3241 Using infant exertion to tailor treadmill intervention

OBJECTIVES/SPECIFIC AIMS: This research examined 3 aims to address the need to understand and quantify exertion in infants. Aim 1: Develop a schema to identify and code exertional behaviors in infants during treadmill stepping. Aim 2: Establish feasibility for the schema’s use with clinical populations. Aim 3: Pilot the schema in a study designed to induce infant exertion. METHODS/STUDY POPULATION: Aims 1 and 2 were achieved using existing treadmill stepping data. The data used in Aim 1 included eight typically-developing infants (age 7-10 months) who were able to sit independently, but not walk. The data used in Aim 2 came from two separate data sets from infants who took more than 10 steps in a 30-second trial: Data set A included six typically-developing infants (age 2-5 months) who were unable to sit independently (developmentally comparable to atypical populations who might receive treadmill interventions). Data set B included six infants with Spina Bifida (age 3-10 months). Aim 3 was addressed with a prospective study using an exertion model. Pre-walking, typically developing infants (age 8-10 months) underwent five total stepping trials. Trial 1 determined the infant’s individualized maximum stepping speed; trials 2-5 were each 60 seconds and alternated between a baseline stepping speed of.20 m/s and the infant’s maximum stepping speed determined in trial 1. All video data were coded for step type, step frequency, and exertional behavior. RESULTS/ANTICIPATED RESULTS: Aim 1: Two behaviors were identified and determined to capture infant exertion: foot dragging and leg crossing. Aim 2: The feasibility of capturing exertion with these two behaviors was established for young infants and infants with neuromotor delays, with exertional behaviors increasing with stepping exposure (p< 0.05). Aim 3: Total exertion (foot dragging + leg crossing) was higher in the maximum speed trials compared to baseline trials (p = 0.005). DISCUSSION/SIGNIFICANCE OF IMPACT: Exertion in infants can be quantified. The exertion schema developed with this study will support the development of dosing guidelines for infant treadmill intervention. The next step in this line of research is to examine the correlation between infant exertion and heart rate, in effort to move from behaviorally-informed protocols to more precise, individualized protocols based on the physiological response of the infant.

Longitudinal changes in muscle activity during infants' treadmill stepping

Previous research has described kinetic characteristics of treadmill steps in very stable steppers, in cross-sectional designs. In this study we examined, longitudinally, muscle activation patterns during treadmill stepping, without practice, in 12 healthy infants at 1, 6, and 12 mo of age. We assessed lateral gastrocnemius, tibialis anterior, rectus femoris, and biceps femoris as infants stepped on a treadmill during twelve 20-s trials. Infants showed clear changes in kinematics, such as increased step frequency, increased heel contact at touch down, and more flat-footed contact at midstance. Electromyographic data showed high variability in muscle states (combinations), with high prevalence of all muscles active initially, reducing with age. Agonist-antagonist muscle coactivation also decreased as age increased. Probability analyses showed that across step cycles, the likelihood a muscle was on at any point tended to be <50%; lateral gastrocnemius was the exception, showing an adultlike pattern of probability across ages. In summary, over time, healthy infants produce a wide variety of muscle activation combinations and timings when generating stepping patterns on a treadmill, even if some levels of muscle control arose with time. However, the kinematic stability improved much more clearly than the underlying kinetic strategies. We conclude that although innate control of limb movement improves as infants grow, explore, and acquire functional movement, stepping on a treadmill is a novel and unpracticed one. Hence, developing stable underlying neural activations will only arise as functional practice ensues, similarly to that observed for other functional movements in infancy.

Heterogenic Feedback Between Hindlimb Extensors in the Spontaneously Locomoting Premammillary Cat

Electrophysiological studies in anesthetized animals have revealed that pathways carrying force information from Golgi tendon organs in antigravity muscles mediate widespread inhibition among other antigravity muscles in the feline hindlimb. More recent evidence in paralyzed or nonparalyzed decerebrate cats has shown that some inhibitory pathways are suppressed and separate excitatory pathways from Golgi tendon organ afferents are opened on the transition from steady force production to locomotor activity. To obtain additional insight into the functions of these pathways during locomotion, we investigated the distribution of force-dependent inhibition and excitation during spontaneous locomotion and during constant force exertion in the premammillary decerebrate cat. We used four servo-controlled stretching devices to apply controlled stretches in various combinations to the gastrocnemius muscles (G), plantaris muscle (PLAN), flexor hallucis longus muscle (FHL), and quadriceps muscles (QUADS) during treadmill stepping and the crossed-extension reflex (XER). We recorded the force responses from the same muscles and were therefore able to evaluate autogenic (intramuscular) and heterogenic (intermuscular) reflexes among this set of muscles. In previous studies using the intercollicular decerebrate cat, heterogenic inhibition among QUADS, G, FHL, and PLAN was bidirectional. During treadmill stepping, heterogenic feedback from QUADS onto G and G onto PLAN and FHL remained inhibitory and was force-dependent. However, heterogenic inhibition from PLAN and FHL onto G, and from G onto QUADS, was weaker than during the XER. We propose that pathways mediating heterogenic inhibition may remain inhibitory under some forms of locomotion on a level surface but that the strengths of these pathways change to result in a proximal to distal gradient of inhibition. The potential contributions of heterogenic inhibition to interjoint coordination and limb stability are discussed.