The Relation Between Plantar Pressure Distribution on the Foot and The Way of Carrying Backpack in Students Between 12 to 18 Years

• Abstract and Core Objective This research presents a detailed biomechanical investigation into the effects of school backpack carriage on foot function in adolescents. The study moves beyond the common focus on spinal posture to analyze the foundational segment: the foot. Utilizing advanced pedobarographic technology, it precisely measures how different loading methods-bilateral (two-strap) and unilateral (one-strap) carriage-alter the distribution of pressure across the plantar surface during static (standing) and dynamic (walking) tasks. The primary objective was to quantify these changes to inform evidence-based recommendations for preventing musculoskeletal strain in a young, developing population.
• Background and Significance Adolescence is a period of significant skeletal and muscular development, making individuals particularly susceptible to external mechanical stresses. The ubiquitous use of backpacks, often carrying loads exceeding recommended guidelines, introduces a chronic, repetitive stressor.

The Kinetic Chain: The human body functions as an interconnected kinetic chain. Alterations in the foundation (the feet) can have cascading effects upward, influencing ankle, knee, hip, and spinal alignment. Therefore, understanding foot pressure is crucial to understanding overall postural health.

The Plantar Pressure Paradigm: Plantar pressure measurement is a sophisticated biomechanical tool that provides a dynamic map of force distribution. It reveals how the body adapts to load, highlighting areas of potential overloading (high pressure) or underloading, which can be precursors to pain, soft tissue damage, or long-term structural changes like flatfoot.

The Unilateral Carriage Problem: Despite widespread knowledge of its negative effects, unilateral backpack carriage remains prevalent among adolescents due to perceived convenience and social trends. This study seeks to quantify the specific biomechanical cost of this habit.

• Detailed Methodology 3.1. Participant Recruitment and Eligibility (Sample Demographics)

A cohort of 92 adolescents was recruited from a mix of public and private schools in Mansoura City, Egypt, to ensure a representative sample. The stringent eligibility criteria were designed to isolate the variable of backpack carriage from other confounding factors:

Age: 12-18 years, capturing the spectrum of pubertal growth and maturation.

Health Status: No history of visual, vestibular, perceptual, neurological, or musculoskeletal disorders. This excluded conditions like cerebral palsy, leg length discrepancy, previous fractures, or diagnosed scoliosis beyond a minimal, normal range.

Anthropometrics: Body Mass Index (BMI) was calculated and checked against the World Health Organization (WHO) 2007 BMI-for-age percentile charts. Only participants within the normal range (typically between the 5th and 85th percentiles) were included. This controlled for the independent influence of body weight and composition on plantar pressure.

Postural Screening: Axial trunk rotation was measured using a scoliometer with the participant in the forward bend test position. Only those with measurements within the normal limit of 0 to 3 degrees of rotation were included, effectively screening out significant undiagnosed spinal asymmetries.

3.2. Instrumentation and Technical Specifications

Foot Scan Plate System (F-SCAN): This is a platform-based pressure mapping system.

Technology: It consists of a flat, rigid plate containing a high-density grid of capacitive pressure sensors.

Data Capture: When a subject stands or walks on the plate, each sensor measures vertical force. The system software collects this data at a high frequency (typically hundreds of Hertz), creating a dynamic, frame-by-frame image of the pressure distribution.

Output Metrics: The system's software automatically calculates numerous parameters. This study focused on three key ones for each foot:

Surface Contact Area (cm²): The total area of the foot in contact with the ground.

Average Pressure (kPa): The mean force per unit area across the entire foot imprint.

Peak (Maximum) Pressure (kPa): The highest pressure value recorded by a single sensor element, indicating a focal point of stress.

Scoliometer: A validated, non-invasive inclinometer used to measure the angle of trunk rotation (ATR), a key indicator for scoliosis screening.

Backpacks and Load Configuration: Two standard student backpacks were used: a double-strap model and a single-strap model. Both were simple, soft-fabric designs without internal frames or hip belts, representing a typical student backpack. The load was standardized at 15% of each participant's body weight using calibrated sandbags, a weight based on previous ergonomic studies (e.g., Chow et al., 2010) shown to induce measurable postural changes.

3.3. Experimental Protocol and Data Collection

The testing protocol was meticulously standardized:

Familiarization: Participants were allowed to familiarize themselves with the laboratory setting and the pressure plate.

Calibration: The F-SCAN system was calibrated according to manufacturer specifications before each testing session.

Testing Conditions: Each participant was tested under four conditions in a randomized order to prevent bias:

Condition 1 (Control): No backpack (unloaded).

Condition 2: Backpack worn bilaterally (on both shoulders).

Condition 3: Backpack worn unilaterally on the right shoulder.

Condition 4: Backpack worn unilaterally on the left shoulder.

Task Execution:

Static Trial: Participants stood motionless on the pressure plate, looking straight ahead, with arms relaxed by their sides. Data was collected over a stable several-second interval.

Dynamic Trial: Participants walked at a self-selected, comfortable speed across the pressure plate, capturing multiple steps. The mean value of several successful steps was used for analysis to ensure reliability.

Data Processing: For each condition and task, the software segmented the foot into anatomical regions (heel, midfoot, forefoot, toes). The values for the entire foot's contact area, average pressure, and peak pressure were exported for statistical analysis.

3.4. Statistical Analysis The collected data was analyzed using repeated-measures Analysis of Variance (ANOVA) to determine if the differences observed between the four carrying conditions were statistically significant (p < 0.05). Post-hoc tests (Bonferroni correction) were then applied to identify exactly which pairs of conditions were different from each other (e.g., Is unilateral right significantly different from no backpack?).

• Detailed Results and Biomechanical Interpretation Surface Contact Area: The analysis revealed a significant increase (p<0.05) in the contact area for both feet, but in a specific pattern.

The right foot contact area increased significantly during bilateral carrying and unilateral right carrying compared to the no-backpack condition.

The left foot contact area increased significantly during unilateral left carrying.

Interpretation: The body's primary adaptation to asymmetrical loading is to increase stability by "splaying" the foot, creating a larger base of support. This is a compensatory mechanism to prevent loss of balance. The fact that bilateral carrying also increased contact area on the right foot may suggest a natural limb dominance or subtle postural shifts even under symmetrical load.

Average and Peak Pressure: Contrary to what might be expected, there were no statistically significant changes in either the average pressure across the foot or the peak (maximum) pressure points under any of the carrying conditions, during both standing and walking.

Interpretation: This is a critical finding. It demonstrates the remarkable adaptive capacity of the healthy adolescent musculoskeletal system. The body successfully manages the added 15% load not by increasing pressure intensity, but by efficiently redistributing it over a larger surface area. This prevents potentially harmful focal points of high pressure that could lead to pain or tissue damage in the short term. This efficient redistribution is likely achieved through subtle adjustments in muscle activation around the foot, ankle, and leg.

Bilateral Comparison: During dynamic walking, no significant pressure imbalance was created between the left and right foot across conditions.

Interpretation: The gait cycle itself acts as a natural equalizer. The rhythmic, alternating pattern of walking appears to mitigate the asymmetrical load imposed during unilateral carrying, preventing a sustained left-right discrepancy in pressure output.

• Discussion, Limitations, and Future Research Discussion: This study provides quantitative evidence that the body's immediate response to backpack load is a strategic increase in foot contact area to maintain stability, rather than an increase in plantar pressure intensity. While this adaptation is effective, it represents an altered and potentially less efficient biomechanical state. Chronic adoption of a unilateral carrying posture could lead to muscular fatigue, joint stress, and long-term postural alignment issues upstream in the kinetic chain (ankles, knees, hips).

Limitations:

The study assessed immediate effects. Long-term longitudinal research is needed to see if these adaptations lead to permanent changes or pathology.

The participants were healthy. Results may differ in adolescents with pre-existing musculoskeletal conditions.

The study was conducted barefoot. Footwear would interact with load carriage and is an important area for future study.

Future Research Directions:

Investigation of heavier loads (e.g., 20% of body weight) to find a potential "breaking point" of this adaptive mechanism.

Electromyography (EMG) studies are used to measure which muscles are activated to facilitate this pressure redistribution.

Long-term studies following adolescents through their school years to correlate carrying habits with the development of pain or postural issues.

• Conclusion and Practical Recommendations In conclusion, this detailed biomechanical analysis confirms that the backpack carrying method significantly influences adolescent posture at the foundational level. The body compensates for uneven loads intelligently, but this compensation comes at the cost of altered natural foot mechanics.

Evidence-Based Recommendations:

Promote Bilateral Carriage: The consistent use of both shoulder straps is the single most effective way to minimize asymmetric biomechanical adaptations.

Monitor Weight: Strictly adhere to the 10-15% of body weight guideline to remain within the body's effective compensatory range.

Ensure Proper Fit: Backpacks should be positioned high on the back, snug against the body, with the bottom above the waist. The use of hip belts and chest straps should be encouraged to distribute the load more effectively to the pelvis.

Educate on Risks: Adolescents and their parents should be made aware that even if no pain is felt, carrying a backpack on one shoulder can disrupt normal body mechanics and may have long-term consequences.

Incorporate into Clinical Practice: Healthcare providers should include backpack habits in their patient history for adolescents presenting with any lower limb or back pain.

Inclusion criteria

• • Their age will range from 12 to 18 years.

• Their body mass index (BMI) is within normal range according to BMI for age percentile score (WHO, 2007).
• All adolescents will be free from postural deviation, the trunk rotation in all students is within a normal degree determined by scoliometer (Bunnel, 1993).

Exclusion criteria

• Visual, auditory, or perceptual deficits.
• Deformities at any joint of the lower limbs and spine.
• Surgical intervention in the foot.
• Deep sensory loss.
• Musculoskeletal disorders.