Exploring infant hip position and muscle activity in common baby gear and orthopedic devices

Abstract

Infant positioning in daily life may affect hip development. While neonatal animal studies indicate detrimental relationships between inactive lower extremities and hip development and dysplasia, no research has explored infant hip biomechanics experimentally. This study evaluated hip joint position and lower-extremity muscle activity of healthy infants in common body positions, baby gear, and orthopedic devices used to treat hip dysplasia (the Pavlik harness and the Rhino cruiser abduction brace). Surface electromyography (EMG) and marker-based motion capture recorded lower-extremity muscle activity and kinematics of 22 healthy full-term infants (4.2 ± 1.6 months, 13 M/9 F) during five conditions: Pavlik harness, Rhino brace, inward-facing soft-structured baby carrier, held in arms facing inwards, and a standard car seat. Mean filtered EMG signal, time when muscles were active, and hip position (angles) were calculated. Compared to the Pavlik harness, infants exhibited similar adductor activity (but lower hamstring and gluteus maximus activity) in the Rhino abduction brace, similar adductor and gluteus maximus activity(but lower quadriceps and hamstring activity) in the baby carrier, similar but highly variable muscle activity in-arms, and significantly lower muscle activity in the car seat. Hip position was similar between the baby carrier and the Pavlik harness. This novel infant biomechanics study illustrates the potential benefits of using inwardfacing soft-structured baby carriers for healthy hip development and highlights the potential negative impact of using supine-lying container-type devices such as car seats for prolonged periods of time. Further study is needed to understand the full picture of how body position impacts infant musculoskeletal development.

Ultrasonographic evaluation of infant hips in the Pavlik harness compared to body‐worn commercial baby carriers

“…The lower Graf's alpha angle and femoral coverage in the inward facing narrow base baby carrier, compared to the Pavlik harness, agree with recommendations set forth by the International Hip Dysplasia Institute to limit prolonged use of narrow-base baby carriers. 17 The similar Graf's alpha angles and femoral coverage in the inward facing wide-base baby carrier compared to the Pavlik harness, combined with the similar hip position and lower-extremity muscle activity recorded in our previous experimental study 15 using the same carrier, indicate possible roles that similar wide-base baby carriers may play in promoting healthy hip development in all infants, and with further research, could eventually function as a supplemental hip-healthy positioning mechanism for DDH populations before and after Pavlik harness treatment. The mean Graf's alpha angles and femoral coverage values for the three infants with mild DDH were in the range of the healthy infants for all measurements except the 49.0°average Graf's alpha angle in the narrow-base carrier which falls below the range of healthy hips (Figure 3).…”

“…Leg support varies from a narrow‐base of support extending only over the pubic area to a wide‐base of support featuring knee‐to‐knee coverage; thus, hip position is also likely variable based on the carrier design. We previously utilized motion capture technology and electromyography (EMG) to demonstrate that infants’ hip position and lower extremity muscle activity were similar in baby carriers featuring a wide‐base or “M‐position” design compared to the hip‐healthy Pavlik harness 15 . However, the use of ultrasound is a more accurate technique to quantify hip position compared to marker‐based motion capture, especially in infants.…”

“…We previously utilized motion capture technology and electromyography (EMG) to demonstrate that infants' hip position and lower extremity muscle activity were similar in baby carriers featuring a wide-base or "M-position" design compared to the hip-healthy Pavlik harness. 15 However, the use of ultrasound is a more accurate technique to quantify hip position compared to marker-based motion capture, especially in infants. Given that positional factors have been shown to influence hip development, it is important to understand how various designs of baby carriers may influence hip position and thus proper hip development.…”

Challenges in Kinetic-Kinematic Driven Musculoskeletal Subject-Specific Infant Modeling

“…De-identified experimental data for one healthy, full-term male infant (2.4 months) was obtained from a study approved by the Institutional Review Board of the University of Arkansas for Medical Sciences [28]. The infant was weighed on an infant scale at 5.35 kg and was measured head to heel (lying supine) at 56 cm.…”

“…Previous studies have attempted to quantify an infant's joint kinematics and kinetics during spontaneous movement, but an infant MCM has yet to be created. The purpose of this work was to develop a novel lower extremity computational musculoskeletal model representative of an infant, using body segment anthropometric measurements and experimental MOCAP data of a single infant taken from previously collected infant biomechanics data [27,28]. This study will provide more insight into biomechanical loadings at the hip joint during a spontaneous kick and will provide a noninvasive technique for evaluating the mechanisms contributing to infant hip development.…”