Recent advancements in technology have greatly enhanced our ability to analyse the biomechanics of extremity joints. This progress has led to the development of targeted training programs aimed at improving motor performance and developing measures to reduce relative risks of injury and implementing appropriate clinical interventions (De Ste Croix and Korff, 2013). The World Health Organization reported that road traffic crashes are the leading cause of injuries, disabilities, and fatalities for children and young adults aged 5–29 years (Organization, 2018). The head is the most frequently injured body region in children due to motor vehicle crashes worldwide and head injuries results in more fatalities compared to injuries in other parts of the child’s body (Hanna, 2010; WHO, 2007). However, recent advancements in vehicle safety and child restraints have significantly helped in reducing these injuries and fatalities (Seidel et al., 2018; Boucher et al., 2016). On the other hand, injuries to both the lower and upper extremities of the child’s body are the second most frequent injuries sustained by children in vehicle crashes worldwide (Hanna, 2010; Jermakian et al., 2007; Fildes et al., 1997). These injuries are usually not life threatening but mostly results in long-term suffering and even disabilities in some cases (Fildes et al., 1997; Boucher, 2014).

Several studies have been conducted on the biomechanics of the vulnerable parts of the child’s body susceptible to injuries in vehicle crashes including the head, chest, abdomen, pelvis, neck, and spine (Jiang et al., 2014; Luck et al., 2013; Margulies and Thibault, 2000; Ouyang et al., 2005; Arbogast and Maltese, 2014). Additionally, numerous studies have been performed regarding paediatric extremity bone biomechanical tolerance (Rockwood, 2010; Miltner and Kallieris, 1989; Chung et al., 1976; Currey and Butler, 1975; Ouyang et al., 2003a; Ouyang et al., 2003b). Furthermore, majority of the literature on extremity injuries in children due to vehicle crashes are focused on bone fractures (Hanna, 2010; Jermakian et al., 2007; Fildes et al., 1997; Arbogast et al., 2003; Bennett et al., 2006). On the other hand, upper extremity joint dislocation has been identified as one of the factors contributing to long-term disability in children due to contact with deploying airbags (Jernigan et al., 2005). Ankle joint kinematics and stiffness have also been found to contribute to ankle and foot fractures and subsequent injury severity (Seidel et al., 2018; Fildes et al., 1997; Boucher et al., 2017). Therefore, studying the biomechanics of the upper and lower extremity joints is essential to provide important information for developing enhanced protection against extremity injuries in children involved in vehicle crashes. Furthermore, the data collected in the proposed study will be valuable to the medical field for diagnosing and treating extremity injuries in children resulting not only from vehicle crashes but also injuries from sports and falls. It will also aid in the assessment and rehabilitation of children with disabilities such as autism, cerebral palsy (CP), and Down syndrome (DS).

This protocol is being developed to help conduct extensive assessments of the pediatric extremity joint function for the upper and lower extremities joints (shoulder, elbow, wrist, hip, knee, and ankle). The protocol will focus on the assessment of the joints’ range of motion (ROM) and stiffness as well as the strength of the muscles surrounding these joints. These biomechanical properties have been recognized as important factors in determining children’s biomechanical tolerance to vehicle-related crash impact (Seidel et al., 2018; Fildes et al., 1997; Boucher et al., 2017). Evaluating these properties will provide a better understanding of the children’s extremity joints’ response to crash forces thereby contributing to the development of more biofidelic pediatric computational models and crash test dummies. This will lead to the development of more effective child vehicle safety systems (i.e., child restraint devices).

ROM measurement is a common clinical assessment among adults and children. It indicates the maximum angular displacement a joint can move between the flexed position and the extended position (Goonetilleke and Karwowski, 2017). The motion resulting in the angular displacement of a joint can be performed either actively or passively. Active ROM (AROM) involves the movement produced by an individual’s voluntary unassisted muscle contractions, while passive ROM (PROM) refers to the motion produced by the application of an external force by an examiner (Norkin and White, 2016). Different instruments have been adopted to measure the ROM such as universal goniometers, electro-goniometers, inclinometers, optical motion capture (OMC) systems, smartphone applications, etc. (Ore et al., 2020; Krause et al., 2013; Keogh et al., 2019; Özüdoğru et al., 2024). In the proposed study, the AROM and PROM of the upper and lower extremities will be measured using goniometers and an inclinometer. The application of these instruments will be further discussed in Section 2.2 (Methodology). Although OMC is considered the gold standard for kinematic measurements (Ore et al., 2020), it is conducted in laboratories using high precision cameras and typically involves small sample sizes (Zhang et al., 2013). The use of a goniometer however, is easy to handle and time effective especially with larger sample sizes (Fraeulin et al., 2020).

The term “muscle function” is often used to describe different aspects of the muscle such as strength and power and its physical fitness. Muscle strength refers to the maximum force a muscle or group of muscles can exert during voluntary movement under specified testing conditions. Power is derived from the product of the force created and the velocity of the movement (Jones and Stratton, 2000). Muscle strength has been evaluated using different methods such as manual tests (Marcolin et al., 2009), hand-held dynamometry (van den Beld et al., 2006), and isokinetic dynamometry (Muñoz-Bermejo et al., 2019; Tsiros et al., 2011; Santos et al., 2013). Muscle function can be evaluated under certain conditions such as isometric, isotonic and isokinetic contractions. Isometric testing refers to the measurement of muscle tension produced at a specific point in the ROM under static conditions, i.e., variable effort to produce a force against an immovable object (Jones and Stratton, 2000). Isotonic testing simply involves movement about the joint axis where the resistance to movement is constant, but the velocity is variable, i.e., lifting an object with a particular weight. Isotonic testing often represents everyday functional activities (Jones and Stratton, 2000). Isokinetic testing, however, involves movement at a constant angular velocity about a joint axis with variable resistance. Isokinetic testing typically requires a device (isokinetic dynamometer) to control the velocity of the motion while applying the force necessary to maintain that velocity (Jones and Stratton, 2000). Both isotonic and isokinetic testing may involve concentric (shortening) or eccentric (lengthening) muscle contractions. Eccentric muscle testing is generally regarded as more stressful on the body as it requires more effort to oppose the resistance acting upon the muscle, therefore it is more likely to result in faster muscle discomfort (Jones and Stratton, 2000). However, the likelihood of muscle injuries during eccentric contractions is minimal provided that the subjects are given adequate warm-up and familiarization (De Ste Croix and Korff, 2013).

From a biomechanical viewpoint, joint stiffness determines how easily a joint can be manoeuvred by the surrounding muscles (Vigotsky et al., 2020). In order to measure stiffness of the muscles and joints, it is important to redefine the notion of stiffness and clarify how it can be measured in an experiment (Latash and Zatsiorsky, 1993). Farley et al. (1998) defined joint stiffness as the change in joint moment M, divided by the change in the joint angle α, denoted by Equation 1 below: K j o i n t = d M d α (1)

The muscle force-stiffness relationship is generally modelled as linear, but the process is highly dynamic requiring a coordinated interaction between the muscles supporting the joint (Brown and McGill, 2005). Latash and Zatsiorsky signified true stiffness of the human joint as the combination of all the individual stiffness values contributed by muscles, tendons, ligaments, cartilages, and bones. Therefore, they introduced the term “quasi-stiffness” to represent the collective behavior of the aforementioned components in a single value (Latash and Zatsiorsky, 1993). However, the mathematical expressions to model all the components that contribute to a certain motion have not been developed yet (Butler et al., 2003). Furthermore, quasi-stiffness cannot be assessed when the system is static as there is no change in the joint angle (Vigotsky et al., 2020).

Joint stiffness has also been defined as the resistance to displacement within a given joint at a specific angle (Struzik et al., 2021; Serpell et al., 2012). For the purpose of this study, only the quasi-stiffness (which will be referred to as dynamic stiffness) will be assessed. In essence, stiffness may be calculated in several ways depending on the research objective and the resources available. This study will focus on the stiffness at the joint level; hence the torsional stiffness will be examined. The methods applied to measure stiffness should be defined and comparisons between studies using different methods should be performed while considering the fact that different methods will likely produce different results (Butler et al., 2003).

While it is important to measure the muscle strength during different types of contractions, the protocol presented will highlight an approach to measure the isokinetic strength of the muscles because isokinetic strength measurements show higher reliability (Danneskiold‐Samsøe et al., 2009). To measure the muscle strength, the speed controlling mechanism in the dynamometer sets the maximum achievable velocity of the moving limb, so any additional force exerted by the participant is absorbed into the device and transformed into increased resistance. Therefore, the benefit of isokinetic muscle strength testing is that the measurement gives information about the dynamic qualities of the muscle tested, as it best mimics real-life scenarios in a safe and controlled process (Danneskiold‐Samsøe et al., 2009; Osternig, 1986). When measuring the dynamic stiffness of the involved joints, the dynamometer offers a reactive mode of operation, i.e., the lever arm of the device is set to move at a selected angular velocity which does not rely on the active muscle contraction, but rather the reactive muscle tension. The muscle strength and joint stiffness will be measured using the Biodex isokinetic dynamometer (Biodex System 4; Biodex Medical Systems Inc., New York). Generally, the isokinetic dynamometer offers a safe and controlled environment to examine muscle function (Gleeson and Mercer, 1996). A study by Tsiros et al. (2011) provided a critical evaluation of the Biodex isokinetic dynamometer for knee strength testing in children. Information regarding the reliability and safety of the device will be considered when performing the tests.

To the best of our knowledge, only a limited number of studies have assessed muscle strength and joint stiffness in pediatric extremity joints, with most of the studies focusing on children with disabilities such as cerebral palsy and Down syndrome. Additionally, these studies primarily examined lower extremity joints only, with particular emphasis on the knee joint (Muñoz-Bermejo et al., 2019; Tsiros et al., 2011; Saarinen et al., 2008; Haberfehlner et al., 2015; Ferreira et al., 2020). While much of the existing research has focused on adult populations, there is a growing body of work emphasizing the importance of extremity joint assessments in both healthy and pathological pediatric populations (Saarinen et al., 2008; Haberfehlner et al., 2015; Ferreira et al., 2020). However, the studies on the biomechanical properties of pediatric extremities often examine one or two properties of a single joint only. Therefore, the aim of this study is to develop a protocol using goniometry and isokinetic dynamometry to 1) measure the AROM and PROM of the paediatric upper and lower extremity joints, 2) evaluate the dynamic stiffness of the paediatric upper and lower extremity joints, and 3) assess the isokinetic strength of the muscles surrounding these joints.

This protocol describes a cross-sectional study aimed at establishing reference values for joint ROM, muscle strength, and joint stiffness. The ROM, muscle strength and joint stiffness measurements will be conducted over 6 days for each subject, with each day allotted to a specific joint (Day 1 – Shoulder, Day 2 – Elbow, Day 3 – Wrist, Day 4 – Hip, Day 5 – Knee, and Day 6 – Ankle), testing 10 subjects per day (See Table 1). All measurements will be performed bilaterally, assessing both dominant and non-dominant limbs. Upper and lower limb dominance will be defined by the participant’s preferred side for throwing and kicking a ball (Ty Hopkins et al., 2007; Richards et al., 2016; Barnes et al., 2001). The potential psychological and physical burden associated with multi-day measurements will be carefully managed by assessing one joint per day. Testing sessions will be scheduled with sufficient rest between days, limited in duration to accommodate the children’s attention span, and will be conducted in a supportive child-friendly manner. Verbal encouragement, breaks, and small incentives (e.g., snacks, fun certificates, toys, and gift cards as appropriate) will be provided to maintain motivation. It will be emphasized that participation will remain voluntary, with the option to withdraw at any time. In addition, the children will perform three repetitions of isokinetic contractions at maximal effort for each joint motion as it has been recommended for adults that two to six repetitions are sufficient to obtain maximal values without resulting in muscle fatigue (Osternig, 1986). Moreover, the velocities set in this protocol will fall below 120°/sec as it is difficult for children to attain velocities beyond this limit (Docherty, 1996). Although the estimated timeline for data collection is approximately 20 weeks, we anticipate that the process may extend over the course of a year due to the possibility of interruptions such as subject availability, scheduling conflicts or any unforeseen delays.

The ROM for each joint motion will be defined by the maximum angular displacements between the flexed and extended positions. For example, the elbow joint ROM will be represented by the angles of maximum flexion and maximum extension.

The muscle strength will be represented by the peak torque produced by the muscle group responsible for each joint motion, with a single maximum value extracted from the torque-angle curve generated by the Biodex system. The joint stiffness will be calculated as the gradient of the torque-angle curve.

The data collected will be securely stored on a computer. The sample of 200 participants will be divided into strata based on three key variables: ethnicity, age and gender. The participants will be grouped into three major ethnic groups: South Asian, Arab and Others. Gender will be grouped into two (boys and girls) and age will be grouped into nine categories (ages 4–12). Each stratum will have an equal number of participants for a proportional representation across all groups.

Normality check will be performed for the data obtained using the Kolmogorov-Smirnov test and parametric or non-parametric tests will be employed accordingly. Descriptive statistical analyses will be conducted to summarize the three parameters investigated in this study. The relationship between categorical variables will be analyzed using either Pearson’s Chi-squared test or Fisher’s exact test as appropriate. Similarly, the association between categorical and numeric variables will be examined using either Mann-Whitney U test or independent samples t-test while the relationship between numeric variables will be analyzed using either Pearson or Spearman correlation analysis. To determine the influence of anthropometric (body mass, height, waist circumference, etc.) and demographic (gender, age, and ethnicity) factors on ROM, strength, and stiffness, a series of multiple regression analyses will be performed to identify the factors that can best predict ROM, strength, and stiffness. Significance level will be set at 5%.

This protocol describes a cross-sectional study aimed at establishing reference values for joint ROM, muscle strength, and joint stiffness. It should be noted that due to the cross-sectional study design, causal relationships between the measured variables cannot be determined. Nevertheless, the findings will provide valuable baseline evidence and inform the design of future longitudinal or interventional studies. The protocol was developed to enable comprehensive assessments of the pediatric upper and lower extremity joints (shoulder, elbow, wrist, hip, knee, and ankle) function focusing on the evaluation of the joint ROM, joint stiffness, and the strength of the surrounding muscles. True stiffness of the human joint can be regarded as the combination of all the individual stiffness values contributed by muscles, tendons, ligaments, cartilages, and bones (Latash and Zatsiorsky, 1993). However, it has been acknowledged that the mathematical expressions to model all the components that contribute to a certain motion have not been developed yet (Butler et al., 2003). While some muscles contribute to a motion by generating a force to stabilize the joint (Escamilla et al., 2009), only the primary (agonistic) muscles which create the movement will be included in the muscle groups under assessment in the proposed study. The primary muscles involved in the specific motions were identified through studies that investigated these muscles (Refer to Supplementary Appendix 1). Isokinetic dynamometry enables the measurement of muscle strength and joint stiffness in both children and adults. The Biodex System 4 allows for the adjustment of the seat and dynamometer heights and orientations to accommodate the different limb lengths of subjects. When examining muscle function in children, it is necessary to consider the gravitational torques that contribute to the motion during testing. Movements in the sagittal and the frontal plane will be either assisted by gravity (e.g., knee flexion) or resisted by gravity (e.g., knee extension). This is particularly important when assessing children as they produce lower muscle torque compared to adults, leading to a higher percentage error (Jones and Stratton, 2000). The Biodex System 4 offers a gravity correcting feature which adjusts the additional torque generated due to gravity.

Several studies have attempted to draw a correlation between extremity joint muscle strength with demographic and anthropometric characteristics. For example, a study by Danneskiold‐Samsøe et al. (2009) investigated the dependency of muscle strength of the upper and lower extremities on gender, age, height, weight and BMI for an adult population while Forthomme et al. (2002) investigated the effects of weight, limb dominance and gender on the strength of forearm pronators and supinators, and wrist flexors and extensors. In addition, Marcolin et al. (2009) assessed the strength of several muscles in children with growing pains and joint hypermobility to make comparisons with healthy children. Wiggin et al. (2006) developed a protocol to establish percentile charts of isokinetic peak torque strength for the quadriceps and hamstrings by gender and age. Thus, the protocol presented in the current study will facilitate the examination of the muscle strength and joint stiffness of the upper and lower extremities of healthy children and correlate these factors with demographic and anthropometric variables. Moreover, the ROM will be correlated with the muscle strength and joint stiffness among healthy children to explore the relation between these variables. A study by Kalkman et al. (2019) identified a correlation between tendon stiffness and limited joint ROM among children with cerebral palsy, suggesting that these correlations may vary across different pediatric populations.

Injuries to both the upper and lower extremities are the second most common types of injuries sustained by children in vehicle crashes globally (Hanna, 2010; Arbogast et al., 2002) with joint kinematics and joint stiffness playing a key role in determining the severity of subsequent injuries (Seidel et al., 2018; Fildes et al., 1997; Boucher et al., 2017).

Understanding the correlation between the biomechanical factors (ROM, muscle strength and joint stiffness) of the upper and lower extremities, and the demographic and anthropometric characteristics is essential for the development of child vehicle safety systems. Research suggests that current standards for evaluating extremity injuries in pediatric crash tests remain limited, as child ATDs (anthropometric test devices) lack adequate instrumentation to measure upper and lower limb injuries (Boucher et al., 2016; Boucher, 2014; Boucher et al., 2017; Boucher et al., 2013). The proposed study will provide essential data to quantify biomechanical tolerance in children and support the development of more biofidelic extremities for child ATDs and computational models, ultimately contributing to more effective pediatric restraint systems. Measuring the pediatric biomechanical tolerances are therefore valuable for improving both injury prediction and child vehicle safety systems. In addition, this data can support clinical applications beyond vehicle safety. It has been suggested that muscle strength in pathological populations can be improved through interventions such as resistance training (Hanssen et al., 2022; Liu et al., 2025), thus an understanding of the biomechanical characteristics of healthy populations is not only valuable for diagnosis, but also for measuring the efficacy of interventions. Moreover, the extremity joints ROM of children with DS are limited compared to typically developing children, which emphasizes the importance of having baseline data from healthy children to guide diagnosis and rehabilitation (Jones et al., 2023). The proposed study will provide normative data that can serve as a reference for clinical assessment and rehabilitation planning.

Abdulazeez et al. (2024), Abdulazeez et al. (2025) conducted a large-scale cross-sectional study on healthy children (1–12 years) demonstrating that knee and elbow ROM varies with anthropometric and demographic factors. Building on this knowledge, our study will extend the scope to include upper and lower extremity joints. Similarly, Scherff et al. (2024) examined the ROM and muscle strength of typically developing children and suggested that future studies should investigate the gender differences across different age groups. In addition, González-Matilla et al. (2025) compared the ROM of the lower extremity joints of healthy children and children with CP, which concluded that children with CP exhibit reduced ROM. However, it was reported that the limited sample size restricted the extrapolation of the results. To the best of our knowledge, only a limited number of studies have assessed muscle strength and joint stiffness in pediatric extremity joints, with most of the recent studies focusing on children with neuromuscular diseases (NMDs). Van Tittelboom et al. (2022) noted that their study was limited by the small sample size. They also reported that the variation between test protocols makes comparison difficult between studies, highlighting the need for standardized protocols (e.g., participant positioning, testing velocities, rest time). Our protocol closely aligns with procedures reported by Van Tittelboom et al. More recently, van der Woude et al. highlighted the usefulness of cutoff values (minimum threshold) to decide whether children with NMDs are strong enough to perform isokinetic testing (van der Woude et al., 2024). In line with this, our study will present the cutoff values in terms of the dynamometer ROM in relation to age or anthropometric factors to determine at what point healthy children are able to perform concentric contractions.

Research on muscle strength and joint stiffness has largely focused on the adult population with the limited number of studies examining pediatric extremity joint and muscle function mostly focused on children with disabilities such as cerebral palsy and Down syndrome. Additionally, most of these studies have concentrated on lower extremity joints only particularly the knee joint. This protocol offers a foundation for future research for conducting comprehensive assessments of the pediatric extremity joint function for measuring the ROM, muscle strength and joint stiffness in pediatric extremity joints (shoulder, elbow, wrist, hip, knee, and ankle) using goniometry and isokinetic dynamometry. Future studies investigating pediatric injury mechanisms and impact forces in vehicle collisions should incorporate extremity biomechanical data in their ATDs and computational models for more accurate evaluation of injury risk. In addition, it would be beneficial to investigate the biomechanical characteristics of pathological populations, such as children with CP or DS, using the same measurement procedures as in healthy children, to allow for comparison between the different populations.

While our proposed study will provide baseline ROM, muscle strength, and joint stiffness measures in typically developing children, pathological populations such as children with CP or DS often demonstrate reduced ROM, lower muscle strength and altered joint stiffness (González-Matilla et al., 2025; Koo et al., 2022; Leister et al., 2024). Therefore, the findings from this study cannot be generalized to children with pathological conditions. Additionally, due to the limited isokinetic dynamometer settings allowing only one configuration per joint, variations in the number of sets, repetitions, machine velocity, etc., to determine the optimal conditions for peak performance in children cannot be explored.