The risk of injury may be related to an ability many women consider beneficial: increased flexibility. Studies have shown that women in general are significantly more flexible and show greater joint laxity (lack of stability of joints) than men . However, joint laxity has been linked to increased incidence of injury. Lax joints are prone to excessive motion and strain, and may require increased muscle activity to provide support. However, the increase in muscle activation places more strain on the surrounding ligaments. For example, muscle activity of the gastrocnemius (the largest muscle of the calf of the leg) works together with the quadriceps and hamstrings to stabilize the knee joint. Gastrocnemius activity has been shown to be higher in women than in men . Because the female knee joint tends to be more lax than the male knee joint, this additional muscle activity is necessary. However, the higher gastrocnemius activation leads to more strain on the ACL, even though it helps protect the knee .
More recent studies have investigated hormonal effects on connective tissue, especially collagen synthesis. Collagen protein is one of the major components of connective tissues like ligaments and tendons. In response to a load – such as an acute bout of exercise – the connective tissue will begin to synthesize collagen at higher rate to repair the tissue. However, estrogen has been reported to inhibit this response , and may explain why women have much lower rates of collagen synthesis at rest and after exercise than men do. The lower rate of tissue repair after a strenuous exercise may lead to decreased recovery and higher injury risk. Furthermore, tendon growth in response to exercise is much greater in men than in women . This suggests that training adaptations are different between the sexes – while men respond to long-term training by increasing tendon size, women do not have any detectable tendon size change. Because of both increased collagen synthesis and tendon growth, men have greater collagen strength than women , which may independently reduce their likelihood of injury. In studies comparing the cartilage in the knee, men had collagen of greater thickness . Therefore it may be that the sheer volume of collagen serves as an injury prevention factor, and female’s lower volume places them at higher risk.
Female athletes are also more likely to develop a stress fracture, a common sports-related overuse injury — some studies indicate that females are at 2-10 times the risk . Bone health is of particular concern for females in general, as they are at risk for developing osteoporosis, a skeletal condition characterized by low bone mass and deterioration of bone tissue leading to increased risk of fracture. Approximately 30% of postmenopausal women have osteoporosis, with projections for the next decade being closer to 50% . Most fractures occur in the hip, spine and wrist and are termed “fragility fractures” because they occur as a result of a fall from standing height or less, indicating extremely fragile bones. In contrast to the sudden, severe nature of these fragility fractures, stress fractures are a different type of injury that are characterized by tiny hairline cracks in the bone. The predominance of stress fractures in young female athletes, as well as the alarming number of fragility fractures due to weak bones in elderly women, has led to more investigation into the mechanisms of bone physiology in women, and its relation to exercise.
Each individual has a maximum bone mass that they are able to reach at skeletal maturity – or around 20 years old – referred to as peak bone mass. Bone mass is primarily determined (60-80%) by genetics . Since almost all bone mass is attained during adolescence, it is critical to maximize bone growth during this time. Thus many intervention studies have targeted teenagers . It has been suggested that the adolescent years offer a “window of opportunity” to build up bone mineral density (BMD), but environmental or lifestyle conditions can impede the process. Inadequate dietary calcium, oligomenorrhea or amenorrhea, low body weight, insufficient energy intake, and low estrogen levels have all been suggested as risk factors for low BMD and minimal bone growth. Weight-bearing exercise can enhance bone mineralization and increase bone mass in this age group, and therefore the role of physical activity in youth in ensuring bone health is crucial .
BMD is measured by a dual X-ray absorptiometry (DXA) bone density scan. A BMD lower than peak bone mass but not low enough to be classified as osteoporosis is called osteopenia. While osteoporosis and osteopenia have been typically defined based strictly on BMD, more information on the quality of bone may lead to changes how bone health is diagnosed. Bone protein content, structure, geometry, and mechanical properties may also play a role in fragility fracture risk . Exercise has two distinct effects on bone. On the one hand bone responds to exercise as a tissue, able to withstand and adapt to increasing load. Muscle contractions place a strain on the bone as it provides internal support to work against gravity. The strain induces remodeling and increased bone strength to compensate for these new loads. The remodeling of bone is caused by the activity of two types of bone cells: osteoblasts, responsible for bone formation, and osteoclasts, responsible for bone breakdown. Together they work on the mineral matrix of the bone, increasing or decreasing bone mass depending on signals sent to the bone cells on the surface. Bone activity thus acts in a negative feedback loop — when the mechanical strain of the bone reaches a certain threshold, adaptation and remodeling occurs . On the other hand, bone is also a material that can be weakened by repetitive stress and microdamage. Bone has repair mechanisms to meet these material failures, but when the bone cannot withstand the accumulation of strain, an overuse injury can occur. The magnitude of the load, how quickly it is introduced, and how often it is repeated are all factors that affect how much damage is accumulated. Without proper care, the continuation of this loading results in a progression of damage from a stress reaction to a stress fracture to a complete fracture. It is not surprising therefore that stress fractures are frequently seen in distance runners, gymnasts, or military recruits, whose bones undergo unusually extreme and repetitive loads .
Kept under a certain threshold and not in excess, exercise can be beneficial to building bone mass. The type of exercise is particularly important in regards to bone health. Studies often distinguish between “weight-bearing” and “non-weight-bearing” types of activities — weight-bearing includes any activity in which there is a load placed on the bone, such as walking or running. These activities are usually touted as being the most beneficial for building bone mass. However, weight-bearing activities can be further divided into impact and non-impact exercise. For example, jumping is both weight-bearing and high-impact, while using an elliptical machine is weight-bearing but non-impact. More recent studies have shown that high-impact exercises provide the most bone density benefit, although all types of exercise put a load on the muscle and therefore are beneficial to bone. Brief bouts of high impact loading, for example in activities like jumping or weightlifting, have been shown to build bone in childhood and adolescence . Non-weight-bearing activities like swimming are associated with lower bone density in the spine, while high impact sports like volleyball, squash, soccer, or track events like hurdling have been shown to improve bone density. Therefore the type of mechanical loading, rather than simply the fact that the activity is “weight-bearing”, is important in determining bone strength and mass.
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