Effects of pregnancy and lactation on muscle‐tendon morphology

Abstract

Pregnancy and lactation hormones have been shown to mediate anatomical changes to the musculoskeletal system that generates animal movement. In this study, we characterize changes in the medial gastrocnemius muscle, its tendon and aponeuroses that are likely to have an effect on whole animal movement and energy expenditure, using the rat model system, Rattus norvegicus. We quantified muscle architecture (mass, cross‐sectional area, and pennation angle), muscle fiber type and diameter, and Young's modulus of stiffness for the medial gastrocnemius aponeuroses as well as its contribution to Achilles tendon in three groups of three‐month‐old female rats: virgin, primiparous pregnant, and primiparous lactating animals. We found that muscle mass drops by 23% during lactation but does not change during pregnancy. We also found that during pregnancy muscle fibers switch from Type I to IIa and during lactation from Type IIb to Type I. The stiffness of connective tissues that has a demonstrated role in locomotion, the aponeurosis and tendon, also changed. Pregnant animals had a significantly less stiff aponeurosis. However, tendon stiffness was most affected during lactation, with a significant drop in stiffness and interindividual variation. We propose that the energetic demands of locomotion may have driven the evolution of these anatomical changes in muscle‐tendon units during pregnancy and lactation to ensure more energy can be allocated to fetal development and lactation.

Muscle strength during pregnancy and postpartum in adolescents and adults

“…As pregnancy progresses, body weight increases, the uterus expands, and the abdomen dilates, bringing the gravity center forward and increasing pressure on the pelvic floor [ 6 ]. Evidence from animal models suggest that there are adaptations in muscle-tendon morphology in response to the hormonal and mechanical changes from pregnancy [ 7 , 8 ]. Pelvic floor muscles adapt increasing sarcomere length and with sarcomerogenesis [ 7 ], type I fibers of lower limb muscles become smaller in diameter and quantity during pregnancy and postpartum, with a reduction in tendon stiffness during pregnancy that may persist through lactation [ 8 ].…”

“…Evidence from animal models suggest that there are adaptations in muscle-tendon morphology in response to the hormonal and mechanical changes from pregnancy [ 7 , 8 ]. Pelvic floor muscles adapt increasing sarcomere length and with sarcomerogenesis [ 7 ], type I fibers of lower limb muscles become smaller in diameter and quantity during pregnancy and postpartum, with a reduction in tendon stiffness during pregnancy that may persist through lactation [ 8 ]. These are likely to affect muscle performance and impair mobility.…”