Development of the infant foot as a load bearing structure: study protocol for a longitudinal evaluation (the Small Steps study)

The stages of infant development from sitting to crawling and eventually independent walking represent a series of important motor control and movement co-ordination milestones [1‐3]. This is the period during which the foot becomes essential for balance, stability and locomotion and therefore central to enabling a child to learn about and engage with their social and physical environments. Foot structures bear load for the first time and progressively adapt to support the entire body weight regularly, cyclically and across different terrains and obstacles. Changes to the dimensions, shape and structure of the feet occur throughout infancy and childhood [4, 5]. However, detailed changes to foot morphology (e.g. more resolution than simply width and length), structure (e.g. tissue characteristics) and function (e.g. gait and pressure), are yet to be quantified during this stage of life, however, especially through longitudinal studies.

Precise knowledge of the development of the paediatric foot and gait is essential to underpin paradigms for clinical practice and locomotion research. However, having very young participants in a gait laboratory is not a trivial task. There are specific ethical and governance requirements (e.g. safe guarding and consent), as well as challenges in terms of the size of the infant, issues of walking trajectory, consistency of movement tasks and the ability of the infant to be directed and follow instructions. Existing literature on gait development has employed both cross-sectional [6, 7] and longitudinal study designs [8‐11]. However, studies generally begin once weight-bearing has commenced, therefore missing the alterations to foot structure and function that occur with the initiation of weight-bearing. Data exists from early walkers (0–5 months of walking experience) including simple temporal-spatial characteristics and some quantification of limb and foot function in the form of plantar pressure, kinematics, electromyography and kinetics [12‐14].

Undertaking standardised gait protocols with infants has proved demanding, with Bertsch et al., 2004 collecting 40–50 walking trials per participant to obtain five suitable plantar pressure data sets [14]. Furthermore, all existing data has been collected within an experimental gait laboratory environment and using protocols which involve leading the infant by the hand, or encouraging them to walk in straight lines [14]. The external validity of these approaches has not been reviewed, but they contrast with an infant’s real-world movements. Specifically, the field of developmental psychology shows us that novice walker infants rarely walk in straight lines, preferring a few steps at a time and in multiple directions [15, 16]. It also demonstrates that this variability is a key feature of motor learning and should therefore be included in our experimental design. This contrasts with existing approaches and indeed this may explain why Bertsch and colleagues had to collect so many trials to achieve straight line walking.

Whilst some aspects of gait have been studied in infants, changes in foot soft tissues due to the commencement of weight bearing have not been documented in longitudinal studies. It is not known whether this phase of infant development would significantly influence the properties of the plantar tissues and the toe nails. Current literature infers changes to skin hydration, viscoelastic properties and surface texture with age [17‐19], in addition to age related differences in skin thickness and composition [18, 20]. Differences in skin characteristics between different sites on the foot are evident in adults, for example dorsal v plantar skin; plantar surface of the metatarsal head versus medial arch skin [21]. The neuromuscular development of the infant is also key; bone growth stimulates muscular adaptation as does increased loading [22]. This is supported by a physiological feedback loop (the Mechanostat) whereby osteogenesis is stimulated by strain between soft tissue and bone as a consequence of movement [23]. The properties of the Achilles tendon and plantar fascia has additionally been explored due to the suggestion that the two are continuous in infants [24]. There also exists a hypothesis that the curvature of nails is due to mechanical forces, but this has not been explored in infants or in the feet as opposed to the hands [25, 26].

In response to this lack of knowledge and information relating to the paediatric foot, the aim of our research is to investigate the changes that occur in the infant foot as it develops from a non-weight bearing to a weight bearing structure. This is part of a wider research initiative (Great Foundations) which is also seeking to understand how parents, professional and industry view children’s foot health.

The study described in this paper will aim to quantify the development of skin, pressure and motion parameters of the paediatric foot during real world tasks.

The external validity and clinical relevance of our research are both highly influenced by the number and timing of the longitudinal visits and therefore how they are defined. The number of longitudinal visits was chosen based on feasibility in terms of asking parents and infants to commit to a long-term study with regular visits. Four was deemed an acceptable number of visits to ask volunteers to attend laboratory spaces for two-hour test sessions over approximately a nine month period. The timing of the visits could be defined by milestone, age or set duration from the first visit. Existing research considering infant gait biomechanics either categorises infants by their age [29] or by attainment of a specific milestone [9, 10]. After significant research and debate we have selected to define infants by milestone. Existing literature shows us that age is not a suitable criteria for differentiation of biomechanical variables of infants, due to high variation in attainment of milestones [30, 31]. We have selected milestones that reflect foot loading specifically, rather than any pre-existing developmental scale. These relate directly to our research questions and are chosen with the assumption that these will relate to the amount of loading that has been undertaken before and between visits. They do not however, represent explicitly the amount of activity between visits as an infant who walks one day may not walk again the next. Age is also a confounding aspect of participants as we can expect changes to the skin characteristics with aging alone in this cohort [17, 19, 32]. To aid the interpretation of our data we will record age and corrected age (both in days) at each visit in addition to duration from milestones (in days).

The intention of the study to incorporate and define a cohort of infants which represent a typical development of babies within the United Kingdom (UK). The two-site design (in the North and South of the UK) enables the capacity to recruit participants which span the demographic make-up of the UK population. Those inclusion criteria relating to milestone attainment aim to reduce the risk of including children whose development later falls outside the typical boundaries.

Inclusion and visit timing relies on parent-reported information. To ensure this information is sound parents will be encouraged to check the child’s ‘Personal Child Health Record’ (supplied by the National Health Service at birth in the UK) and also to send video records to the researcher regularly to identify milestone achievement. Guides and regular phone calls are scheduled to inform parents as much as possible, which have been developed through pilot work and checked for comprehension by groups of parents. The questionnaire at the start of the laboratory visit also provides a further opportunity to discuss and define dates at which milestones were attained and to gather information relating to foot loading prior to visit one and between later longitudinal visits.

The collection of data from infants is challenging for many reasons including the researcher being unable to instruct them to do a task and their inconsistent locomotion choices. The infant walkers at visit three are likely to not want to walk during the test sessions and to opt to crawl when encouraged to move around the laboratory area. Our methods are flexible to allow supported walking for this visit if the infant does not voluntarily walk alone. This is a modified approach in attempt to incorporate motion strategies within the data collection which have high external validity and keeping in mind that bouts of infant walking are most commonly short in terms of steps [15]. This enables us to fully capture the changes in locomotor strategy and the trajectory of the inter- and intra-individual variability across visits [2]. It does mean that we need to consider the comparison of our data across milestones carefully as existing literature demonstrates that supported walking results in different foot trajectories in infants [9] and gait kinematics with more mature levels of coordination [33]. Our skin and ultrasound protocols are not influenced extensively by the infants’ behaviour their inability to follow instructions may mean they move during testing. If this happens the data will be recollected. The skin characterisation devices have not been utilised within this patient group before. However, they have been utilised in published research in adults [21, 27]. Extensive pilot work and comparison to other devices has been undertaken to establish this protocol as feasible, in addition to repeatability work at each laboratory.

It is evident that the infant’s level of emotional and comprehensive development is a challenge to their participation in a standard experimental protocol. Additionally, physical characteristics such as sensitive skin, high subcutaneous fat content and small dimensions should be considered, and standard data collection protocols adjusted. For example, the size and resolution of a pressure platform is designed for testing adults. Infant step length is 25 cm compared to 77 cm in adult walking [34], leading to multiple contacts on the pressure platform. This paired with the variable walking pattern requires manual extraction of collected pressure records, which adds some subjective error to an otherwise automated data extraction protocol. The resolution of the pressure platform is 4 sensors/cm² and the sensitivity measuring range 10–1270 kPa. Therefore, the resolution of infant pressure records are comparatively lower than that achieved from an adult foot with the same technology, however, this resolution is equivalent to adult pressure records from some of the lower resolution pressure platforms utilised for research and therefore can be considered sufficient for comparison. The sensitivity may mean errors in quantification of contact areas (where pressures 10 < kPa are not considered), especially when the soft pliable nature of the infant foot is considered, however this is the current gold standard in this technology. A foot size of approximately 12 cm at age one [5, 14] compared with 24 cm at 17 years of age [35] also allows substantially less surface area for the placement of markers and accurate detection and tracking by 3D motion cameras. The infant’s body composition makes the correct placement of markers on anatomical sites challenging [36]; skin motion artefact may increase due to increased fat mass and less stiff tissue and the non-ossified bones hinder palpation and rigid-body assumptions. Limb dimensions also influence the available distance between markers [37] and body composition the signal achievable through electromyography analysis. Noise and cross-talk from nearby muscles become significant risks and the determination of an appropriate inter-electrode distance is also a challenge. Significant pilot work has enabled the final protocol to minimise such errors and quality checking of data will be continuous.

Despite the challenges addressed above, the importance of this data set warrants the extensive two-site longitudinal study design which is planned. The strengths of this study will be the array of data providing an extensive database of the development of the infant foot as a load bearing structure. The environment and the inclusive approach to the use of data will enable the quantification of the trajectory of variability across these visits in terms of kinematics and pressure. As part of the Great Foundations Initiative, the results of this Small Steps study will contribute to better knowledge and understanding being shared with industry, health care professionals, researchers and parents.