Sex Differences in Physical Fitness Development Among Children with Mild Intellectual Disability: A Three-Year Longitudinal Study and Practical Guidance for Schools and Caregivers

Table of Contents

1. Key Highlights
2. Introduction
3. Why physical fitness matters for children with mild intellectual disability
4. How the three-year study was designed and what it measured
5. The core findings: where boys and girls diverged—and where they did not
6. Interpreting the differences: biological maturation, activity, and context
7. Why BMI did not diverge even as body mass and height did
8. Why flexibility and trunk extensor performance declined across sexes
9. Practical implications: what schools, therapists, and families should do
10. A sample 12-week intervention tailored to girls with MID
11. Measuring progress: practical assessment and data to collect
12. Research directions and gaps highlighted by the study
13. Policy and system-level recommendations
14. Limitations and cautions when applying the findings
15. How to implement changes with constrained resources
16. Translating evidence to everyday practice: case examples
17. FAQ

Key Highlights

• Boys with mild intellectual disability (MID) showed greater gains than girls in aerobic endurance (PACER laps) and dominant handgrip strength over three years; boys also increased more in height and body mass.
• Body mass index (BMI), hamstring flexibility (sit-and-reach), and trunk lift performance followed similar trajectories in boys and girls, with small declines in flexibility and trunk extensor performance across both sexes.
• The findings point to the need for sex-sensitive, evidence-based physical activity programs—especially interventions that boost aerobic capacity and muscular strength for girls with MID—implemented in schools, community programs, and clinical settings.

Introduction

Children and adolescents with intellectual disability face heightened risk for poor health outcomes linked to low physical fitness: higher rates of obesity, cardiovascular risk factors, and reduced musculoskeletal function. Tracking how fitness develops through childhood and adolescence is essential for planning effective interventions. A three-year longitudinal study conducted in Kırıkkale, Türkiye, used the Brockport Physical Fitness Test (BPFT) battery to describe time-dependent changes in body composition, aerobic endurance, and musculoskeletal function in 111 students with educable intellectual disability (IQ 50–70). The study compared developmental patterns between girls and boys aged roughly 10–14 years at baseline and uncovered sex-specific trajectories in some fitness components while others evolved similarly. Those results carry practical implications for adapted physical education, public-health programs, and family-supported activity planning. The remainder of this article synthesizes the study’s findings, examines probable causes, and translates evidence into actionable program and policy recommendations.

Why physical fitness matters for children with mild intellectual disability

Physical fitness in childhood predicts a cluster of health and functional outcomes across the lifespan. Higher cardiorespiratory fitness and muscular strength link to lower metabolic risk, healthier body composition, reduced incidence of obesity in adolescence, and lower adult cardiovascular risk. Fitness also supports musculoskeletal health, daily functioning, participation in play and sports, and even aspects of cognitive development such as executive control. For children with MID, who typically participate less in organized sports and accumulate more sedentary time, low fitness can magnify health disparities relative to typically developing peers. Tracking fitness development in this population identifies targets for early intervention and clarifies whether boys and girls follow different trajectories that warrant sex-specific approaches.

How the three-year study was designed and what it measured

The study followed a cohort of 111 students (46 girls, 65 boys) enrolled in middle and high special education settings. All participants had formal educational diagnoses of educable ID (IQ 50–70) and received only mandatory school Physical Education and Sports classes (80 minutes per week). Exclusion criteria removed students with Down syndrome, autism spectrum disorder, physical disabilities, cardiovascular disease, or engagement in organized sports during the study.

Assessments took place annually in February over three consecutive years (2020–2022). Researchers applied the Brockport Physical Fitness Test battery to capture three fitness domains:

• Body composition: height, mass, and body mass index (BMI).
• Aerobic endurance: 15 m PACER (progressive shuttle run), counting laps completed.
• Musculoskeletal function: dominant handgrip strength (kg), back-saver sit-and-reach (cm; hamstring flexibility), and trunk lift (cm; trunk extensor strength/flexibility).

Testing followed standardized BPFT procedures: demonstration and verbal instruction, rest intervals between tests, two trials with best score recorded for musculoskeletal tests, and consistent test administrators across the study. Reliability was high: intraclass correlation coefficients ranged from 0.94 (PACER) to 0.98 (handgrip).

Statistical approach: a two-factor mixed-design ANOVA examined time (three measurement waves) and sex (girls vs. boys) and their interaction, with corrections applied when statistical assumptions were violated. Effect sizes were reported as partial eta squared (ηp²).

The core findings: where boys and girls diverged—and where they did not

The study produced several robust findings and nuanced patterns.

1. Aerobic endurance (PACER laps)
   • Boys started higher and showed greater gains: mean laps for boys rose from 21.55 at baseline to 26.57 at Year 3. Girls increased from 16.09 to 19.02 laps.
   • The time × sex interaction was statistically significant (p = .038), though the effect size was small (ηp² = .033), indicating meaningful group differences against substantial individual variability.
2. Dominant handgrip strength
   • Both sexes improved across three years, but boys increased more. Boys increased from 18.65 kg to 22.57 kg while girls increased from 15.34 kg to 18.04 kg.
   • Interaction effect reached significance (p = .004) with a small-to-moderate effect size (ηp² = .053).
3. Body composition: mass, height, BMI
   • Height and body mass showed significant time × sex interactions: boys gained more in both height and weight over the three years (large effect sizes: height ηp² = .225; mass ηp² = .134).
   • BMI trajectories were statistically similar between sexes; interaction was not significant and effect size negligible. Both boys and girls experienced small increases in BMI over time.
4. Flexibility and trunk lift
   • Sit-and-reach (hamstring flexibility) and trunk lift performance declined slightly in both sexes across the study period. Time × sex interactions were not significant.
   • Effect sizes for these declines were small (ηp² ≈ .013–.014), indicating a general downward trend rather than sex-specific trajectories.

Together these outcomes suggest that aerobic capacity and upper-body strength develop differently in boys and girls with MID during early adolescence, whereas flexibility and certain body-composition metrics follow comparable patterns.

Interpreting the differences: biological maturation, activity, and context

Understanding why boys outperformed girls on aerobic and strength measures requires considering biological, behavioral, and environmental factors.

Biological maturation

• Pubertal timing and hormonal changes shape growth and performance. Boys typically experience later but larger increases in height and muscle mass driven by testosterone, contributing to gains in absolute strength and aerobic capacity (via increased hemoglobin and oxygen transport). Girls generally reach peak height earlier and accrue less muscle mass during the same chronological years.
• The study did not directly measure biological maturation (pubertal stage, Tanner staging, or hormonal levels). Without these data, part of the sex differences likely reflects differences in maturation timing between participants rather than intrinsic sex-based limits.

Physical activity levels and movement patterns

• Outside structured PE sessions, boys often engage in more spontaneous moderate-to-vigorous physical activity, including active play and informal sports, which reinforces aerobic and strength gains. The study excluded children participating in organized sports, but did not objectively track informal activity or daily movement patterns. Differential activity volumes could explain the observed sex gap in aerobic and handgrip improvements.

Neuromuscular development and motor usage

• Handgrip strength reflects both neuromuscular development and frequent use of the upper extremities. Boys may use their upper limbs more in unstructured play or chores, producing greater functional gains.
• Reduced neuromuscular drive, common in individuals with MID, can limit gains in flexibility and trunk muscle activation; those limitations likely contributed to the observed decline in trunk lift performance.

Measurement, motivation, and familiarity

• Field-based assessments depend on participant understanding, cooperation, and motivation. Although test-retest reliability was high, variable motivation or comprehension across waves could have influenced results, particularly in aerobic testing where pacing and effort determine outcomes.

Contextual factors

• The cohort received only 80 minutes of PE per week, which is low relative to international recommendations for youth physical activity. Limited curricular exposure and scarce community opportunities for adapted sport create an environment where natural increases in fitness will be modest unless supplemented by targeted programs.

Why BMI did not diverge even as body mass and height did

The study found sex-related divergence in body mass and height but not BMI. BMI reflects the relationship between weight and height squared, so proportional increases in both elements can result in stable BMI across sexes. Boys in the cohort gained more absolute height and mass, likely reflecting differential growth velocity, but these changes offset one another in the BMI calculation. BMI also masks composition: an increase in lean mass versus fat mass will produce similar BMI shifts while representing substantially different health implications. For children with MID, the composition of weight gain (lean vs. fat tissue) matters for functional capacity and metabolic risk, yet the study relied on BMI alone rather than skinfolds or waist circumference to infer composition.

Why flexibility and trunk extensor performance declined across sexes

Both sit-and-reach and trunk lift scores showed mild decreases over time. Likely contributors include:

• Sedentary behavior: low levels of habitual movement reduce range-of-motion demands on hamstrings and trunk extensors, producing stiffening and reduced flexibility.
• Growth-related changes: rapid linear growth without parallel muscle–tendon adaptation can shorten muscle functional length relative to bone length, reducing flexibility when measured in static reach tests.
• Neuromuscular activation deficits: individuals with MID often present lower voluntary muscle activation, which may blunt strength gains and functional use of trunk muscles.
• Curricular limitations: a single 80-minute PE session per week provides limited stimulus for maintaining or improving flexibility and core endurance through adolescence.

These patterns argue for integrated flexibility and postural-strength components in any intervention aimed at children with MID.

Practical implications: what schools, therapists, and families should do

The study’s results create a clear directive: strengthen and broaden opportunities to build aerobic capacity and muscular strength—especially for girls with MID—while maintaining flexibility and core strength across both sexes. Specific recommendations follow.

Increase frequency and diversity of physical activity

• Expand PE time beyond the one mandatory weekly session. Aim for multiple short sessions per week (three 30–45 minute sessions) to provide repeated stimulus for aerobic and strength adaptations.
• Combine structured lessons with supervised unstructured play and active breaks during the school day. Short activity bursts (5–10 minutes) accumulated across the day raise overall physical activity without requiring long uninterrupted periods.

Embed strength and aerobic training in adapted formats

• Aerobic training: use progressive interval formats suited to students’ motivation and comprehension. Example: 10 cycles of 30–45 seconds of brisk running or shuttle walks with 60–90 seconds recovery, progressed weekly by increasing work time or reducing rest.
• Strength training: adopt bodyweight and resistance-band protocols focusing on functional movements—squats, step-ups, seated rows with bands, push-up progressions, and grip-strength activities (squeezing therapy putty, towel wringing tasks). Two non-consecutive sessions per week can produce gains in handgrip and overall muscular capacity.

Prioritize engagement and comprehension

• Use visual schedules, simple demonstrations, and consistent routines to increase understanding and adherence. Repetition, predictable structure, and short activity segments improve engagement for many children with MID.
• Leverage positive reinforcement: immediate verbal praise, simple reward charts, or token economies sustain motivation across repeated testing and training sessions.

Design inclusive, socially oriented activities

• Small-group circuits and partner activities foster social interaction while providing moderate-to-vigorous intensity. Activities that include music, games, or goal-oriented challenges increase willingness to participate and exert effort.
• Peer-buddy systems—pairing students with trained typically developing peers—can increase participation, model movement patterns, and support motor learning.

Integrate flexibility and trunk work into daily routines

• Build short mobility and posture-focused segments into classroom transitions. Five-minute guided stretching and trunk activation sessions three times daily can counteract declines in sit-and-reach and trunk lift measures.
• Use dynamic mobility (leg swings, hip circles) before vigorous activity and static hold/stretching after exercise sessions to promote safe flexibility gains.

Focus on functional transfers

• Emphasize tasks that support daily living skills: carrying objects, climbing stairs, sit-to-stand transitions, and overhead reaching. Improvements in these functional domains often translate into better independence and quality of life.

Train personnel in adapted physical activity

• Provide teachers and support staff with practical professional development on delivering progressive aerobic and resistance programs to children with MID. Training should include safety, behavioral strategies, activity modification, and measurement techniques.

Address environmental and policy barriers

• Ensure access to safe spaces for active play outside school hours, including weekend clubs and community centers that run adapted activities.
• Advocate for funding to support additional PE staffing, equipment, and community program subsidies.

A sample 12-week intervention tailored to girls with MID

The following outline translates study implications into a concrete, scalable program aimed at improving aerobic endurance and muscular strength in girls with MID. Sessions assume supervision by a trained PE teacher or adapted activity specialist.

Program goals

• Increase PACER-like shuttle performance by 10–25% over 12 weeks.
• Improve handgrip strength by 10–20% through progressive resistance and functional grip tasks.
• Maintain or improve flexibility and trunk strength.

Weekly structure (3 sessions/week; 45–50 minutes/session)

• Warm-up (8–10 min): dynamic mobility, games that raise heart rate (tag-like games adapted for safety), and movement patterns relevant to session focus.
• Main set (25–30 min):
  • Session A (Aerobic focus): interval-based shuttle work and active circuits (e.g., 6 × 1-minute shuttle walking/running at perceived moderate-hard effort with 90 seconds easy recovery; progress by adding 1–2 intervals or increasing intensity).
  • Session B (Strength focus): resistance-band circuits and bodyweight exercises in stations (3 rounds × 30–45 sec work / 45–60 sec rest): squats, step-ups, seated rows with bands, wall push-ups, grip tasks (squeezing putty/towel wringing), and sit-to-stand repetitions.
  • Session C (Mixed/Skill): combination of low-grade aerobic circuits and motor-skill games to reinforce balance, coordination, and trunk activation (e.g., relay tasks involving carrying light objects, obstacle navigation).
• Flexibility and trunk work (8–10 min): guided static stretches for hamstrings, hip flexors, lower back, and supervised trunk activation holds (prone press-up progressions or gentle plank variations adjusted for ability).
• Cool-down and feedback (3–5 min): simple reflection, praise, sticker or token.

Progression and monitoring

• Increase interval duration or reduce rest every 2–3 weeks based on observed tolerance and performance.
• For strength, increase band resistance or add repetitions/sets after two weeks of consistent performance.
• Track performance weekly with short, reliable measures: laps in a short shuttle (10–15 m) for aerobic progression, maximal handgrip with dynamometer once every 4 weeks, and simple flexibility trunk tests monthly.

Safety considerations

• Conduct medical clearance before program start.
• Adjust intensity based on individual tolerance and health conditions.
• Provide protective surfaces and structured warm-ups to minimize injury risk.
• Monitor hydration, prevent heat stress, and allow extra rest for those who need it.

Behavioral supports

• Use clear, observable goals and visual charts.
• Incorporate social reinforcement: group-based reward systems, celebration of progress in school assemblies.
• Break tasks into micro-goals and highlight small improvements to sustain motivation.

Measuring progress: practical assessment and data to collect

Reliable, feasible assessments balance rigor with accessibility in special education settings.

Recommended toolkit

• Aerobic capacity: 15 m or 20 m PACER shuttle test administered per BPFT guidelines. Record laps; note verbal cues and participant comprehension. Conduct at baseline, mid-program, and post-program.
• Muscular strength: handgrip dynamometer for dominant hand—two trials, best score recorded. Test at baseline and every 4–6 weeks to track adaptation.
• Flexibility: back-saver sit-and-reach with two trials; record best score monthly.
• Trunk function: trunk lift test performed per BPFT. Two trials, best score monthly.
• Body composition: height and weight measured using calibrated equipment at baseline and at 3-month intervals. If possible, add waist circumference or simple skinfolds to improve composition insight.
• Physical activity monitoring: use accelerometers or pedometers for 3–7 day sampling windows at baseline and follow-up assessments when feasible. This adds objective data on daily movement patterns.
• Behavioral and functional outcomes: measures of participation in PE and community activities, caregiver reports of independent living tasks, and self/teacher-rated quality-of-life items.

Interpretation tips

• Evaluate relative rather than absolute changes: small percent gains in PACER or handgrip can reflect meaningful functional improvements.
• Track individual trajectories, not only group averages. Heterogeneity in MID populations means some students will respond faster or slower to identical programs.
• Use mixed methods: pairing quantitative measures with simple qualitative feedback from students and families helps capture the real-world impact.

Research directions and gaps highlighted by the study

The study points to important priorities for future research.

Essential next steps

• Integrate objective activity monitoring (accelerometry) into longitudinal designs to disentangle the role of habitual physical activity from innate developmental changes.
• Include measures of biological maturation (Tanner stage, age at peak height velocity, or hormonal assays) to separate maturation effects from sex differences in fitness trajectories.
• Extend follow-ups into later adolescence and early adulthood to map long-term tracking of fitness relative to health outcomes.
• Test randomized controlled interventions that compare standard PE with tailored, higher-frequency programs to quantify causal effects on aerobic and muscular adaptations.
• Examine girls with MID specifically in intervention trials to identify program components that offset observed deficits.
• Use body-composition assessments beyond BMI (e.g., waist circumference, skinfolds, bioelectrical impedance) to clarify whether mass gains reflect lean tissue or adiposity.

Broader population coverage

• Expand samples beyond educable MID to include borderline and moderate ID groups; report stratified results to tailor recommendations.
• Include socioeconomically and culturally diverse cohorts; environmental context shapes access to activity and nutrition, which influences development.

Qualitative work

• Conduct interviews with students, caregivers, and teachers to understand barriers to activity participation, motivational levers, and feasible program formats within schools and communities.

Policy and system-level recommendations

Translating research into practice requires system changes in education, healthcare, and community planning.

School-level policy

• Mandate higher minimum weekly PE minutes in special education settings and ensure sessions include structured aerobic and strength elements.
• Fund training for special-education PE teachers in adapted physical activity methods and behavior management for activity settings.

Healthcare and family support

• Include physical activity counseling for families of children with MID within pediatric and community health services, emphasizing practical, low-cost activities adaptable to home environments.
• Provide subsidized access to community adapted-sport programs and caregiver respite that enables participation.

Community and inclusion

• Partner school districts with community sports organizations and Special Olympics programming to create inclusive, supervised activity opportunities.
• Invest in safe play spaces and transport options for children who depend on caregivers for access to community facilities.

Data and monitoring

• Integrate fitness and activity metrics into school health records to enable longitudinal monitoring and referrals to targeted supports.

Limitations and cautions when applying the findings

The study contributes valuable longitudinal insight but must be interpreted within its constraints.

Population and generalizability

• Sample drawn from special education schools in a single Turkish city; cultural and systemic differences may limit transferability to other countries or to community-based populations.
• Only children with educable MID were included (IQ 50–70), excluding those with Down syndrome, autism spectrum disorder, or moderate-to-severe ID. Findings may not generalize to those groups.

Measurement gaps

• The study did not assess biological maturation, habitual physical activity, dietary intake, or body composition beyond BMI. Each of these factors influences fitness development.
• Test performance depends on motivation and comprehension; although reliability was high, individual fluctuations in effort affect outcomes.

Design considerations

• The absence of a typically developing (TD) control group limits direct comparison to normative adolescent trajectories.
• Group-level analyses obscure individual variability and do not identify subgroups that may follow different developmental patterns.

Use the findings as guidance

• Apply the study’s results as a starting point for program design and hypothesis generation rather than prescriptive mandates. Local assessment and adaptation remain essential.

How to implement changes with constrained resources

Budget and staffing constraints are common. The following low-cost strategies can produce meaningful gains:

• Micro-dosing activity: add two daily 5–10 minute activity breaks to classroom routines led by existing staff.
• Use resistance bands and bodyweight exercises to develop strength without expensive equipment. Bands cost little and are durable.
• Recruit and train parent volunteers or community students to run supervised after-school activity clubs.
• Embed activity into existing curricular goals (math, reading) via movement-based tasks to gain administrative buy-in.
• Leverage online repositories of adapted-PE lesson plans (from recognized organizations) and adapt them to local needs.

Translating evidence to everyday practice: case examples

Example 1: A primary special school in a mid-sized town assigned two 30-minute movement sessions per week in addition to regular PE. Teachers used a 12-week interval-based game program and a twice-weekly resistance band routine. After 12 weeks, girls in the program improved PACER-equivalent shuttle performance by an average of 18% and handgrip strength by 12%, with teachers reporting better participation in playground games.

Example 2: A community health clinic integrated a family coaching model where caregivers received a simple home-activity pack (bands, foam ball, visual activity cards) and weekly phone coaching. After three months, caregivers reported increased daily activity frequency and improved confidence in guiding strength and flexibility work for their children with MID.

These practical adaptations illustrate that modest increases in structured activity, delivered consistently, yield measurable improvements and functional benefits.

FAQ

Q: What is mild intellectual disability (MID)? A: MID typically refers to individuals with an IQ in the range of 50–70 who have limitations in intellectual functioning and adaptive behavior that appear before age 22. Children with MID may learn more slowly than peers and often benefit from adapted teaching, including in physical education.

Q: How big were the fitness gains observed in the study? A: Over three years, boys improved their PACER laps from an average of 21.6 to 26.6 laps; girls increased from 16.1 to 19.0 laps. Handgrip strength rose in boys from about 18.7 kg to 22.6 kg and in girls from about 15.3 kg to 18.0 kg. These changes were statistically significant for aerobic capacity and handgrip strength, with small-to-moderate effect sizes.

Q: Do these results mean girls with MID cannot improve their fitness? A: No. Girls improved across several measures; their gains were smaller than boys’ in aerobic and handgrip tests but were nevertheless real. The findings indicate that girls may require different or additional program components, higher frequency, or targeted strategies to achieve parity in certain fitness domains.

Q: Can schools safely implement higher-intensity or resistance programs for children with MID? A: Yes, when programs are adapted and supervised by trained staff. Safety requires baseline medical clearance, gradual progression, proper warm-up/cool-down routines, appropriate exercise selection, and adaptations for individual needs. Emphasize functional movements and use low- to moderate-intensity resistance sources (bodyweight, bands) initially.

Q: Should BMI be the main outcome to monitor health improvements? A: BMI is a convenient index but has limitations, especially in growing children. It does not distinguish fat from lean mass. Adding simple measures like waist circumference or waist-to-height ratio, and functional outcomes (endurance, strength) provides a fuller picture of health and fitness.

Q: What are the most critical program elements to prioritize? A: For this population and based on the study, prioritize (1) progressive aerobic activities that increase heart rate in structured intervals, (2) regular resistance-based strength exercises focused on functional movements and grip tasks, (3) flexibility and trunk-strength segments to counteract declines, and (4) strategies to boost motivation and comprehension (visual cues, repetition, group support).

Q: How should progress be measured in school settings with limited time? A: Use short, reliable field tests that are feasible to administer: a short PACER shuttle for aerobic capacity, handgrip dynamometer for strength, sit-and-reach for hamstring flexibility, trunk lift for trunk function, and regular height/weight measures. Collect data at baseline, mid-program, and post-program, and track simple activity logs if possible.

Q: Where can teachers and families find ready-made resources for adapted physical activity? A: Look for programs from established providers of adapted PE curricula, materials from Special Olympics (Healthy Athletes and youth programs), and reputable adapted-PE organizations. Local university departments in physical education or disability studies often share open-access lesson plans and training modules.

Q: What are the research priorities to better serve children with MID? A: Priority areas include studies that combine objective activity monitoring with maturation measures; randomized controlled trials testing intervention formats, frequency, and content; longer-term follow-ups that link adolescent fitness to adult health outcomes; and qualitative work exploring barriers and facilitators to participation for families and schools.

Q: How should policymakers respond to these findings? A: Policymakers should support increased PE time in special education settings, fund teacher training in adapted physical activity, subsidize community access to inclusive sports, and integrate fitness monitoring into school health services. These investments address both preventive health and functional independence.

Sustained improvement in fitness among children with MID requires consistent, tailored programming built into school and community systems. The three-year longitudinal evidence shows both opportunity and challenge: aerobic and strength capacities can grow during adolescence, but girls may need focused support to achieve comparable gains. Practical, evidence-aligned interventions—delivered with clear progression, inclusive pedagogy, and objective monitoring—will translate measurable performance improvements into daily functional benefits for children and adolescents with MID.