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Babies are born with muscles, but the extent of their muscularity depends on several factors. For instance, babies born with congenital muscular dystrophy (CMD) often experience muscle weakness and poor muscle tone, which can cause delays in reaching motor milestones. On the other hand, some babies are born with an unusually high amount of muscle mass due to genetic mutations. The growth of skeletal muscle in newborns is influenced by factors such as birth weight, nutrition, and overall development. Research suggests that larger size and muscle mass at birth are associated with greater muscle strength in childhood.
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What You'll Learn
Muscle growth in neonates
The growth of skeletal muscle in neonates is dependent on the stage of muscle development and birth. The neonatal period covers a wide range of developmental stages, especially in humans due to the advances in the clinical management of premature babies. The close of the neonatal period is defined as the stage when an organism no longer depends on its caretaker for nutrition, which for most species is when they are weaned.
The growth of skeletal muscle in neonates is highly sensitive to variations in overall nutrient intake. Feeding stimulates muscle protein synthesis in neonates, and this response decreases with age. The feeding-induced stimulation of muscle protein synthesis is influenced by the body's sensitivity to the postprandial rise in insulin and amino acids. Insulin and amino acid signaling components have been identified as contributors to the stimulation of protein synthesis in neonatal muscle.
The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis and the rapid accumulation of muscle nuclei. The growth rate of neonatal muscle is also attributable to the high rate of protein synthesis, which is dictated by the abundance of ribosomes in a tissue and the efficiency with which they translate mRNA into protein. The rate of growth during the neonatal period is higher than at any other stage of postnatal life, and the majority of the mass increase is comprised of skeletal muscle.
The extent to which factors that influence fiber hypertrophy also affect maturation depends on an interaction between the muscles' stage of development and the underlying cause and severity of the growth deviation. Research has shown that larger overall size and muscle mass at birth are associated with greater muscle strength in childhood. For example, a study of 9-year-old Indian children found a positive association between birth weight and grip strength.
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Birth defects of the muscles
Muscle defects at birth can vary in severity and may occur in isolation or as part of a broader genetic syndrome. For instance, a baby may be born missing individual muscles or groups of muscles, or they may exhibit incomplete muscle development. These defects can be present anywhere in the body, including the neck and back, and can be caused by injuries to soft tissues or bones. One example of a spinal defect is scoliosis, which is rarely apparent at birth but can progress rapidly as a child grows.
Prune-belly syndrome is another example of a muscle defect characterised by missing layers of abdominal muscles, resulting in a wrinkled appearance of the abdominal wall. This condition is also associated with cryptorchidism, where the testes do not descend into the scrotum, and urinary tract defects. The exact cause of prune-belly syndrome remains unclear, but it can be diagnosed through ultrasound before birth or physical examination after birth.
The development of skeletal muscle is closely linked to muscle connective tissue (MCT), and defects in this interaction can lead to congenital birth defects in head and limb muscles and the diaphragm. Recent studies have identified MCT fibroblasts as crucial regulators of muscle development, influencing myogenesis and muscle morphogenesis. The diaphragm, unique to mammals, is essential for respiratory capacity, and defects in its development can have significant implications for respiratory function.
The growth of neonatal muscle is a dynamic process influenced by various factors. It is primarily driven by the high rate of protein synthesis, which exceeds the rate of protein degradation. Feeding plays a significant role in stimulating muscle protein synthesis in neonates, and this response decreases with age. Additionally, the neonatal period can encompass a wide range of developmental stages due to advancements in the clinical management of premature babies.
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Muscle strength in childhood
Muscle strength in children is a topic that has been widely studied, with a variety of factors influencing muscular performance in childhood. Firstly, it is important to understand the role of genetics and prenatal development in muscle strength. For instance, a baby born with a double dose of a specific genetic mutation exhibited unusually well-defined and bulging muscles, a condition that has been observed to cause immense strength in mice and cattle. Fetal development and pre-natal nutrition have also been linked to muscle function in later life, with larger overall size and muscle mass at birth associated with greater muscle strength in childhood.
During the neonatal period, skeletal muscle growth is highly dependent on the stage of muscle development and birth, which can vary significantly among different species. In humans, advances in the clinical management of premature babies have resulted in a wider range of developmental stages during this period. Skeletal muscle growth in neonates is influenced by protein synthesis, muscle nuclei accumulation, and feeding, which stimulates muscle protein synthesis. As children grow, their muscle strength and endurance can be developed through various activities such as swimming, animal walks, and throwing bean bags. These activities help build strength and endurance by working against resistance.
Studies have shown that muscle strength in children is influenced by factors such as muscle size, birth weight, and postnatal growth. For example, a study of 9-year-old Indian children found a positive association between birth weight and grip strength, suggesting a link to prenatal nutrition and growth. Additionally, muscle strength has been linked to height, body mass index, and arm muscle area, with larger muscle mass resulting in greater strength. However, it is important to note that children's muscular performance differs from that of adults in several attributes, including size-normalized strength, endurance, and fatigability.
Furthermore, children's maximal volitional muscular force, contractile velocity, and muscular power are typically lower than those of adults, especially in males. This difference is not solely due to body and muscle size but also to other factors such as agonist-antagonist muscle cocontraction, which can detract from the measured force and power output. Children's type-II muscle fibers are generally similar or smaller in diameter than type-I fibers, indicating under-use during prepubescence. These differences in muscle composition and motor-unit activation between children and adults contribute to the variation in muscular performance.
Overall, muscle strength in childhood is influenced by a combination of genetic, prenatal, and postnatal factors, and it continues to develop through various activities and exercises. Understanding these factors can help promote healthy muscular development and address any potential issues early on.
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Genetic mutations and muscularity
Babies are born with muscle, but their muscles are typically soft and underdeveloped. However, in rare cases, babies have been born with exceptionally well-defined and bulging muscles due to genetic mutations.
The extent of an individual's muscularity is influenced by their genetic profile, particularly genes related to the myostatin protein, also known as the MSTN gene. Myostatin is a negative regulator of muscle mass, meaning it inhibits muscle growth. Mutations in the MSTN gene can cause a reduction or complete loss of myostatin function, leading to muscles growing much larger than normal. This phenomenon has been observed in various species, including mice, cattle, dogs, and even fish.
In humans, there have been rare cases of children born with genetic mutations resulting in increased muscularity. One notable case is that of a German boy, first reported by pediatric neurologist Dr. Markus Schuelke, who was born with protruding muscles in his thighs and upper arms. Before the age of five, he displayed extraordinary strength, capable of holding 7-pound weights with arms extended, a feat that many adults cannot match. Another boy, Liam Hoekstra, was diagnosed with a clinically similar condition.
The genetic mutation associated with these cases is a double dose of a mutation in the MSTN gene, specifically a two-base-pair deletion in the third exon. This mutation leads to a premature stop codon at amino acid 313, resulting in the myostatin protein being non-functional or present in very low levels. The absence or reduction of myostatin's negative control on muscle growth allows muscles to grow beyond what is typical, resulting in increased muscularity and strength.
The discovery of these mutations has significant implications for both medicine and sports. In medicine, there is hope that understanding this genetic mutation can lead to the development of drugs to help people with muscle-wasting diseases such as muscular dystrophy. In sports, the potential for performance enhancement through genetic manipulation has raised ethical considerations.
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Muscle development and birth weight
Muscle development in the womb and a baby's weight at birth are both factors that can influence a child's muscle strength later in life.
Primary muscle development begins in the human fetus at 8-10 weeks of gestation. The regulation of skeletal muscle growth is dependent on the stage of muscle development and birth, which defines the start of the neonatal period and occurs at different stages of development among species. The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis accompanied by the rapid accumulation of muscle nuclei. Feeding stimulates muscle protein synthesis in neonates, and the response decreases with age.
A baby's weight at birth can be an indicator of their muscle strength in later life. Several studies have found a positive association between birth weight and muscle strength in men, women, and children across the lifecourse. For example, a study of 574 Indian children found that weight, length, and arm muscle area (AMA) at birth were positively related to 9-year grip strength. The children with a larger overall size and muscle mass at birth had greater muscle strength in childhood. Similarly, a study of young adult women found that grip strength increased by 1.10kg per kg of birth weight.
However, it is important to note that birth weight is a poor summary measure of fetal growth and development, and other factors such as body size, physical activity, and age also influence muscle strength. For example, a study of 9-year-old Indian children found that while birth weight and AMA were positively related to grip strength, the associations between birth size and grip strength were attenuated after adjusting for 9-year size.
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Frequently asked questions
Yes, babies are born with muscle, but they are usually soft and underdeveloped.
Congenital muscular dystrophy (CMD) is a group of muscular dystrophies that become apparent at or near birth.
Poor muscle tone, or hypotonia, is the most common symptom of CMD. Babies with CMD often feel "floppy" when held or picked up and may be delayed in reaching motor milestones such as sitting and walking.
CMD is typically diagnosed through a series of physical, neurological, and muscle exams, as well as genetic testing and muscle biopsies.
While there is currently no cure for CMD, treatment focuses on the patient's function and wellbeing, as well as that of their family or caregivers. Physiotherapy and orthotics services can help improve joint movement and prevent deformities.
