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Invited eCommentaries |
JC Heathcock, PT, PhD, is Assistant Professor, Division of Physical Therapy, School of Allied Medical Professions, Ohio State University, Atwell Hall, 453 W 10th Ave, Columbus, OH 43210 (USA).
Address all correspondence to Dr Heathcock at: jill.heathcock{at}osumc.edu
Grant-Beuttler et al1 present an interesting report describing the measurement of gastrocnemius-soleus muscle tendon unit (MTU) lengths from term to 12 weeks of age in infants born preterm and infants born full term. This work joins that of other authors2 in suggesting that infants born preterm have a different developmental trajectory than infants born full term and they have identifiable and measurable impairments during early infancy.
This study examined the differences in muscle-tendon tautness, length, and stretch over time. The authors note that a limitation of their study was the lack of measurements of knee flexion angle of newborn infants, both full term and preterm, to demonstrate extension limitations of the knee. Because the gastrocnemius is a 2-joint muscle, any degree of knee flexion may be important. It is very difficult to stretch the gastrocnemius muscle at the ankle with any degree of knee flexion. In this population, it is unlikely that any measurements were taken with the knee in full extension. As such, the measurements taken in this study were largely of the soleus muscle.
It is intriguing that infants born preterm show biases toward plantar flexion during passive range of motion, within kicking patterns, and during early gait. The direction of abnormal movement patterns at the ankle during kicking and stepping is constant with the direction of musculoskeletal abnormalities at the ankle in this article. However, the gastrocnemius muscle is a large contributor to plantar flexion. Toe walking may be habitual, neurological, or orthopedic, as the authors suggest, but it seems unlikely that the soleus muscle is a primary contributor.
Another interesting question is how to design the most effective intervention given the MTU differences at the ankle of preterm infants. The authors suggest splinting the infant's ankle as early as possible and provide 2 reasons that splinting may be effective in re-creating the MTU length in full-term infants: (1) studies in animal models have demonstrated a change in the number of sarcomeres with immobilization, and (2) infants born preterm are missing the constrained uterine space during a full-term pregnancy that promotes prolonged dorsiflexion.
First, splinting and positioning of the ankle are primarily passive interventions. As with any kind of passive immobilization, the result will likely be a weaker muscle with a possible change in length of the MTU that may (or may not) result in improvements in functional skills. In addition, splinting and positioning of the ankle may affect the soleus muscle, but infants—especially preterm infants—kick a lot. As a knee flexor, the gastrocnemius muscle will likely play a large role in leg movements. Therefore, it is unlikely that there will be any morphological changes of the gastrocnemius muscle during immobilization of the ankle.
Second, in the last 4 weeks of gestation, full-term infants may be confined, but their ankles are in no way immobilized. In addition to a prolonged stretch, the uterine environment provides a wealth of sensory information to the infant and to the legs. The experience these infants born preterm are "missing" is more active and reciprocal than just confinement. The infants’ own body size changes rapidly (also causing a stretch to musculature), and all movements (big or not) result in a variety of sensory stimulation from the uterus that is dynamic and adaptive. What's more, half of the infants born preterm who participated in this study were from multiple births. Twin and triplet pregnancies result in confined quarters earlier in gestation. It is not clear whether these infants from multiple births received more dorsiflexion stretch earlier in gestation and how that might affect their MTU length from birth to 12 weeks of age. Confinement and prolonged stretch are likely not the end of the story.
Because the developmental trajectory of infants born preterm is likely different from that of full-term infants, re-creating the MTU length in full-term infants may not be the right goal. Our challenge may be to find ways to constrain the movement without taking away the ability to learn how to move.
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  M. Grant-Beuttler, R. J Palisano, D. P Miller, B. Reddien Wagner, C. B Heriza, and P. A Shewokis Author Response to Invited Commentary by Heathcock Physical Therapy, February 1, 2009; 89(2): e2 - e4. [Full Text] [PDF]
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