physical fitness through the COVID‑era and its immediate recovery period. Height rose modestly, body mass increased slightly, and BMI remained broadly stable. Performance measures, however, followed divergent paths: lower‑limb explosive power decreased in both sexes, boys’ pull‑up counts dropped, and girls’ sit‑up performance jumped markedly.

These patterns are not uniform across fitness components. The dataset allows school leaders, physical educators, and local policymakers to identify which measures are most sensitive to recent change and which training emphases might yield the biggest returns in a school setting. The dataset’s strengths — large scale, consistent November testing, standardized protocols and examiner training — enable a practical translation of findings for school practice while the study’s design imposes limits on causal interpretation and external generalizability. The following analysis synthesizes the Jinan results, compares them to broader trend evidence, and presents specific, actionable recommendations for school‑based monitoring and programming.

What the Jinan data show: body size, BMI and short‑term fluctuations

The dataset includes 18,966 male and 18,101 female test records collected annually from students who completed the NSPHS battery in November of each study year. Height increased by roughly 1.26–1.27 cm across the 2019–2025 interval for boys and girls respectively. Body weight rose overall, and BMI remained largely unchanged — mean BMI shifts were trivial (boys −0.07 kg/m²; girls −0.06 kg/m² between 2019 and 2025). Both sexes experienced a transient dip in weight and BMI around 2020, followed by recovery and modest increases thereafter.

Interpretation

• The small upward change in height reflects continued gains in linear growth that have been recorded in recent decades across many Chinese urban and peri‑urban populations.
• Stable mean BMI over six years suggests that, at this late‑adolescent age and within these boarding school settings, population‑level body mass did not change substantially despite year‑to‑year fluctuations; the 2020 dip plausibly reflects short‑term COVID‑related lifestyle disruption rather than a persistent trend.
• Because the dataset is school‑ and city‑specific, these morphological findings should not be extrapolated to the provincial or national level without caution.

Practical implication for schools

• Continued, routine recording of height and weight remains important because even small shifts at the population level can mask widening dispersion (more students at the extremes). Schools should pair mean‑level surveillance with categorical reporting (e.g., percentages of underweight, normal weight, overweight, obese by NSPHS thresholds) so that rising numbers at risk are visible even when means are stable.

Muscular fitness trends: lower‑limb power down, abdominal endurance up, upper‑body strength down for boys

Key numeric signals

• Standing long jump declined by about 4.04 cm in boys (2019 → 2025) and 4.44 cm in girls. Cohen’s d indicates small effects for boys (d ≈ −0.20) and small‑to‑modest effects for girls (d ≈ −0.28).
• Boys’ pull‑ups fell by an average of 1.53 repetitions (d ≈ −0.31), a small but notable decline. By contrast, girls’ 1‑minute sit‑ups increased by 7.15 repetitions — a very large effect (d ≈ 0.92).
• These changes reflect divergent directions across different muscular domains within the same schools and time window.

Interpreting the divergence

• Standing long jump measures explosive lower‑limb power and is sensitive to neuromuscular coordination, plyometric capacity and overall leg strength relative to body mass. A roughly 4‑cm decline over six annual cohorts suggests deterioration in lower‑limb power that is measurable and may be functionally meaningful at the population level.
• Pull‑ups require relative upper‑body strength; their decline among boys could reflect insufficient targeted training, increases in relative body mass among weaker students, or reductions in habitual activities that develop pulling strength (climbing, field games). The decline of 1.53 repetitions is small in effect size but matters because pull‑up counts are already a challenging test for many adolescents; losing one or two reps can shift performance categories for a substantial number of students.
• Girls’ large increase in sit‑ups points to one of two non‑exclusive explanations: focused practice and test‑oriented preparation in the school environment, and greater responsiveness of abdominal endurance to regular low‑effort practice. Sit‑up performance can improve markedly with relatively modest, repeated training compared with gains in explosive power or maximal strength.

Why different components moved in different directions

• Some fitness components are more amenable to modest, regular practice and assessment preparation (sit‑ups, short sprints). Others, such as explosive power and maximal strength, require structured progressive overload, technical coaching, and often more varied movement exposure (plyometrics, resisted movements). If routine PE or informal practice emphasizes volume and test rehearsal rather than progressive strength development, improvement will be asymmetric across fitness domains.
• Boys’ pull‑ups being sensitive to lean mass and relative strength makes them especially vulnerable to concurrent small weight gains without matching strength increases.

Action points for PE teachers

• Implement progressive lower‑limb power programs: short, supervised plyometric circuits (box jumps, broad‑jump progressions), single‑leg strength work and technical coaching on jumping mechanics. Use periodized progressions across the school term rather than ad‑hoc test drills.
• Restore or expand structured upper‑body strength opportunities for boys: ring or bar work, assisted‑to‑unassisted pull‑up progressions, and body‑weight rows. Ensure technique and volume progression to reduce injury risk.
• Keep routine, low‑cost sit‑up practice where it is working, but allocate class time to maintain balance across strength qualities rather than over‑prioritizing any single test that is easy to coach.

Speed, cardiorespiratory endurance and flexibility: nuanced signals

Speed (50‑m sprint)

• Mean 50‑m sprint times showed small fluctuations and a marginal overall decrease (slightly faster sprints), with tiny effect sizes (d between −0.01 and −0.03). Statistically significant trends were detected due to large sample size but the practical change was minimal (e.g., main changes of 0.02–0.09 seconds across years). These small shifts are unlikely to alter fitness classifications or everyday functioning.

Cardiorespiratory endurance (1,000 m boys / 800 m girls)

• Endurance times fluctuated year‑to‑year and showed a slight overall increase (worsening) across the period: boys’ 1,000‑m increased by ~0.72 s and girls’ 800‑m by ~3.53 s between 2019 and 2025. Effects were small (girls r² = 0.002 for relative change) though the direction indicates a drift toward slower times across cohorts. Year‑to‑year variability around 2020 suggests transient pandemic‑linked disruption in activity patterns may have temporarily reduced endurance capacity, followed by partial recovery.

Flexibility (sit and reach)

• Flexibility remained largely stable with trivial mean changes; girls showed a small improvement, boys a small decline, but neither change was likely to be practically consequential.

Interpretation for practice

• Speed and flexibility appear stable or slightly improved; endurance and explosive power have shown less favorable movement. Schools should avoid assuming PE time devoted to sprint activities will automatically maintain cardiovascular fitness. Endurance requires sustained, structured training stimulus (progressive intervals, tempo runs, aerobic conditioning) rather than episodic or test‑only training.

Program implications

• Introduce weekly endurance sessions for late adolescents with graduated intensity and volume, tailored to academic calendars to minimize conflict with exam periods. Interval training (e.g., 4×4 minute high‑intensity intervals or repeated 2–3 minute tempo runs) can be time‑efficient and effective in boarding school timetables.
• Use mixed models: combine plyometric or strength elements with endurance intervals to maintain strength while building aerobic capacity, avoiding double‑down on single domains only.

Why standing long jump and endurance run make effective sentinel indicators

The study authors recommend standing long jump and the 1,000‑/800‑m endurance run as practical sentinel indicators within routine school monitoring. The recommendation rests on three features:

1. Sensitivity: both indicators showed consistent unfavorable movement across the study period in this population (standing long jump declined in both sexes; endurance worsened slightly).
2. Practicality: tests require minimal equipment — a tape measure or jump mat and a track or measured course — and can be administered in class‑scale group testing with modest staff resources.
3. Interpretability: results map directly onto functional capacities (lower‑limb power and aerobic endurance) that have bearing on everyday movement competence and sports participation.

How schools should use them

• Schedule sentinel tests 2–3 times per school year (e.g., start of term, middle, end) to detect short‑term shifts and monitor training effects.
• Pair mean‑based reporting with categorical thresholds (NSPHS performance classifications) to identify students who require targeted intervention.
• Use sentinel results to trigger programmatic responses: if standing long jump declines across cohorts, implement a lower‑limb strength block; if endurance drops, prioritize structured aerobic conditioning.

Pitfalls to avoid

• Overemphasizing a single test for grading or ranking can encourage narrow test‑preparation without genuine fitness gains. Use sentinel monitoring as a diagnostic tool to inform curriculum adjustments rather than as an end in itself.
• Do not ignore measurement quality: ensure consistent testing surfaces, footwear policy, and examiner training to limit artificial year‑to‑year variation.

Translating findings into school practice: concrete program examples

Designing interventions that match observed deficits requires matching training specificity, progression, and feasibility in the school context. The following examples are adaptable to boarding schools or any secondary school with constrained PE time.

Lower‑limb power block (8–12 weeks)

• Frequency: 2 sessions per week (20–30 min each) integrated into PE lessons or morning activity time.
• Structure:
  • Warm‑up: dynamic mobility, 5–8 minutes.
  • Plyometric progression: week 1–2: double‑leg hops and squat jumps; week 3–6: box jumps (low box), standing long jump progressions focusing on technique; week 7–10: single‑leg hops, bounding. 6–10 sets of 3–6 reps per movement, with full recovery between explosive reps.
  • Strength accessory: body‑weight or loaded squats, Romanian deadlifts (light), lunges for 3×8–12 to develop strength underpinning power.
• Load control and safety: emphasize landing mechanics, knee alignment, and soft surfaces. Use low volumes initially and progress based on competency.

Upper‑body strength progression for boys (10–12 weeks)

• Frequency: 2 sessions weekly.
• Structure:
  • Assisted pull‑up work: band‑assisted pull‑ups or partner assists (3–5 sets of 6–10 reps).
  • Horizontal pulling: inverted rows or TRX rows (3×8–12).
  • Isometric holds: dead hangs and scapular pulls to build scapular control.
  • Progression: reduce assistance or increase reps/sets gradually; move to negative‑only reps and partial‑range to full pull‑ups.
• Integration: combine with general upper‑body strength exercises (push variations) to maintain balanced development.

Endurance development microcycle (6–8 weeks)

• Frequency: 2 sessions per week plus one optional light aerobic block.
• Session types:
  • Interval day: e.g., 6×3 minute runs at moderate‑hard intensity with 2‑minute recovery.
  • Tempo day: 20 minute sustained effort at comfortably hard pace.
  • Long day (optional): 30–40 minute steady run for base building.
• Monitoring: use time‑based sentinel (800/1,000 m) at baseline and post‑block to gauge change.

Assessment and feedback loop

• Baseline test: standing long jump, 50‑m sprint, 800/1,000 m run, pull‑ups/sit‑ups and sit and reach.
• Mid‑block mini‑tests: standing long jump and short jump progressions to check neuromuscular adaptation.
• Post‑block test: repeat sentinel measures and report class‑level and individual changes. Use small‑group remediation for those who fall behind.

Real‑world example — a practical timeline

• Term 1 (weeks 1–6): Lower‑limb power emphasis, twice weekly; light endurance session weekly.
• Term 1 (week 7): Sentinel testing to capture initial changes.
• Term 2 (weeks 1–8): Upper‑body strength emphasis for boys; maintain lower‑limb maintenance; introduce interval endurance sessions for all.
• Term 2 (week 9): Full NSPHS battery in November with systematic data capture and analysis.

Resource cost and staffing

• Most interventions require minimal equipment (plyo boxes, resistance bands, cones, stopwatches) and basic staff training in progressive overload and technique. Investment in examiner training pays dividends in data reliability.

Measurement quality, statistical approach and what the numbers mean

Data collection strengths

• Tests were administered following the 2019 NSPHS manual, with standardized equipment and trained examiners.
• All annual assessments were held in November, reducing seasonal bias.
• Data underwent double entry and verification; implausible values were screened using a mean ±3 SD rule by sex and year.

Analytic design

• The study used repeated annual cross‑sections rather than linked longitudinal tracking of the same students. Analyses describe cohort‑level year‑to‑year change, not within‑person trajectories.
• Sample‑weighted linear regression quantified temporal trends across the seven observation years (2019–2025). Effect sizes (Cohen’s d) and r² values were used to judge the magnitude and practical relevance of changes, not only p‑values, given the very large sample size.

How to interpret effect sizes and small mean changes

• Large datasets can make trivially small changes statistically significant. The authors prioritized effect sizes and absolute unit changes. For example:
  • Standing long jump: ~4 cm decline represents a small effect but is measurable and directionally consistent across cohorts. Repeated small declines across years can have practical implications for athletic readiness and movement competence.
  • Girls’ sit‑ups: +7.15 repetitions with a large effect (d ≈ 0.92) indicates a clear, meaningful improvement. Such a large change is unlikely to be a measurement artifact.
  • Sprint and flexibility changes were statistically significant but minute in absolute terms (fractions of a second or fractions of a centimeter) and unlikely to drive programmatic change on their own.

Limits that matter for policy translation

• Absence of individual linkage prevents evaluation of whether the same students improved or declined over time. This limits identification of within‑student response to interventions.
• Lack of school identifiers and contextual data (socioeconomic mix, PE curriculum specifics, extracurricular sport participation) prevents multilevel modelling that could separate school‑level from cohort‑level effects.
• No direct measures of physical activity, sedentary time, nutrition or sleep restrict causal interpretation of observed trends. The 2020 dip is plausibly COVID‑related but cannot be attributed definitively.

Comparisons with regional and international evidence

Regional parallels

• Earlier studies in Shanghai and Yunnan reported parallel patterns: continued height gains alongside increases in body mass and mixed trajectories for fitness components. The Jinan results extend this evidence into late adolescence and across the COVID period.

International context

• Large international syntheses have documented long‑term declines in standing broad jump performance in many countries since 2000. The approximately 4‑cm decline in Jinan over six years aligns directionally with global concerns about explosive power deterioration among youth, though international comparisons require caution due to differing time spans and sample representativeness.

Practical takeaway

• The Jinan data fit a broader narrative: some fitness qualities (flexibility, short sprints, and easily trained endurance or abdominal endurance) may remain stable or improve locally, while explosive power and some strength qualities are susceptible to decline absent specific training emphasis. Boards and education authorities should prioritize resource allocation toward the components showing negative movement.

Policy and curriculum recommendations

1. Adopt sentinel monitoring and mixed reporting
   • Combine mean‑based reporting with categorical breakdowns by NSPHS thresholds (pass/fail bands) so that gains or losses among lower‑performing subgroups are not masked by stable means.
2. Build sex‑sensitive programming into PE curriculum
   • Allocate time for progressive lower‑limb power training for all students.
   • Provide targeted upper‑body strength progressions and remedial pathways for boys whose pull‑up performance is low.
   • Maintain abdominal endurance practice that is showing success among girls, while balancing training time across components.
3. Use a periodized approach rather than test‑only preparation
   • Replace short bursts of test rehearsal with blocks of progressive overload and periodized skill and strength development that carryover to test performance and overall functional capacity.
4. Create assessment‑to‑intervention pathways
   • Sentinel declines should trigger a standard response: identify underperforming classes or cohorts, deploy an 8–12 week targeted intervention, and reassess. Embed this cycle into annual PE planning.
5. Strengthen surveillance systems
   • Where feasible, retain unique student identifiers to allow longitudinal linkage. Collect contextual data (extracurricular sport, commuting mode, sleep, screen time, family socioeconomic status) to enable causal modelling and more precise program targeting.
6. Train examiners and PE staff
   • Maintain standardized training and recertification for staff conducting NSPHS assessments. High inter‑examiner reliability is essential if annual changes are to be trusted and used for policy decisions.

Limitations that affect interpretation and next research steps

Key limitations

• Non‑random sample: three public boarding senior high schools in a single city were purposively selected for complete records; findings reflect school‑ and city‑specific patterns, not national trends.
• Cross‑sectional design: absence of student‑level linkage prevents within‑person change analysis and limits causal claims about the effect of school practices.
• Missing contextual variables: no direct measures of physical activity, screen time, dietary intake, sleep, or socioeconomic position. These variables are critical to explain fitness changes.
• Potential measurement error: while protocols and equipment were standardized, field testing in school settings always carries some residual measurement variability.

Recommended research priorities

• Implement longitudinal cohort surveillance with unique identifiers to track within‑student trajectories and better quantify training responses.
• Add routine collection of lifestyle and social determinants to identify modifiable drivers of fitness change.
• Pilot randomized or quasi‑experimental school interventions that target the deficits identified here (e.g., lower‑limb power blocks) and evaluate their effectiveness using both mean and categorical outcome reporting.
• Expand surveillance beyond urban boarding schools to represent wider socioeconomic and geographic diversity.

What these findings mean for parents, coaches and students

• Parents: watch for declines in functional movement and encourage varied physical activity beyond test‑rehearsal routines. Activities like recreational jumping games, climbing, and supervised resistance play build explosive power and upper‑body strength.
• Coaches: focus program design on balanced development. Incorporate technical work, progressive overload and recovery planning to support gains in the more difficult‑to‑improve domains (explosive power, maximal strength).
• Students: consistent, structured training that progresses logically across weeks — even brief sessions done regularly — produces greater and more durable gains than last‑minute test practice.

FAQ

Q: What is the NSPHS battery and why is it used here?
A: NSPHS is China’s National Student Physical Health Standard, a standardized set of physical fitness tests used in schools. This study used NSPHS items (height, weight, BMI, sit and reach, 50‑m sprint, 1,000/800‑m run, pull‑ups/sit‑ups, standing long jump) as they are the routine, policy‑relevant measures available in school records.

Q: Are these results representative of all Chinese adolescents?
A: No. The data come from three senior high schools in Jinan and were purposively selected for complete annual records. The results are credible and informative for similar school settings but should not be assumed to reflect national trends without broader, representative surveillance.

Q: Why did girls’ sit‑up performance improve so much while other measures declined?
A: Sit‑ups respond well to frequent, low‑skill practice and can improve quickly with repeated exposure. Schools may have emphasized abdominal endurance in PE or test preparation, producing a large, measurable improvement. By contrast, explosive power and pull‑up strength require progressive overload and specific modalities that may not have been emphasized.

Q: Could the COVID‑19 pandemic explain changes around 2020?
A: The dataset shows a transient decline in weight/BMI around 2020 and some year‑to‑year fluctuation in fitness; these patterns are consistent with short‑term pandemic disruption of activity and schooling. However, because the dataset lacks direct measures of lifestyle change and the assessments continued in November each year, the evidence supports plausibility rather than a definitive causal attribution.

Q: How often should schools test and monitor fitness?
A: Sentinel measures like standing long jump and the 1,000/800‑m run can be done 2–3 times per school year to detect meaningful cohort changes and to evaluate interventions. Full NSPHS batteries are commonly administered annually; combining annual full batteries with interim sentinel checks balances data needs with resource constraints.

Q: What immediate actions can schools take based on these findings?
A: Start with sentinel monitoring and triage: identify cohorts or classes with declining standing long jump or endurance. Introduce time‑efficient, structured programs: short plyometric and strength sessions for lower‑limb power; assisted pull‑up progressions for boys; interval endurance sessions. Prioritize examiner training and maintain consistent testing conditions to ensure reliable trend detection.

Q: How should data be reported to be most useful?
A: Report both means and category‑based distributions (e.g., percentage above/below NSPHS thresholds). Include sex‑disaggregated results and track changes among the lower‑performing quintiles to ensure interventions target those who need them most.

Q: What research would strengthen future monitoring?
A: Longitudinal tracking with unique student identifiers, collection of physical activity and sedentarism measures, nutritional and socioeconomic data, and randomized evaluations of school‑level training interventions would clarify causal pathways and provide stronger evidence for targeted policies.

Q: Can small mean changes really matter?
A: Small mean changes can be meaningful when they reflect consistent directional change over time or when they disproportionately affect the lower end of the performance distribution. Pairing mean changes with category shares avoids overlooking rising numbers of students who fall below functional thresholds.

Q: Where should a school begin if resources are limited?
A: Begin with low‑cost sentinel monitoring (standing long jump and endurance run), minimal‑equipment training blocks (body‑weight and band progressions for pull‑ups, simple plyometrics), and teacher upskilling on progressive coaching and safe implementation. Prioritize practices that target the deficits the school observes.

This analysis uses the Jinan annual NSPHS records to identify practical priorities: arrest declines in explosive lower‑limb power and boys’ upper‑body strength, retain what is working for girls’ abdominal endurance, and deploy sentinel indicators to guide timely, evidence‑based school responses.