# Towards a fuller integration of respiratory rhythms into research on infant vocal and motor development

Susanne Fuchs, Elina Rubertus, Laura L Koenig, Aude Noiray

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Susanne Fuchs, Elina Rubertus, Laura L Koenig, Aude Noiray. Towards a fuller integration of respiratory rhythms into research on infant vocal and motor development. Annals of the New York Academy of Sciences, In press. hprints-05422089

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# Towards a fuller integration of respiratory rhythms into research on infant vocal and motor development

Susanne Fuchs<fnref n="1" />

Elina Rubertus<fnref n="1" />

Laura L. Koenig<fnref n="2" />,<fnref n="3" />

Aude Noiray<fnref n="4" />

**Correspondence**

Corresponding author is Aude Noiray, Université Grenoble-Alpes, Laboratoire de Psychologie et NeuroCognition, CNRS UMR 5105, Grenoble, 38400, France.

Email: aude.noiray@univ-grenoble-alpes.fr

**Funding Information**

This research was supported by the French-German ANR-DFG grant to the Voc2Speak project: FU791/10-1 to S.F. and ANR-24-FRAL-0004-01 to A.N. and a DFG grant for the FLESH project: FU791/9-1 to S.F.

**Abstract**

Rhythm organizes many human motor activities from before birth and continues to shape development throughout infancy. In this review, we examine the role of rhythmic processes in early vocal development, drawing on research from motor control, physiology, speech and language acquisition. We propose that respiration functions as a crucial core of early rhythmic coordination, linking vocalizations and bodily movements into an integrated system. At present, our understanding of how infant breathing for speech develops during the first year of life remains imprecise. However, respiration, an inherently flexible and adaptive system, may provide a temporal framework within which speech articulation and motor actions become progressively aligned. During canonical babbling, a key milestone in language acquisition, repetitive adult-like syllables emerge from rhythmic motor actions. The advent of this behavior presumably reflects developing coordination among motor, respiratory, and vocal subsystems. This three-way coordination creates the multimodal foundation of language. In this perspective, the respiratory rhythm is fundamental to early vocal development. Along with reviewing past work and its limitations, we suggest directions for future work to better address how the respiratory rhythm subserves developing linguistic and non-linguistic actions in infant development.

**KEYWORDS**

respiration, vocalization, multimodality, language acquisition, rhythm, babbling, coordination

**Graphical Abstract:** From birth, respiration constitutes an intrinsic rhythm. We suggest that vocalizations and bodily movements are interactively coordinated with this respiratory rhythm, providing a temporal framework for multimodal language development.

<figure><img src="image_2.png" /></figure>

<footnote n="1">¹Leibniz Zentrum Allgemeine Sprachwissenschaft,
Berlin, Germany</footnote>

<footnote n="2">²Adelphi University, Brooklyn, New York, USA</footnote>

<footnote n="3">³Yale Child Study Center, New Haven CT, USA</footnote>

<footnote n="4">⁴Laboratoire de Psychologie et NeuroCognition,
CNRS UMR 5105Université Grenoble-Alpes,
Grenoble, France</footnote>
# 1 INTRODUCTION

From the very beginning of life, infants encounter rhythm. Prenatally, fetuses are exposed to their mother's heartbeat, breathing, walking, and speech rhythms. They also produce rhythmic body movements such as jaw openings, head movement, body jerks, leg kicking<sup>1,2</sup> and diaphragmatic actions comparable to what they will need for breathing after birth.<sup>3,1</sup> Once they are born, the babies begin to generate their own breathing rhythm and more extensive body movements. Throughout the first year of life, infants display rhythmic body movements such as kicking their legs, rocking their whole bodies, and waving their hands.<sup>4,5</sup> A rhythmic activity that has received considerable attention, because it is a cornerstone for the emergence of language, is canonical babbling. Babbling involves rhythmic movements of multiple components of the upper vocal tract to produce speech-like utterances such as "bababa".

In this review, we argue that the rhythmicity of babbling may reflect more than a developing communication system in isolation. Rhythmicity is an intrinsic property of the human body organizing biological signals such as the heartbeat,<sup>6</sup> respiration, motor and vocal behavior.<sup>7</sup> These perspectives are also congruent with the notion that language is a multimodal phenomenon, not limited to vocalization alone but including various bodily motions. Along these lines, a central aim in this review is to propose that respiration, as a flexible and adaptive rhythm, is closely connected to rhythms in vocal and motor development. For clarity, we separate vocal and bodily motor development terminologically while recognizing that speech and other vocalizations are also a form of motoric behavior. We suggest that learning the coordination between respiration and speech articulation and that between respiration and bodily motions is important for language development. We assume that rhythmic motions may facilitate learning this coordination.

In the following sections, we first review the literature on various developmental rhythms with an emphasis on the first year of life. We then outline developmental studies examining the coordination among different motor and vocal subsystems. We provide arguments for why respiratory rhythms could provide a bridge between vocal and motor systems, and why these rhythms are important for coordinated actions. Finally, we discuss the literature in light of limitations and challenges and propose directions for future research.

# 2 DEVELOPMENTAL RHYTHMS

Definitions of rhythm are manifold. In this paper, we focus on the production of different potentially interacting rhythms of the human body. The physical signal we are interested in involves repetitiveness, regularity and predictability,<sup>8</sup> and this signal builds sequences of successive events of similar duration. Such rhythms may be called "quasi-isochronous", since they approach isochrony, but also vary to some degree.<sup>8</sup>

Most research on rhythm in early vocal development has focused on the phonetic sequencing of reduplicative babbling. However, it is now widely accepted that language is a fundamentally multimodal phenomenon,<sup>9</sup> not confined to sound. It also incorporates gestures, postures, and facial expressions. This perspective shifts the focus beyond the vocal domain to include bodily expressions that often bear rhythmic properties, as part of language in communication. Therefore, the presence of rhythmic patterns in both vocal and motor development may not only arise from a general backdrop of potentially shared developmental motor and vocal trajectories or control mechanisms; it also reflects the embodied nature of language. Within this view, it is necessary to unify the literature on rhythmic body motion with that on vocalization related to language development. Building on previous research, we further suggest that the lifelong physiological rhythm of breathing could serve as a crucial link between the vocal and motor systems, connecting both during the acquisition of multimodal language.

## 2.1 Rhythmic body motions in infancy

Infants commonly produce specific motor actions multiple times in short time intervals of one second or less. These repetitive sequences have been termed motor stereotypies by Thelen.<sup>10</sup> Stereotypical rhythms of this sort occur in many motor systems: Kicking or cycling actions of one or both legs; rattling or banging with arms and hands; rocking or bouncing; shaking or nodding the head.<sup>11,12,13,14,15</sup> These various forms of rhythmic behavior unfold in different stages of development and are conditioned by bodily position (e.g., supine position, sitting, standing), as Thelen showed. Whereas stereotypical motor behaviors have often been associated with neurodivergence,<sup>16</sup> Thelen's pioneering research demonstrated that rhythmic motor stereotypies play a critical role in typical human motor development, especially as transitions between different developmental stages. Together
with colleagues,¹⁰ she observed 20 infants longitudinally on a biweekly basis from 4–52 weeks of age for an hour per session and found more than 16,000 rhythmical stereotypies. These stereotypies did not occur randomly; instead the rhythmic actions for particular bodily systems increased in frequency and then declined over time, with a peak of total rhythmic activity at about 6–7 months. Rhythmic behavior generally served to refine coordinative behavior, but with individual variation in the specifics wherein each infant "learn[ed] to tune his or her own springs" (p. 373).¹³

Thelen suggested that stereotypical motions are intrinsic rather than a consequence of interacting with the environment, but some recent research indicates that early rhythmic movements may in fact be shaped by sensory input. For example, newborns are capable of producing rudimentary crawling-like motions when their head is supported.¹⁴ These motions have previously been regarded as reflexive, but may involve more complex processes. Hym and colleagues¹⁴ demonstrated that newborns produced a greater number of steps and more pronounced head rotations toward a loudspeaker when exposed to their native language compared to a foreign one. Additional evidence for an interaction between early motor rhythms and external auditory input is provided by Ejiri.¹⁷ She reported that after a period of no preference for either a sound-inducing or a silent rattle, infants start to shake the audible rattle more frequently than the silent one. Interestingly, this preference for the sound-evoking rattle develops around the onset of canonical babbling. Not only auditory but also visual feedback affects rhythmic motions and can play a role in early language development. For instance, Petitto and colleagues¹⁸ found that infants whose caregivers communicate with sign language produce rhythmic manual motions in two frequency ranges, while infants whose caregivers communicate with vocal speech only produce the faster manual rhythm. The slower frequency rhythm shared many features of vocal babbling and was therefore termed 'silent babbling'. The authors suggested that there is a genetically hard-wired sensitivity to specific rhythmic patterns and a capacity to use them, either in the manual or in the vocal modality. More examples of early rhythms reacting to sensory feedback can be found in Provasi and colleagues,¹⁹ who provide a comprehensive overview of rhythm production and perception in infancy. Taken together, the reported findings reveal aspects of sensorimotor integration in early development.

## 2.2 | Vocal rhythm in infancy: Canonical babbling

During the first year of life, infant vocalizations transition from mostly reflexive sounds to long vowel-like coos and then to more complex sequences shaped by the phonetic structure of the ambient language.²⁰,²¹,²²,²³,²⁴,²⁵ One of the universal cornerstones of language development within this period is the emergence of canonical babbling,²⁶,²⁷,²⁸ which is characterized by repeated consonant-vowel syllables that resemble mature speech. Importantly, the repetitive nature, either in multiple productions of the same syllable (i.e., reduplicated babbling /bababa/) or of varying syllables (i.e., variegated babbling /badada/) creates a rhythm that resembles other rhythmic motor patterns typically found in infants.²⁹ Canonical babbling, therefore, provides insight into both motor and linguistic development.

From a motor perspective, the developing coordination between supralaryngeal speech articulators is of special interest (Figure 1). Specific attention has been paid to the rhythmic jaw movements of babbling: Open-close jaw movements without phonation (i.e., silent jaw wagging) are often found in infants and peak in frequency between 8 and 13 months of age.³⁰ When coordinated with phonation, rhythmic syllable-like structures emerge.²⁴ Control over these oscillatory jaw movements was shown to precede the independent control of the lips, the tongue, and the soft palate.³¹,³²,³³ In the frame-content theory,³¹ jaw oscillations are, therefore, interpreted as (syllabic) frames to which segmental “content” is added subsequently.

From a linguistic perspective, canonical babbling marks the first appearance of syllables that bear a resemblance to adult-like acoustic-phonetic patterns of timing, prosody, and formant transitions.³⁴ Canonical babbling builds the foundations for early word and utterance production.³⁵ Whereas earlier stages of vocal development seem to be independent of auditory feedback, more babbling is observed in normally-hearing and less in hearing-impaired children.³⁶ The findings of Petitto and colleagues¹⁸ are also instructive in this context. These authors demonstrated that infants who are exposed to sign language develop manual babbling, that is, rhythmic hand movements with a frequency of around 1 Hz that were not found in their speech-exposed peers. Altogether, these findings indicate that linguistic input in the form of speech or sign can shape both vocal and manual canonical babbling.

While advocates of a strong motoric hypothesis view babbling as merely a side effect of stereotyped movements associated with general motor development,³¹,³⁷ proponents of a linguistic perspective on babbling would posit that it is an active discovery process through which infants explore the phonological inventory and patterns of their native language.²⁷,³⁸,³⁹ Based on a review of empirical findings gathered in the last decades,⁴⁰ we would like to highlight, instead, that canonical babbling reflects important motoric as well as linguistic developments which interact. The similarity in structure and timeline between early babbling and other typical rhythmic motor behaviors gives rise to the interpretation of jaw oscillations as one of many highly stereotyped
motor patterns found in infant development.¹⁰ In fact, several of these stereotyped movements begin to show effects of sensory input around the onset of canonical babbling. Ejiri,¹⁷ for example, provides evidence from 28 infants that the occurrence of rhythmic hand and arm activities (rattle shakes) peaks around the onset of canonical babbling. A developing preference for audible over non-audible rattles indicates the increasing importance of auditory feedback at this time. This underscores the above proposal that after a period of rather reflexive, intrinsically organized rhythmic motor patterns, a fuller integration of sensory feedback may lead to more precise motoric coordination and developmental progression.

## 2.3 | Respiratory rhythm and development

Whereas some developmental rhythms are transient and may serve as a transition into more complex behavior, the rhythmicity of breathing is obviously present from birth and persists throughout life. As a basic biological function, breathing has been studied extensively in the context of medical evaluation.⁴¹,⁴² Although individual differences are considerable, particularly when the infant is awake and active, healthy infants’ breathing development in the first three months of life is characterized by a rather rapid decrease in breaths per minute and increased regularity of the breath cycle, with fewer instances of apnea, that is, gaps in the breathing rhythm.⁴³ Similar to other rhythmic movements, the respiratory rhythm has been found to react to external sensory feedback. Sammon and Darnall,⁴⁴ for example, found rhythmic entrainment of preterm infants’ breathing rate to the tempo in which they were rocked.

In infants, as in adults, quiet breathing (also called rest or tidal breathing) is rather sinusoidal,⁴⁵ with approximately equal durations of inhalation and exhalation. However, the inspiratory-expiratory cycle is modified⁷ even in early infancy in service of phonation during cry (see section on coordination below).⁴⁵,⁴⁶ Although patterns may be less regular or controlled than observed in mature speakers,⁴⁷,⁴⁸ the exhalation phase for phonated, speech-like events lengthens, ultimately leading to a more asymmetrical breath cycle.⁴⁸ Increasing modification of the breath cycle for speech-like utterances has been documented for infants from 32 to 41⁴⁹ and from 53 to 90 weeks of age.⁵⁰ Specifically, syllable production, whether "unarticulated" or in canonical babbling, is characterized by slightly shorter, steeper, and deeper inspirations, and longer, more extensive exhalations compared to tidal breathing. These asymmetries are a key feature of breathing during read and connected speech in adults.⁵¹,⁵² Insofar as the respiratory system needs to fulfill basic biological functions, the temporal domain of the breath cycle ultimately shapes utterance length and phrasing.⁵³ It has long been known that typical adults do not inhale randomly during speech, but tend to do so at major linguistic boundaries (sentence or phrase boundaries),⁵⁴,⁵⁵thus coordinating linguistic and respiratory needs.

Figure 1 provides an illustrative summary of empirical evidence (see reference numbers) of the main respiratory, vocal and (speech) motor developments occurring during the first two years of life, as well as the coordination among these systems. However, note that the ages provided are based on averages across infants. Although infants’ patterns are often described in relation to specific ages, numerous studies have shown that speech, motor as well as language-related developments unfold along a continuum rather than in clear-cut stages.³³,³⁵,⁵⁶,⁵⁷,⁵⁸,¹²,⁵,⁵⁹ In line with these findings, we conceptualize development as a continuous, dynamic set of processes. We do not expect the acquisition of new (vocal) behaviors to follow a linear developmental trajectory over time, or to develop uniformly across infants. Instead, we expect individual trajectories to reflect developmental spurts, plateaus, overlaps, and periods when some patterns may peak or even regress. For instance, rhythmic body motion and stereotypies are observed from the first months of life but they may peak at slightly different times depending on the infant. This variability is instrumental to the acquisition and refinement of (speech) motor control. Despite this variability, the findings drawn from separate lines of literature intriguingly suggest that rhythmic behavior in the respiratory, vocal and motor domains peak at around 7 to 10 months of age. It will be important in future work to determine whether this period is of importance for the development of (speech) motor coordination involving rhythmic actions that engage motor systems of the whole body. We elaborate further on this in the next section.

## 3 | THE DEVELOPMENT OF COORDINATION AMONG DIFFERENT SUBSYSTEMS

An important issue to consider in motor development in general, and speech in particular, is that the same muscles or systems may be recruited for multiple actions. The muscles of the lips, tongue, jaw, larynx, and respiration must work together as functional synergies for actions like chewing and swallowing as well as for speech.⁶⁰ Some authors emphasize how early speech behaviors like babbling can be seen as the outgrowths of earlier stereotypical movement patterns.⁶¹,⁶² There is also considerable empirical evidence, however, that patterns of coordination among muscles and systems are task-specific.⁶³,⁶¹,¹² Without assuming that
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** Visual summary of the main respiratory, vocal and (speech) motor developments occurring during the first two years of life. Ages are provided for average reference but clear-cut developmental stages are not imposed.</figcaption></figure>

motor actions are simply borrowed from non-speech behaviors into early speech behaviors, we do believe that anatomical and physiological developments contribute to changes in coordination of multiple subsystems and that a valid assessment of infants' changing vocalization abilities should at least put these developments in the context of other physical actions. This is especially the case if we assume that linguistic expression is accomplished in a multimodal fashion and is not restricted to vocalization or speech alone.

## 3.1 | Anatomical and physiological changes that could affect coordination

As indicated above, studies of babbling, particularly those with a linguistic perspective, have focused heavily on characterizing the phonetic repertoire in terms of consonant- and vowel-like sounds of infant vocalizations. While the coordination of supralaryngeal articulators has been addressed in many studies of speech motor control development,³²,³¹ less attention has been paid to phonatory features apart from⁶⁴,⁶⁵ with often little more than a nod to the role of the respiratory system in the form of a mention of breath groups.⁶⁶,⁵³ Part of this emphasis on supraglottal articulation presumably reflects the fact that consonant-vowel sequences bear a stronger auditory resemblance to adult speech than the earlier coos, grunts. To some degree it may also have to do with the fact that the widely-discussed laryngeal descent in human infancy and early childhood massively reshapes the upper vocal tract.⁴⁵,⁶⁷,⁶⁸

Figure 2, upper inset, shows the extent and nature of these developmental changes. In young infants, the high position of the larynx puts the epiglottis, a flexible cartilage that protrudes above the larynx, just beneath the soft palate. The tongue mass is concentrated within the horizontally-oriented oral cavity, restricting shaping of the supralaryngeal cavities. Developmental lowering of the larynx-hyoid complex pulls the epiglottis and tongue root inferiorly, allowing for a wider range of oral and pharyngeal cavity shaping,⁴⁵ and therefore expanding infants' vocalic repertoire beyond a neutral schwa-like vowel to a wider vowel space as measured by vowel formant frequencies.⁷⁰,⁷¹ Thus, this aspect of anatomical development can be seen as ushering in the (supralaryngeal) phonetic characteristics of mature speech.
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** A schematic comparison of infant (left) and adult (right) overall body shape and proportions (e.g., the head-body relation), vocal tract, and ribcage. In the vocal tract, the size relation between pharynx (dark blue shade) and oral cavity (light blue shade) changes along with the descent of the larynx from infants to adults. The ribcage as well as the diaphragm undergo changes in shape. The ribcage and diaphragm drawing was adapted from Devlieger and colleagues.⁶⁹</figcaption></figure>

What may be less widely appreciated is the degree to which the respiratory system is also reshaped in humans, beginning in infancy and continuing through late adolescence.⁷²,⁷³,⁷⁴,⁷⁵,⁷⁶,⁷⁷ These changes involve overall enlargement; for the respiratory system this yields increased lung volume, which can, in turn, support longer utterances.⁷⁸ The protracted nature of learning to coordinate breathing with speech must arise, in part, from linguistic and cognitive development related to utterance complexity and planning as has been reported for adults.⁵⁴,⁷⁹,⁸⁰,⁸¹ More interestingly, and parallel to upper vocal tract development, the respiratory system also undergoes system-wide restructuring and changes in physiological capacities, partly brought on by increasing chest wall stiffness with age, leading to stronger static recoil forces that contribute to or must be controlled for expiration⁷⁷ and using respiratory muscles during speech-related exhalation.⁸² Whereas the lower ribs assume an angular orientation in adulthood, in infants they lie mainly on the horizontal, so that action of the intercostal muscles between the ribs yields relatively less change in lung volume.⁸³,⁸⁴ This is illustrated in Figure 2, lower inset. Similarly, the diaphragm is a dome-shaped muscle in adult humans, but is flatter in infants, providing for less vertical displacement overall.⁶⁹ Infants have therefore been described as "belly-breathers" (p.224),⁸³ and abdominal and thoracic contributions to breathing for vocalization evolve well into childhood.⁷³ Finally, paradoxical, i.e. oppositional movement between the rib cage and abdomen, is common in infancy, but becomes rare in early childhood and adulthood.⁸⁵,⁶² In short, the act of breathing involves different mechanisms in infancy than in older children and adults. Prominent researchers evaluating the early development of speech breathing have pointed to Thelen's Dynamical Systems framework in arguing that learning respiratory patterns for vocalization, like other early motor actions, involves early exploration and practice followed by a period of refinement which can last some years.⁸⁶,⁶²

At present, little is known about the early stages of coordinating respiration with other bodily activities, including speech, which both shape the breath cycle and increase ventilatory demands. The respiratory system may play a crucial role in cross-system coordination because it must simultaneously serve the requirements of speaking and those of other bodily and even cognitive tasks. In the next sections, we will present empirical evidence for coordination among different subsystems.
## 3.2 Coordination among vocal subsystems

Mature speech production requires precise coordination among respiratory, laryngeal, and supralaryngeal actions. Since empirical evidence on the coordination between respiratory and supralaryngeal motions in the first year of life is limited, we mostly concentrate on respiratory-laryngeal coordination.

It is self-evident that physical growth accompanies an increasing age in infants and children. More important for present purposes is that these growth patterns are non-linear, occur at different times for different bodily systems,⁸⁷,⁸⁸ and ultimately require reorganization of coordination among respiratory, laryngeal, and supralaryngeal actions. As pointed out earlier, the neonatal act of crying represents an early form of developing coordination between the respiratory and laryngeal systems, with the exhalation phase modified in order to sustain the phonated phase of cry achieved by rapid and precise vocal fold adductory movements.⁴⁵ The preliminary nature of respiratory-laryngeal coordination is evident, however, in the fact that early cry vocalizations can be observed also during inspiration.⁴⁵,⁴⁶,⁸⁹ Mature speech, in contrast, is almost exclusively produced on the exhalation phase aside from occasional short ingressive vocalizations used in some contexts and cultures.⁹⁰,⁹¹

A similar progression has been observed for rhythmic laughter bouts: Whereas adult human laughter occurs primarily on exhalation, young infants produce laughter on both inhalation and exhalation, as has been observed for non-human primates.⁹²,⁹³ Specifically, from three to 18 months, there is a developmental progression so that adult listeners judge infants’ laugh bouts as being increasingly more produced on exhalations than inhalations.⁹⁴ This developmental change in respiratory-vocal dynamics was also observed in infants’ speech-like vocalizations. In a recent longitudinal case study of a prelinguistic bilingual infant,⁴⁹ phonation was initially (at 7 months and 15 days, in the infant literature spelled as 7;15) realized on inhalation, but by 9;15 months predominantly on exhalation. The developmental model of Stark,⁹⁵ which considers how vegetative and other "pre-speech" behaviors may contribute to later speech function, points to cooing as a phase during which infants begin to show coordination between phonation and egressive airflow, though supraglottal vocal tract activity may remain reflexive, i.e., not yet actively controlled.

Although most authors have focused on the precision of laryngeal control in crying and emerging coordination between respiratory and laryngeal systems,⁴⁵,⁴⁸ rudimentary aspects of respiratory-laryngeal-supralaryngeal organization can also be observed in cry. The mouth opens⁴⁵ and infants may switch from their regular pattern of nasal breathing⁷¹ to oral inhalation.⁴⁵ Closure of the velopharyngeal port and tongue furrowing may be observed as air is channeled through the oral cavity.⁴⁵ However, speech-like infant babbling utterances involve more extensive and coordinated actions of the upper articulators. These may begin primarily with broadly coupled articulatory routines that become more complex and result in greater independent control of individual articulators over time.³¹,⁹⁶,³³,³²

If the temporal properties of respiratory kinematics change in parallel to those associated to infants’ developing vocalizations, the relation is not straightforward. In a study of 7-to-11-month-old infants, Reilly and Moore⁸³ found significant developmental changes in chest wall movement which could not be attributed to infants’ vocalization duration because those did not change over time. While the authors suggested changes in respiratory movement to mostly be related to biomechanical factors, they also noted a change in the quality of infants’ vocalizations during the 7-to-11 month period, with the emergence of reduplicative babbling. The authors postulated that this new rhythmic pattern may change the way biomechanical constraints affect the development of respiratory control for vocalization.

## 3.3 Coordination among bodily motions

This section reviews empirical and theoretical work that describes coordinated actions among different parts of the body. Although unimanual exploring or reaching tasks may at first glance appear to have nothing to do with the coordination of different limbs, they are nevertheless interesting for general discussion, as younger children often move with coordination across limbs. Joint activities of the limbs while only one limb is needed for a task is called ‘motor overflow’.⁹⁷,⁹⁸ Thus, exploring objects by shaking (the only rhythmic activity), dropping, or rotating them, becomes a whole body phenomenon.⁹⁷ Soska and colleagues⁹⁷ investigated 27 infants between 4.5 and 7.5 months of age, either in supine or sitting position, while they were encouraged to play with various toys. No task differences were found among the shaking, dropping or rotating activities, hence we consider the results relevant for rhythmic development in general. In about 72% of the cases, unimanual tasks were produced simultaneously with additional movements of the free hand and in 47% with movements of a leg. The authors argue that this symmetry in motion may be the default in early development and could be explained by a lack of inhibition in one of the motor cortices of the two brain hemispheres.⁹⁹
Similarly, D'Souza and colleagues¹⁰⁰ studied motor overflow in 9- to 12-month-old infants testing developmental changes and the potential role of attention. In this 3-month age range, they found that younger infants exhibited more extraneous arm movements compared to older infants. Moreover, infants who demonstrated lower attention showed increased motor overflow, suggesting a potential link between attentional mechanisms and motor specialization. These findings raise the possibility that in early infancy motor control is initially broadly tuned toward goal achievement and involves multiple limbs. Over time, motor control becomes more refined: Limbs that are unrelated to the task are progressively inhibited and decoupled and only the limb movements that are crucial to fulfill a task become more differentiated, specialized, and efficient. Although the initial broadly tuned system may appear insignificant for communicative purposes due to its less goal-directed nature and reduced involvement of attention and voluntary control, the findings presented here demonstrate that it serves as a precursor to intentional, goal-directed manipulative actions. In principle, motor overflow observed at this early stage could later give rise to purposeful body movements, such as co-speech and pointing gestures (multimodal language).

Locomotion provides another example of developing coordination between breathing and bodily actions. Noah and colleagues¹⁰¹ investigated the entrainment of respiratory frequency with movement in 52 infants at about 9 months of age. The infants were not able to walk on their own. Movement conditions involved sitting, then stepping on a treadmill with vertical support by the experimenter and sitting again. Treadmill motion was varied with different speeds (1-2m/s or 4-12m/s). The authors showed that breathing frequency changed at the beginning of sitting to stepping and back to sitting. More importantly, breathing frequency adapted in response to stepping frequency, rising with increased stepping rates and falling with decreased ones. This adaptation was especially striking in the slow stepping condition, where, unexpectedly, breathing frequency dropped below the resting (sitting) level despite the increased physical demands. It would be interesting to know whether this coupling between locomotion and breathing changes over developmental time.

Contrary to previous literature, breathing may also be coordinated with sign language¹⁰² and its acquisition. Ming compared breathing in signing (Swiss German Sign language) with speaking. Her findings show that breathing in sign language is aligned with syllable level movements, even if airflow is not needed to produce signs. Exploring breathing in learning a sign language or learning to be bilingual signer/speaker are unknown but would be interesting for future research.

## 3.4 Coordination between vocal and bodily motions

Many early theories of language development have separated language and motor behavior.¹⁰³ One reason for this historical separation is probably that language has mostly been associated with cognition and vocal output, but not with body motions (signs, gestures, facial expressions, postures). However, there have been acknowledgments¹⁰³ of "remarkable synchronization of speech milestones with motor-developmental milestones" (p. 232) citing Lenneberg.¹⁰⁴ Since it is now widely accepted that language production is multimodal and includes body motions as well as vocal output,⁹ it should not be surprising to find co-occurrences in development.

Several attempts have been made to investigate the developing coordination between vocal and motor actions. As noted above, rhythmic activities such as hand banging and rattle shaking are particularly frequent around the onset of canonical babbling.¹⁰⁵,¹⁷,¹⁰⁶ Moreover, before the onset of canonical babbling, rhythmic activities of the arms and/or legs are more frequent than other motor actions (banging or swinging arms up and down once, mouthing: putting objects or a finger in the mouth, handling: touching, grasping, pulling objects).⁴ These rhythmic activities decline later on. They also affect the acoustic output: Syllables are shorter in duration with activity and longer without. Ejiri and Masataka⁴ state that "infant's rhythmic activity, which occurs frequently during the transition period from the pre-canonical stage to the canonical stage, may play a role in helping infants learn the articulatory skills required to produce CB [canonical babbling]" (p. 47). Although it cannot be determined at present whether or not there is a causal link between motor activity and vocalization, it is nevertheless justified to regard babbling as "one accomplishment in a series of overall developments of infants' rhythmic behavior" (p.227) in Ejiri,¹⁷ and several other studies.¹⁰⁷,⁷,¹⁰⁸,¹⁰³,¹⁰⁹ Additional evidence for vocal-motor coordination can be seen in the work by Borjon and colleagues,¹¹⁰ who showed a coordination among head and hand motions and vocalizations. Motions started prior to vocalizations in infants between 9 and 24 months of age, speaking for a vocal-motor coordination. This coordination tightened, or more precisely, the velocity peaks of the respective motion became closer to the onset of vocalization with developmental time. The authors were not explicit about whether hand and head motions were rhythmic, but we assume this is the case, at least for the earlier age (at 12 months) based on their Figure 1b. Going one step further, Walle and Campos¹¹¹ showed a correlation between the onset of walking and receptive and productive vocabulary. Infants who were walking at the age of 10 to 13 months had a higher productive and receptive vocabulary than crawling infants of the same age.
Finally, Pouw and Fuchs¹¹² used the model of Speech-Gesture Physics¹¹³ to interpret biomechanical links connecting vocalization and rhythmic limb motions in vocal-motor babbling. Within this model, rapid motions of the upper limbs leave their imprint on the respiratory system, since the lungs are embedded in the ribcage and any kind of ribcage motion affects the lungs to some extent. Depending on motion direction and which part of the arm moves,¹¹⁴,¹¹⁵ small changes in subglottal pressure can be expected. Subglottal pressure changes in turn, have an impact on acoustics of speech, leading to higher intensity and fundamental frequency peaks when the arms are moving down, as in a beat gesture. That means that physical impulses due to fast motion yield changes in speech output via the respiratory system.

# 4 | THE POTENTIAL ROLE OF RESPIRATORY RHYTHMS IN COORDINATIVE ACTIONS

So far, we have introduced respiratory rhythms in development, anatomical and physiological changes that the respiratory apparatus undergoes and how respiration is coordinated with other motor and vocal movements. This section is dedicated to motivate why breathing would be a suitable rhythm to be used for learning to coordinate speech production and bodily motions. Although our earlier focus was on the infant literature, we will also incorporate adult data, given that infant data are not always available.

## 4.1 | Breathing is adaptive

The primary role of respiration is life-sustaining, that is, to supply the muscles and brain with oxygen and maintain homeostasis. It entails both an automatic and a voluntary control component: it is governed by the autonomic nervous system, yet also subject to voluntary cortical control.¹¹⁶,¹¹⁷ As such, respiration is a physiological rhythm that may remain stable over days and even years within an individual.¹¹⁶,¹¹⁸ However, being physically and functionally connected to other body parts, the respiratory system is also highly adaptable to task demands, as summarized by Serré and colleagues.¹¹⁹ Under physical effort when more oxygen is needed, breathing frequency increases. At the same time, pulmonic airflow provides the energy source for phonation. Therefore, the shape of the breathing cycle changes from a symmetrical shape at rest to an asymmetrical shape for speech production indicating a shorter and steeper inhalation phase and a longer exhalation phase. During speaking, breathing also adapts to linguistic structure e.g., for pausing at constituent boundaries.⁵¹ Four examples of task-specific breathing patterns are provided in Figure 3. The figure displays respiratory cycles of an adult in different vocal and motor tasks. Breathing at rest and biking have the most symmetrical shapes in comparison to speaking or speaking while biking with physical effort. Speaking alone shows the largest variability in breath duration, while under physical effort, long respiratory cycles are not common. In general, breathing rate varies substantially, with higher rates under physical effort.

## 4.2 | Breathing as an internal clock

Importantly, as we have argued in a previous review,¹²² the adaptation between speech and breathing can be bidirectional. For example, while breathing adapts to linguistic structure, speech itself can (and even must) adapt to breathing and the limits of its flexibility. Towards the end of a speech chunk in long exhalation periods,¹²³ for example, speakers adjust their laryngeal resistance to prevent air loss and match the speech material to the respiratory cycle. Similarly, locomotion and breathing interact. It has been shown that human limb and respiratory frequency tend to be phase-locked in various frequency relations, especially during running,¹²⁴,¹²⁵,¹²⁶ creating a stable temporal scaffold for movement. Respiratory-locomotion coupling occurs in other mammals as well, and it may be classified as automatic, especially in quadrupedals where each forward step of the upper limbs on the ground pushes on the respiratory system. Bipedals may have more freedom for the coupling frequency, since their upper limbs are not affected by ground reaction forces.

There is also evidence for the coupling of respiration with fine-tuned motor control such as button presses and cognitive activities. Participants who were free in their timing, substantially more often pressed the button during their expiration than during their inspiration phase.¹²⁷ From a neuroscientific perspective, this is explained by the amplitude of the so-called readiness potential that precedes any voluntary action to reflect breathing-induced fluctuations of neuronal background activity. Relatedly, reaction times in several perceptual tasks were shown to vary significantly with the respiratory state.¹²⁸ The same study also indicated that participants adapt their breathing cycle to inhale around stimulus presentation and exhale when responding.¹²⁹
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** Example from an adult respiratory behavior in different conditions (from top to bottom): breathing at rest, speaking, biking under moderate effort (resistance = 70 W), biking and speaking. Data are from a study by Fuchs et al.¹²⁰ All figures are made using *praatpicture*.¹²¹</figcaption></figure>

This bidirectional neural-respiratory phase-locking supports the hypothesis that respiratory events are coordinated with both cognitive and motor actions. Whether this coupling is a specific human motor-respiratory-cognitive organization, or simply the result of oxygen in the brain preparing for the upcoming task, is so far unclear.¹³⁰,¹³¹

**4.3 | Breathing coordination in infancy**

As reviewed above, the literature on adults presents indications of close bidirectional interrelations among breathing and both speech and motion, which underpins the central role of the respiratory rhythm. The physical coupling between the respiratory system, postural biomechanics and vocal production links speech to other bodily motions, as described in the Gesture-Speech Physics model.¹¹³ Since breathing is a lifelong available rhythm accompanying a human from the moment of birth, it may offer temporal anchor points for developing coordination with speech articulators, arm gestures, and locomotion. However, the literature on the role of breathing coordination in infants and young children is sparse.

Because breathing is a requisite and omnipresent rhythm, it does not need to be initiated for specific tasks. However, it is necessary for infants to gain voluntary control over the respiratory system. Already at a young age, respiratory rhythms change in rate depending on activity¹³² also differ in cycle shape.⁷² Preliminary data from one infant indicating very different breathing patterns while listening, clapping her hands during play, and crying are presented in Figure 4. This high variability in respiratory rhythms is in line with Wolff's description¹³³ that "infants are "noisy" systems whose frequent and unpredictable changes of behavioral state radically alter their spontaneous behavior and responses to environmental events" (p.119). Regarding speech production, the "breath group"¹³⁴ has long been used as a reference unit for structuring utterances¹³⁵ indicating that lung volume and breathing capacity determine the length of utterances. Infants must learn to coordinate phases of inhalation and exhalation with laryngeal and supralaryngeal actions for rhythmic vocalizations like babbling and later speech. With respect to locomotion (more precisely, its precursor), Noah and colleagues,¹⁰¹ as mentioned above, provide evidence for very early adaptations of breathing to movements: respiratory frequency varies with stepping rate when infants move on a treadmill with assistance. Importantly, breathing frequency in the lowest stepping condition fell even beneath that during rest. According to the authors this is a surprising finding as it contrasts with the expectation that any kind of motor action induces an increase in breathing frequency.
<figure><img src="image_6.png" /></figure>

**FIGURE 4** Example of respiratory data from a 7;15 years-old infant in different activities (from top to bottom): listening to an experimenter producing a sound repetitively, clapping with both hands on a table, parts of a longer cry. Data are from the study by Fuchs and colleagues.⁴⁹

**4.4 | Rhythm, coordination and practice**

Why should breathing and rhythmic speech and/or body motions be a good match for coordination? Rhythmic motions can serve as an efficient form of practice, providing sensorimotor feedback to refine actions over short time frames and promote the learning of sensorimotor routines as well as train voluntary motor control. Practice effects have been studied extensively in motor learning and in the long term give rise to actions that are faster, less variable, and have more efficient coordination. ¹³⁶,¹³⁷,¹³八年,¹³九年,¹¹,¹⁴⁰ In the speech literature, trends towards faster rates and less variability have been widely documented from about three years of age and extending into adolescence, ¹⁴¹,¹⁴²,¹⁴³,¹⁴⁴,¹⁴⁵ but they are hardly documented below the age of three.

Although rhythmic actions may not always be explicitly goal-directed,¹⁰ they can nevertheless support the development of sensorimotor coordination. Kent⁷ suggests that early rhythmic behavior may serve as a foundation for the emergence of more complex, coordinated movement, and points to central pattern generators as a possible neurological source of rhythmicity.

"In analogy with the control of limb movements, the innate programs to control different sound sequences in animals may be generated through a control of the activity in the respiratory brainstem pattern generator and fractionation of the central pattern generators (CPG) for mastication and swallowing" (p. 227). ¹⁴⁶ In other words, breathing is a natural physiological rhythm that may provide a foundation for coordinated action with other movements.

Coordinating actions and respiratory cycles is both mechanically convenient and neurologically wired; it also makes physio-
logical sense. Oxygen intake directly supports the metabolic demands of both muscles and the brain. As described above for
adults, neural activity preparing for motor actions or cognitive tasks becomes phase-locked to breathing, thus shaping perception
and action timing. ¹⁴⁷,¹²⁸,¹⁴⁸ This suggests that respiratory–neural coupling may not be merely reflexive but can also be refined
through experience and learning specific motor actions.
# 5 | LIMITATIONS AND CHALLENGES

## 5.1 | Theoretical limitations

For decades, empirical studies of early speech development have concentrated on describing the consonantal and vocalic nature of infants' suparalaryngeal articulation, turning the spotlight on canonical babbling, which more closely resembles adults' speech than the earlier coos and grunts. Part of this focus presumably reflects the tendency to describe infants' utterances through the lens of an adult speech system (e.g., using IPA transcriptions<sup>72,22</sup>). However, infants' vocal productions may not be organized along the same phonetic categories as adults'. Furthermore, it places primary emphasis on phonology and vocal behavior, overlooking the contribution of bodily motions.

Most of the research leading to the frame/content theory,<sup>31</sup> for instance, has focused almost exclusively on alveolar /n,d,t/, labial /m,b,p/, and velar /g,k/ consonants and their co-occurrence with vowel categories. While this line of work certainly highlights motor aspects and the coupling of tongue and jaw movement in language development, it largely neglects the contributions of laryngeal and respiratory activity to the broader coordination of speech actions. In addition, it does not fully account for the highly variable and exploratory nature of infants' vocal productions, which may not adhere to an adult categorical system yet. The theory has also been challenged by reports of consonant-vowel patterns that a jaw-dominant motor scheme would not predict,<sup>149</sup> as well as by kinematic investigations.<sup>150</sup>

Other theoretical frameworks have put forth alternative explanations of infant vocal development. Grounded in dynamical systems theory, Articulatory Phonology<sup>151</sup> holds that speech is associated with gestural goals (or gestures) as units of both motor and linguistic organization<sup>152</sup>. Those are organized within a dynamic coupled-oscillator model (the Task Dynamics model) which specifies timing relations among gestures.<sup>153,154</sup> While Articulatory Phonology provides an organizational scheme for describing speech gestures without relying on traditional phonemes, it currently does not incorporate an independent role for the jaw in articulation, though studies have documented segment-specific mandibular contributions to consonant constrictions.<sup>155</sup> Further, of particular importance in the current context, it does not fully account for the contributions of respiratory activity to the broader coordination of speech actions. However, some recent attempts have been made to integrate respiration into movements of speech articulation of the whole vocal tract using real-time MRI<sup>156</sup>.

Importantly, explorations of coordination of rhythmic body motions, early vocal rhythms, and infants' breathing behavior in early development are scarce. To our knowledge there is only one model addressing the close interactions of the three players, the gesture-speech physics model.<sup>113,112</sup> This model accounts for the physical impulses gestures have on the respiratory system and vocalization. In this sense, it is undirectional, but effects of speech on respiration and gesture may also be possible. While the gesture-speech physics model unifies the three components, it is unspecific about supralaryngeal articulation.

None of the major developmental theories of language have integrated respiration thus far. Data-driven analyses of infants' movements, vocalizations, and breathing patterns that avoid an adultcentric view and instead accept that infants are "noisy" systems<sup>133</sup> may pave the way for a more holistic theory of infants' development, acknowledging the role of rhythm and multimodal aspects of communication.

## 5.2 | Data reduction and lack of specific data types

Studies on the relation between body motions and respiration in early infancy hardly exist. Many authors have used acoustic or auditory indications of breathing to define infants' utterances.<sup>22,157,66</sup> Within those 'breath groups', each voiced period produced on a single expiration is counted as one utterance.<sup>66</sup> This reduces the dataset because any vocalization on ingressive phonation is discarded. However, as discussed above, ingressive phonations occur in infancy and may provide insightful information on the development of a synergistic relation between the respiratory and phonatory systems. Our recent kinematic investigation of breathing during vocal expressions showed repetitive ingressive vocalizations in earlier productions and more frequent egressive phonation in later ones.<sup>49</sup>

Additionally, infant vocalizations are often only analyzed if they are produced in comfort and without simultaneous body movements,<sup>23,72</sup> leading to tremendous data reduction. As a case in point, Boliek and colleagues<sup>72</sup> analyzed vocalization and breathing kinematics in 40 healthy infants, and followed ten of these longitudinally from five weeks of age to 12 months. Altogether, 16,259 vocalizations were recorded of which only 11.5% were analyzed. The inclusion criteria leading to this reduction were: 1) vocalization being produced without additional movements and 2) two additional tidal breath cycles around the vocalization event. It is unclear how much the first criterion contributed to the data exclusion. These strict criteria are
certainly understandable in light of the fact that limb movement may be registered in respiratory movement signals. However, infants often move and vocalize at the same time (e.g., so-called vocal-motor babbling). Fuchs and colleagues,⁴⁹ for example, observed that 43.8% of 672 vocalizations were accompanied by motions. This form of data reduction may, therefore, limit the ecological validity of studies on respiratory behavior in early language acquisition not only resulting in substantial data loss, but also risking overlooking an entire class of typical infant vocalizations necessary to better understand how motor and vocal development are related.

Averaging across individuals is, of course, a common form of data reduction. Yet the high inter- as well as intra-individual variability characteristic of infant behavior oftentimes precludes straightforward general conclusions based on group data. There is growing evidence that speech motor control¹⁵⁸ and breathing¹⁵⁹ do not develop uniformly across children. The four infants investigated from 8 to 48 months of age in Connaghan's study,¹⁵⁹ for example, differed in their developmental trajectories. At the same time, intra-individual variability could reflect transitional phases instead of being mere noise in the data. Dissecting individual differences and following possible developmental trajectories longitudinally are both necessary to differentiate typical vs. atypical development.

## 5.3 | Methodological challenges in studying infant vocal and motor development

It must be acknowledged that studying language development in infants, particularly in a multimodal context, presents many challenges. Auditory evaluation of young infant vocalizations is complicated by the fact that listeners may impose their own linguistic categories on the acoustic output of a vocal tract that differs not only in size but in overall configuration from that of older children and adults.¹⁶⁰,¹⁶¹,⁶⁷ The high fundamental frequencies generated by the infant larynx, and the consequent wide harmonic spacing, introduces large error into the accuracy of estimating the vocal tract formant values that can provide insight into articulation.¹⁶² Further, it has long been established that acoustic patterns do not uniquely specify articulations,¹⁶³,¹⁶⁴ that is, a clear understanding of articulation requires physiological measurements. While articulatory recordings in young children are challenging, in recent years ultrasound has emerged as a method of assessing tongue configurations even in children as young as three years.¹⁶⁵,⁵⁹,¹⁶⁶,⁵⁷

Although phonation and glottal aperture can be studied using minimally invasive methods such as electroglottography¹⁶⁷ or photoglottography,¹⁶⁸,¹⁶⁹ they still require strapping electrodes to the neck or pressing a light source against the neck, procedures that may not be well-tolerated for more than brief periods of time in the population of interest. An understanding of infant vocalizations, therefore, probably has to be based on the acoustic signal.

Many investigators have derived the respiratory behavior of infants from auditory and acoustic parameters to allow a naturalistic environment and avoid invasive or confining procedures. Although acoustically derived measures of speech-related breathing provide preliminary insights into the developing relationship between breathing and vocalization, those remain indirect measures, yielding an incomplete, coarse-grained assessment. Some studies have relied on listener judgments of ingressive or egressive phonation⁹⁴ but the extent to which listeners can reliably assess the direction of airflow is not known. There are some acoustic differences between ingressive and egressive phonation⁹¹ and airflow in voiceless sounds,¹⁷⁰ and machine-learning methods can differentiate inhalation and exhalation¹⁷¹ but have not, to date, considered ingressive and egressive phonation per se. Only after having collected enough empirical evidence about the relation between acoustic output and respiration can we confidently rely on auditory or acoustic analyses of infant breathing. The most commonly-used direct method of studying respiratory movement is inductance plethysmography, which involves placing elastic bands on the thorax and abdomen. It can be used with very young infants, but body movements introduce perturbations into the signal, which makes it difficult to disentangle respiratory and bodily movements at our current level of knowledge. Overcoming these challenges with methodological innovation, tool advancement and empirical data is crucial for theory building in the field of language acquisition.¹⁷²

# 6 | DIRECTIONS FOR FUTURE RESEARCH

Given the challenges and limitations discussed above, how should research on language development proceed? As Fogel and Thelen¹⁷³ note: "development can be envisioned as a heterochronic series of different developmental processes, each with separate but interacting timetables [...], however, the behavior of the organism as a whole cannot be predicted by the developmental status of only one component, because it is the systems interactions among the components that is crucial" (p.750). Along with this
view, we believe that future work needs to adopt a more inclusive view of the many components that contribute to different stages of motoric and linguistic development, regardless of theoretical perspectives. What does this approach entail concretely?

First, we propose that research on language development should integrate more fully the respiratory rhythms and its interactions into empirical analyses, theoretical approaches, and modeling. This would enhance our understanding of the evolution of coordinative actions among different subsystems of the motor and vocal systems, in particular those with the supralaryngeal articulators for which empirical investigations are scarce. Relatedly, thorough direct comparisons of respiratory and acoustic data would help to determine where the acoustic signal carries information about respiration and where additional direct respiratory data are needed, because the acoustic signal is incomplete. This may be particularly important for defining phonation in inhalation versus exhalation periods in early infancy.

Similarly, inclusion of body motion within theories of speech development is crucial. Although calls for integrating vocal production with whole-body movement date back to at least the 1970s,¹⁰ empirical work on the first year of life has lagged behind. Early rhythmic arm and leg movements, often coupled with vocalizations, may be a precursor to multimodal language. Over time, such couplings can weaken or even decouple (see above discussion on motor overflow), suggesting the need for a broad view of bodily precursors to language. Restricting attention to a narrow set of movements and ignoring potential interactions between modalities, risks missing important developmental dynamics.

Physical growth adds another layer of complexity that has rarely been considered with respect to coordination among different systems. Different body parts do not mature at the same rate, e.g., changes in ribcage compliance or shifts in the position of the larynx–hyoid complex may affect coordination. Likewise, pharyngeal and oral structures do not grow linearly over time nor uniformly across infants.⁸⁸ Future research should disentangle how asymmetrical growth trajectories affect coordination among subsystems and to what extent coordination emerges as a self-organizing system. Physical modeling could play an important role here by systematically varying the size of the vocal apparatus or bodily system and evaluating the consequences for coordination and language output. Several studies provide examples of modeling the vocal system in human adults and nonhuman infants.¹³³,¹⁷⁴,¹²³,¹⁷⁵

Finally, methodological progress is essential. Much prior work on respiration in the context of vocalizations has relied on acoustic data which provides insufficient information on the underlying articulatory controls associated with breathing while vocalizing. The development of methods such as inductance plethysmography or thermal cameras now allows for direct measurements of respiration in infants. Future research can benefit from these methodological advances and ultimately provide finer-grained measurements of breathing development.

Most prior work on respiration has also relied on substantially reduced data sets, potentially obscuring our understanding of language development. Future work should consider critically whether the extensive data reduction that has been carried out previously is truly necessary and which types of data have been excluded. For example, a better understanding of the impact of upper body movements on respiratory measurements would be a step forward in this direction. If we consider bodily motions within language, the co-occurrence of vocalization with motions may tell us something about development. Time series analyses of motion and respiratory data could help us to identify the motor perturbations that are detectable on the respiratory signals.

Altogether, future research programs that integrate physical growth, multimodality, and respiration will provide a more comprehensive account of how language emerges from the coordination of developing bodily systems. The three components of bodily motion, vocalization, and respiration may serve as a foundation for a more integrative framework of infant development.⁵⁶ We have begun recording infants’ vocalizations and bodily behavior alongside their respiratory rhythms. Our early findings together with the literature suggest that around the period of babbling lies an important transition for the developing coordination between breathing, laryngeal and supra-laryngeal systems.⁴⁹ In the next step, we will further illuminate to what extent respiratory control develops together with speech motor coordination and body motion during the first two years of life. Ultimately, this research should provide evidence-based knowledge to advance models of communicative development.

## AUTHOR CONTRIBUTIONS

The authors have equally contributed to this review.

## ACKNOWLEDGMENTS

We thank Šarka Kadavá and Wim Pouw for input on an earlier version of the paper and Rasmus Puggaard-Rode for adding a new feature in an updated version of praatpicture.

## CONFLICT OF INTEREST

The authors declare no potential conflict of interests.
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