Differences in Lower Limb Co-Contraction Calculations Vary Clinical Interpretation of Aquatic Treadmill Walking in Typically Developing and Children with Cerebral Palsy

Joseph W. Harrington ¹ Brian A. Knarr ¹ Vivek Dutt ² David C. Kingston ^1*

• ¹ Department of Biomechanics, College of Education, Health and Human Sciences, University of Nebraska Omaha, Omaha, Nebraska, United States
• ² Department of Orthopaedic Surgery and Rehabilitation, College of Medicine, University of Nebraska Medical Center, Omaha, Nebraska, United States

The purposes of this study were to 1) investigate muscle co-contraction during aquatic (Wet) and conventional (Dry) treadmill walking at various speeds in typically developing (TD) and children with cerebral palsy (CP) and 2) explore how the clinical interpretation of co-contraction, using co-contraction indices (CCI), may vary depending on the method employed.Methods: Fifteen TD children (30 limbs, 7M | 8F, 11.3 ± 4.1 yrs., 1.46 ± 0.18 m, 44.2 ± 16.8 kg) and ten children with CP (20 limbs, 6M | 4F, 13.1 ± 3.5 yrs., 1.54 ± 0.18 m, 53.2 ± 26.2 kg, 7 GMFCS I and 3 II) participated in this study. Muscle activity of the tibialis anterior (TA), rectus femoris (RF), medial gastrocnemius (MG), and semitendinosus (ST) was recorded during three 3-minute walking trials on a Dry treadmill followed by a Wet treadmill. Muscle co-contraction was calculated using three common CCI calculation methods for the RF/ST and TA/MG muscle pairings. Separate linear mixed-effects models examined the influence of population (TD vs. CP), walking speed (Slow, Normal, Fast), and treadmill environment (Dry vs. Wet) on CCI for each equation and muscle pairing.Results: CCIUnnithan and CCIRudolph demonstrated that aquatic treadmill walking reduced muscle co-contraction in TD (p < 0.001) and CP (p < 0.012) populations for the RF/ST muscle pairing. Additionally, CCIUnnithan and CCIRudolph showed significant differences between speeds in both environments (p < 0.001) except for the Slow-Normal comparison in the aquatic treadmill (p > 0.423). All methods had a significant CCI reduction in the TA/MG muscle pairing for both populations. For the RF/ST muscle pairing, CCIF&W showed that only TD children had lower muscle co-contraction in the aquatic treadmill (p = 0.023). CCIF&W also showed no speed effect for the muscle pairings.This study shows the potential of aquatic treadmill walking to reduce muscle cocontraction; however, caution is recommended as clinical implications can vary due to the computation method. Future studies should aim to report values from multiple methods to account for the variability within methods and validation of results.