Objective To evaluate growth of the triceps surae muscle over the first 12 months of life in neonatal intensive care unit (NICU) graduates compared with typically developing (TD) infants.

Design Longitudinal, prospective study.

Participants Sixty-one infants, n=24 TD (15 male) and n=37 NICU graduates designated as intermediate-risk (NICU-IR; n=14, 9 male) or higher-risk (NICU-HR; n=23, 11 male) based on additional risk factors (including <28 weeks gestational age; <1000 g; neonatal encephalopathy; abnormal neuroimaging; small for gestational age).

Outcome measures Sequential assessments at corrected ages 3, 6 and 12 months of muscle volume (freehand three-dimensional ultrasound) and gross motor development (Peabody Developmental Motor Scale-2, Gross Motor Quotient (GMQ)). Linear mixed models analysed muscle volume trajectories.

Results Triceps surae growth trajectories differed significantly by group (p<0.001). Between 3 and 12 months, triceps surae increased on average by 18.1 cm³ (95% CI 16.1 to 20.2 cm³), 13.3 cm³ (10.6 to 16.0 cm³) and 12.5 cm³ (10.5 to 14.6 cm³) in TD, NICU-IR and NICU-HR infants, respectively. Soleus was significantly smaller at 6 and 12 months for both NICU groups, and lateral gastrocnemius was smaller at 12 months for NICU-HR (p<0.001). At 12 months of age, 8% of NICU infants and 30% of the TD infants were walking, the GMQ was >90 in all TD infants, and all but 5 (14%) NICU infants. Muscle volumes at 12 months were positively associated with both gestational age and birth weight.

Conclusion Reduced soleus growth from 3 to 12 months led to a 25% smaller triceps surae muscle at 12 months in NICU graduates.