Documentation:FIB book/The Influence of Playground Surface Materials on Fall Biomechanics in Children

1.0 Introduction

Playgrounds provide children with opportunities for physical activity, exploration, and skill development, but they also present a risk of injury. Understanding how children interact with playground equipment, how falls occur, and how surface materials affect the forces transmitted to the body is essential for designing safer environments. This review examines the influence of playground surface materials on fall biomechanics. By linking surface mechanics, such as energy absorption and impact force attenuation, to the tolerance of pediatric bones, this review aims to inform both injury prevention strategies and future playground design standards.

1.1 Epidemiology

Playground injuries represent a significant and persistent public health concern among children. Each year, over 200,000 children in the United States are treated in hospital emergency departments for injuries associated with playground equipment, with the majority resulting from falls to the ground . The severity of these injuries is strongly influenced not only by the mechanics of the fall, such as height and body orientation, but also by the impact-absorbing characteristics of the playground surface .

Fractures are the most common injury outcome, representing over 80% of fall-related cases, and occur most frequently in children aged 5 to 12 years . Despite the establishment of safety standards by ASTM International and the U.S. Consumer Product Safety Commission (CPSC), the overall incidence of playground injuries has remained relatively stable over several decades . This trend suggests that while head injury risk has been reduced, current standards have not significantly decreased the far more common injuries.

Existing standards, including the ASTM F1292-04, primarily focus on mitigating the risk of life-threatening head injuries by regulating the impact attenuation properties of playground surfaces. The standard specifies that impact acceleration should not exceed 200 g and that the Head Injury Criterion (HIC) should remain below 1000 to reduce the likelihood of brain injury . However, recent epidemiological evidence indicates that concussions account for less than 2% of all playground injuries, whereas upper limb fractures are overwhelmingly more common. This discrepancy highlights a critical gap in current safety design criteria, which inadequately address the mechanisms that lead to the most frequent and clinically significant injuries, specifically, wrist fractures resulting from falls onto rigid or insufficiently cushioned surfaces.

The epidemiology of playground injuries thus underscores the need for research beyond head injury prevention. Understanding how different surface materials influence the biomechanical response of a child’s body during a fall, including energy absorption, impact distribution, and limb loading, can provide essential insights for designing safer playground environments. By focusing on the mechanisms and material interactions that lead to fractures, future safety standards could more effectively target the most prevalent injury types and reduce the overall incidence and severity of playground-related injuries.

1.2 Injury Mechanisms

Buckle (Torus) Fracture

The upper extremity, which includes the upper and lower arms, shoulders, elbows, wrists, hands, and fingers, accounts for about 45% of all playground equipment-related injuries. Among these, distal radius, or wrist, fractures represent the most common serious injury sustained on school playgrounds. Approximately 90% of these cases result from falls, most often from playground equipment, during which children instinctively extend their arms to break the fall, leading to upper limb impact . While other injuries, such as upper-arm fractures, shoulder dislocations, and soft tissue injuries, can also occur during falls, wrist fractures are by far the most frequent and clinically significant, making them the primary focus of this review.

Understanding the common types and causes of wrist fractures is essential for understanding the injury biomechanics, prevention mechanisms, and reviewing existing research on fall-related injuries. The type and severity of the fracture depend largely on the direction, magnitude, and distribution of these forces, as well as the inherent properties of the pediatric bone, which is softer, more pliable, and protected by a thick periosteum .

Greenstick Fracture

Buckle (Torus) Fractures: These are the most common fractures in children, representing over 80% of unmanipulated distal radius fractures. They occur when compressive forces act on the metaphyseal region of the radius, causing the bone to bulge without disrupting the cortical layer. Buckle fractures are inherently stable due to the intact periosteum and low displacement risk.

Greenstick Fractures: Involves disruption of the cortex on the tension side of the bone while the opposite cortex remains intact. They usually result from bending forces applied to the pliable pediatric radius .

Complete Fractures: Involves disruption of both cortices of the bone and typically results from higher-energy impacts. These fractures are relatively rare in children but are highly unstable, often requiring manipulation and immobilization to achieve proper alignment .

2.1 Surface Types

Example of sand as a playground surface material

Fall height and surface energy absorption are the primary risk factors for severe playground-fall injuries . Most common playground surfaces, such as sand, rubber mats, wood fibre, and gravel, help mitigate the risk by providing cushioning during falls. Falls onto non-recommended or non-resilient surfaces, such as compacted soil or hard asphalt, are associated with a significantly higher risk of severe injury, including fractures and head injuries . Because sand is a commonly used playground surface material, research has examined how its properties and maintenance affect impact attenuation during falls. Eager et al. (2021) found that playground sand degrades with use, but this degradation can be substantially reduced by selecting well-rounded sand rather than sand with high fines or clay content. To prevent compaction and a resulting decrease in fall-mitigation performance, a uniformity coefficient (the ratio of particle sizes in soil or granular material) below 2.75 is also recommended . These findings highlight the nuances involved in selecting suitable playground surfacing materials, where mechanical properties must be carefully evaluated and quantified.

2.2 Material Mechanics

The mechanical behaviour of a surface under impact is quantified by its ability to absorb energy, often measured by g-max, the peak deceleration during standardized drop tests . Surfaces with lower g-max values absorb more energy and reduce the forces transmitted to the body. The depth, composition, and compaction of the material influence its mechanical properties and, in turn, its energy-absorption ability. Thicker and more deformable surfaces provide greater cushioning, while compacted or worn surfaces exhibit higher stiffness and lower energy absorption .

To represent the mechanical behaviour of common playground surfaces, Davidson et al. (2004) conducted standardized drop tests following ASTM procedures, using a hemispherical missile to measure the force-displacement response of some materials. The compliant surfaces, including wood fibre and rubber, were modelled as exponential spring elements because their stiffness increased with penetration depth, whereas grass and earth were modelled as linear springs with constant stiffness. These surface stiffness profiles are used in biomechanical modelling .

3.1 Voluntary Human Subject Testing

One experimental approach to studying fall biomechanics involves controlled laboratory simulations using human participants to quantify the forces experienced during impact. A notable example is the study conducted by Choi et al. (2014), which examined how different playground surface materials affect impact forces during forward falls onto outstretched hands . Twelve healthy adult males (mean age 25 years) participated in a series of torso-release experiments, in which subjects, by use of a harness and electromagnet, were suspended in a prone position and released to simulate a 5 cm vertical drop onto their hands. This setup allowed for consistent fall height and impact configuration while safely replicating upper limb fall arrests. The experimental system integrated a force plate sampling at 500 Hz to capture high-resolution, three-dimensional ground reaction forces throughout the impact event. Trials were performed on a reference baseline surface of a rigid aluminum plate and five common playground surface materials: rubber tile, sand, gravel, wood mulch, and wood fibre, each at standardized depths based on field measures (10-15 cm).

Force data were analyzed in MATLAB to extract peak resultant, vertical, and horizontal forces, as well as time to peak force and angle of the resultant force. Force attenuation was computed relative to the rigid surface to quantify the protective performance of each material. Statistical analysis was conducted using repeated measures ANOVA with Bonferroni corrections to evaluate differences between surface types and arm configurations (20° and 40° from vertical). This method was chosen because repeated measures ANOVA determines if there are statistically significant differences between the means of 3 or more groups where the same participants were tested under multiple conditions. This approach provided detailed insight into how surface compliance and impact geometry influence force transmission through the upper limb during falls. Overall, human subject testing methods offer a valuable, controlled means of assessing fall biomechanics and the performance of impact-attenuating materials.

3.2 Biomechanical Modelling

Biomechanical modelling provides a controlled and quantitative method for analyzing the mechanical stresses that occur during playground falls and for predicting the likelihood of fracture under different impact conditions. Davidson et al. (2004) developed and validated a rheological-stochastic biomechanical model of arm impact in children, integrating both experimental surface testing and computational simulation to estimate distal radius fracture risk .

The modelling framework was based on a combination of real-world fall data and controlled laboratory testing. The dataset, including the age, body mass, fall height, and the type and condition of the surface impacted, was drawn from a case-control study. Surface properties were experimentally derived and incorporated into the computational simulations to ensure that the predicted impact forces reflected realistic playground conditions.

In the computational model, the child’s body was represented by a simplified two-mass rheological system, consisting of the outstretched arm and the suspended body connected by spring elements at the wrist and shoulder joints. Vertical compression loading simulated the fall, with joint stiffness and damping drawn from statistical distributions based on human impact experiments. For each child in the dataset, 1,000 Monte Carlo simulations were run, generating a distribution of impact force values that captured the probabilistic nature of real-world falls.

The second component of the model estimated the fracture force of the distal radius. This was derived from age-dependent data on bone geometry and mineral density obtained from pediatric bone studies. The compressive strength of bone was expressed as a function of volumetric bone density and strain rate, based on the empirical relationship. The Factor of Risk (FR), defined as the ratio of the estimated impact force to the fracture force, was an indicator of injury likelihood, with higher FR values corresponding to greater fracture risk. Logistic regression analysis confirmed a strong positive relationship between FR and the probability of fracture occurrence, demonstrating that the model accurately distinguished between cases in which fractures occurred and those in which they did not.

The tools and analytical methods used in this study combined both experimental and computational elements. Laboratory testing provided quantitative data on the mechanical properties of different playground surfaces, while dynamic simulation software was employed to implement the two-mass spring-damper model and perform stochastic sampling. Statistical analysis using logistic regression further validated the predictive power of the model by comparing simulated fracture probabilities with real injury outcomes. This integrated approach enabled researchers to link physical surface properties and fall conditions with the biomechanical responses that lead to injury.

3.3 Biomechanical Testing

Anthropomorphic test devices (ATDs) are commonly used in impact biomechanics to quantify human injury potential under controlled conditions. A study by Sherker et al. (2005) conducted in-situ biomechanical testing to replicate and measure the impact conditions of children’s playground falls . The researchers combined epidemiological data with physical testing by reconstructing each child’s fall at the original playground site, thereby capturing real-world variability in equipment design, surface depth, and substrate composition.

The ATD experiments are the main physical biomechanics study, providing direct measurement of the impact loads that are difficult or unsafe to obtain from human participants. Two specialized instrumented surrogate devices were used to simulate and measure fall impacts. First, a standard headform is used as a benchmark for head impact severity in accordance with playground safety standards. Second, a custom anthropometric child arm load dummy was developed to model the biomechanical loading experienced by a child’s upper limb during a fall onto an outstretched arm.

The arm dummy featured a mechanical limb structure integrated with force sensors to record peak axial loads transmitted through the arm upon impact. Both devices were dropped from multiple heights, including the measured fall height and equipment height, to obtain key biomechanical parameters such as peak deceleration, Head Injury Criterion, and arm load. By providing precise, repeatable measurements, these ATD tests offered critical data on upper limb loading, enabling direct comparison of surface materials’ protective performance and validation of computational models.

This dual-device approach enabled the simultaneous evaluation of headform-based injury metrics, which underpin existing playground safety criteria, and upper limb impact loads, which are more representative of the most common playground injury of arm fractures . The ATD study bridges the gap between theoretical models and real-world injuries, highlighting how surface properties translate into biomechanical forces and fracture risk, and underscoring the importance of incorporating such experimental data when assessing playground safety.

4.1 Injury Criteria

Injury criteria provide a quantitative framework for interpreting biomechanical data in the context of real-world injury risk. They define thresholds or predictive measures that relate external loads, motions, or stresses to the likelihood of tissue failure. In the study of playground falls, injury criteria are essential for linking simulated or measured forces to the probability of fractures. By establishing clear biomechanical thresholds, injury criteria enable both the evaluation of safety interventions and the development of evidence-based standards for playground design.

In playground safety research, the Head Injury Criterion and peak deceleration are the most widely adopted standards, forming the basis of current safety regulations for head impacts. These metrics are derived from anthropomorphic headform drop tests, in which an impact resulting in HIC<1000 and a peak deceleration below 200G is considered to represent an acceptable risk of serious head injury . However, these traditional headform-based injury criteria were developed primarily for assessing head trauma and may not adequately capture the most common injury mechanisms, upper limb fractures resulting from outstretched-arm falls. There is a need for arm fracture-specific injury criteria to better represent limb injury mechanisms. Through reconstructed falls and measured loading data, there was a 90% probability of arm fracture when arm loads exceeded approximately 3.0 kN, suggesting this value as a preliminary arm fracture threshold .

Implemented in the rheological-stochastic model when FR exceeds 1, the predicted force surpasses the estimated fracture tolerance, indicating a high likelihood of injury. This ratio accounts for both the external loading conditions, such as fall height and surface stiffness, and the internal biomechanical properties of the child, including bone geometry, density, and joint mechanics. By incorporating stochastic variations in joint stiffness and damping, the FR provides a probabilistic estimate of fracture risk rather than a single deterministic value, making it a robust tool for assessing real-world fall scenarios.

4.2 Results

Playground surface material is found to have a measurable effect on fall biomechanics and associated injury risk. Non-recommended surfaces increased the risk of severe injury by 2.3 times compared to recommended surface materials. G-max of surface materials is a good indicator of fall injury risk, independent of severity. However, the risk of injury is 1.8 times greater when g-max is in the range of 150-199 g, compared to <150g, indicating that the current allowable threshold of g-max<200g could be lowered to decrease the risk of injury during playground falls. This data suggests that materials which currently pass the standards for playground safety may still permit impact loads within the range associated with injury.

Experimental investigations comparing common surface types demonstrated that while the common playground surface materials attenuate impact forces relative to rigid baselines, the degree of reduction varies substantially between materials . Relative to a rigid aluminum plate, reductions in peak resultant force were observed as follows: 17% for sand, 16% for gravel, 7% for mulch, 4.5% for wood fibre, and 1.5% for rubber tiles .

In contrast, another study found that rubber surfaces outperformed wood bark coverings in reducing impact forces , which conflicts with the earlier findings. These inconsistencies likely reflect the influence that environmental conditions have on material performance. Factors like compaction, moisture content, and surface depth, as well as the underlying material used in construction, can cause significant differences in a certain material’s stiffness and impact attenuation . This is particularly true for variable, loose surfaces like sand, mulch, and wood fibre. As a result, differences in installation and environment can lead to major variations in measured force reduction, which contributes to the conflicting rankings of studied materials. In general, studies suggest that sand provides the greatest degree of impact attenuation , but inconsistencies remain across the literature due to variations in experimental design, surface conditions, and testing protocols.

These inconsistencies underscore that direct comparison between studies is challenging without standardization. As a result, the magnitude of force reduction reported for a given surface should be interpreted relative to each study’s test conditions rather than as definitive rankings across all playground environments. Even so, several biomechanical patterns can be inferred based on the literature. Loose, highly deformable materials generally offer greater energy absorption, as opposed to unitary surfaces, which transmit higher loads, particularly when thin or compacted . Overall, these findings suggest that while all playground surface materials demonstrated some capacity to absorb impact energy, the level of biomechanical protection varied substantially between materials and was generally insufficient to prevent common fracture injuries .

4.3 Strengths and Limitations

The studies investigating fall biomechanics for different surface materials use both real-world fall data and controlled laboratory testing. Real-world data in the studies offer several strengths, including high response rates and the integration of direct playground observations with detailed injury data. However, there is potential selection bias from hospital-only data, underrepresentation of minor injuries, and measurement proxies for fall height and g-max that may have introduced some imprecision.

Ethical and safety limitations inherently constrain experiments. The low drop height of the human subject testing cannot replicate the higher-energy impacts of real playground falls . Although adult participants were used for ethical reasons, and the study argued that relative differences in force attenuation between surfaces would likely be similar in children, this substitution does not accurately reflect the anatomy, bone strength, or neuromuscular responses of children who are most susceptible to distal radius fractures. Children have distinct biomechanical characteristics, including greater joint flexibility, lower bone density, and different bone geometry compared to adults, which can influence their impact response and fracture risk. Furthermore, the simplified fall conditions, fixed elbow position, single impact, and uniform surface containment limit generalizability to the diverse and dynamic nature of real-world falls.

The use of Anthropomorphic Test Devices (ATDs) provides a controlled and repeatable means of evaluating injury risk under standardized conditions. ATDs enable precise measurement of kinematic and kinetic responses during impacts, which are essential for comparing surface materials. Their use allows testing at impact energies and orientations that would be unsafe for human participants, thereby overcoming ethical and safety limitations associated with voluntary subject testing. However, the mechanical fidelity of the custom arm dummy presented notable limitations. Specifically, the surrogate lacked realistic joint stiffness and damping characteristics, which are critical for accurately representing the dynamic response of a child’s arm during impact. The absence of these biomechanical features in the dummy likely resulted in stiffer impact responses and higher measured loads, potentially overestimating the true injury risk. Furthermore, the simplified construction limited its ability to reproduce complex rotational and bending behaviours that occur in natural falls .

A further limitation related to existing research on playground material is the difficulty in relating reported forces and accelerations from studies to defined injury thresholds. Several studies quantify material performance using percentage reduction of peak forces or g-max values, which is useful when interpreting surface properties, but fail to provide realistic arm loads when impacted on said surfaces. As a result, these studies demonstrate relative differences between playground surfaces well, but provide more limited insight into whether the forces from an impact will result in a wrist fracture. This gap in research highlights the need for more standardized testing conditions, particularly in defining impact energies that reflect realistic fall scenarios. 

The rheological-stochastic model offers several notable strengths that make it a valuable tool for studying fall biomechanics and predicting distal radius fracture risk in children. One of the key strengths is its integration of multiple data sources, combining epidemiological fall data, laboratory surface testing, and computational simulation. This approach allows the model to reflect real-world fall conditions while maintaining a controlled, quantitative framework for analysis. The experimental characterization of playground surfaces using standardized drop tests provides reliable force-displacement data, which are incorporated directly into the model to simulate realistic interactions between the falling child and different surface types. Moreover, the model’s probabilistic approach accounts for natural variability in biomechanical properties such as joint stiffness and damping, improving its predictive power compared to models that rely solely on mean values. By estimating the Factor of Risk, the model links external fall conditions with internal stresses and fracture probability, offering a mechanistic understanding of injury risk and practical guidance for playground safety design.

Despite these strengths, several limitations should be acknowledged. The model simplifies the human body into a two-mass system with linear spring-damper elements, which captures vertical compressive forces but does not account for complex multi-joint dynamics, rotational motion, or bending and shear stresses that often contribute to distal radius fractures in real falls. The model also assumes passive limb behaviour, ignoring active muscular responses that children may employ to mitigate impact forces. Surface testing, while rigorous, may not fully represent real-world playground conditions, which are influenced by wear, compaction, moisture, and seasonal changes. Similarly, bone fracture thresholds were estimated from adult-based empirical relationships adapted for children, which may not capture the true variability and structural properties of pediatric bones.

4.4 Controversies

Playgrounds offer children a place to engage in creative play and, through a challenging and stimulating environment, develop cognitive, motor, and social skills . Such environments, combined with the playful and exploratory nature of children, are inevitably associated with some risk of injury. This raises the question: How does the risk of injury compare to the developmental and health benefits of engaging in risky play on playgrounds? And how should playground design and development reflect this balance? Whereas some researches highlight the developmental benefits of risk-taking, others emphasize the importance of limiting hazards, with several studies suggesting that safety standards have prevented children from engaging in risky play, which would otherwise help them learn about risk management, increase playtime, and promote healthier lifestyles .

The current method of using head drop tests and HIC injury criteria to assess the safety of playground surface materials could relate to this issue, since upper limb fractures, although more common in playground accidents, are not as severe a safety concern as head trauma . If injuries are inevitable, playgrounds should provide children with opportunities to explore their capabilities and manage fear while maintaining as much safety as possible. The surface materials might reflect this by focusing on preventing the most severe and potentially life-altering injuries, such as head trauma, rather than upper limb fractures.

A point of contention lies in the age inclusivity of playground standards. Most surface impact tests and injury thresholds are derived from adult or hybrid surrogate models, which fail to represent the biomechanical and behavioural differences between toddlers, school-aged children and adolescents . As a result, some surfaces that meet ASTM and CPSC requirements for head injury protection in adults might pose excessive limb injury risks for younger children due to their lighter mass and smaller limb sizes.

Another contributing controversy is the variability in impact-force reductions reported across studies for the same surface types. These discrepancies are likely driven by differences in surface depth, moisture content, compaction, aging, installation quality, and testing protocols, all of which can substantially alter how a material behaves under impact . As a result, studies have led to inconsistent or conflicting findings.

4.5 Future Directions

Future research on playground injury biomechanics should focus on bridging the gap between laboratory-based impact testing, computational modelling, and real-world injury outcomes. Further studies using ATDs and computational human body models are needed to simulate realistic fall scenarios. These models should incorporate variations in body orientation, impact velocity, and contact distribution across different surface materials. In particular, the development of age-specific and more biofidelic child surrogates, including wrist and upper-limb specific models with standardized fall protocol, is essential to capture the unique biomechanical properties of children.

While computational and biomechanical modelling approaches have shown promising accuracy in replicating child fall dynamics, further refinement is required to account for the diverse mechanisms underlying different fracture types, including various distal radius fracture patterns. This may involve improving model complexity to include multi-body dynamics, tissue-level strain responses, and realistic joint constraints during falls.

Future research should prioritize the design and evaluation of innovative playground surface materials that not only minimize head injury risk but also address the persistent issue of upper limb and wrist fractures, which continue to occur even when current safety standards are met. Expanding investigations into a range of modern surfacing materials will provide a broader understanding of how different materials perform across a range of impact energies and fall conditions. It is also essential to assess the long-term durability and performance of these materials under different environmental exposure and conditions, as these factors significantly influence their real-world protective effectiveness over time.

From a policy and standards perspective, future research should explore how biomechanical insights can inform updated playground safety guidelines. Integrating validated biomechanical injury thresholds and material response data into regulatory standards could lead to more evidence-based height limits, surface criteria, and maintenance protocols. Ultimately, collaboration between engineers, researchers, and standards organizations will be critical in ensuring that playgrounds are biomechanically safe environments for children.

5.0 Conclusion

This review shows that while playground surfaces influence fall biomechanics, current safety standards focus mainly on head injury prevention and do not adequately address the far more common upper limb fractures. Research across epidemiology, experimental testing, and biomechanical modelling consistently demonstrates that although compliant surfaces reduce impact forces, they often provide insufficient protection against wrist and arm injuries.

Limitations in existing methods, such as simplified fall conditions, adult surrogates, and headform-based criteria, highlight the need for more child-specific and realistic approaches. Moving forward, improved biofidelic models, age-appropriate surrogates, and advanced simulations are crucial for better representing real-world falls. Updating standards to incorporate limb injury criteria and material performance data will help create playgrounds that more effectively reduce injury risk while supporting healthy, active play.