Injury Risks for the Female Athlete - Part 3

Ok... last section!

Interestingly, repetitive loading sports like distance running are not correlated with higher bone density, although the high impact nature of running would suggest otherwise [11]. This may be due to other factors specific to distance runners, such as the high prevalence of disordered eating or menstrual irregularities. Amenorrhea or other menstrual irregularities are correlated with low bone density, and the risk of a stress fracture in athletes with amenorrhea is almost four times greater than athletes without [23]. It has been hypothesized that estrogen can modify the threshold for damage accumulation of the bone, offering a clue into how menstrual function is linked to bone health. Estrogen may exert its effects on the metabolically active trabecular bone, the porous type of bone found in the spine and all joints. Furthermore, studies have indicated that even without any menstrual dysfunction, energy deficits and disordered eating are related to low BMD and a higher risk of stress fracture [11]. Runners, who commonly have high training volumes and restricted eating patterns, may be at a higher risk of energy deficit than other athletes. Running is also a sport that places immense value on leanness and low body weight, which has independently been found to be a predictor of BMD. Therefore, female athletes with amenorrhea who strive to reach or maintain a low body weight through restrictive eating are at a very high risk for developing stress fractures and osteoporosis later in life. It is crucial that this population in particular be aware of the dangerous and lifelong effects of low bone density.

Although sports like running and gymnastics that emphasize leanness and very low body weight can be dangerous, the majority of Americans are on the opposite end of the weight spectrum. Overweight or obese individuals, while at risk for many other life-threatening conditions like diabetes, heart disease and cancer, have relatively high bone mineral density. Increased body weight is associated with a decreased risk of any type of fracture [22], and has a positive effect on bone turnover and bone density [16]. While not completely understood, the protective effect of higher body weight is possibly due to the increase in skeletal loading (due to more weight on the bones) as well as higher levels of certain hormones like insulin. Weight loss has been found to decrease BMD, but studies suggest that exercise incorporated into a weight loss program may help prevent this bone density decrease. Weight loss through dieting has been repeatedly shown to cause rapid bone loss, but weight loss achieved through exercise alone showed none of these harmful effects. Therefore as obesity is confronted as a nation-wide problem and more people are attempting to lose weight, it is important to consider the impact of the method of weight loss on bone health.

Several nutritional factors are critical in maintaining proper bone health. Calcium is perhaps the most well known mineral to be associated with osteoporosis, and it is true that calcium plays a large role in the disease. If not consumed in the diet, calcium will be leached from the bones, where it forms an integral part of the bone matrix. Other nutritional factors that play a role in bone health are Vitamin D, which must be present for calcium to be absorbed, Vitamin K, phosphorous, potassium, and sodium.

Bone mass can be maintained during adulthood, but there are very few treatments that can reverse bone loss. Current treatments for osteoporosis include estrogen replacement therapy or biphosphonates, which block or slow down the breakdown of bone, or agents like fluoride or parathyroid hormone, which promote the formation of bone [1]. No treatment can “cure” osteoporosis, but some can maintain a sufficient bone mass for normal everyday function and activity.

Overall, women are prone to many of the same exercise-associated injuries as men, such as patellofemoral pain syndrome, iliotibial band friction syndrome, medial tibial stress syndrome, Achilles tendonitis, ACL tears, plantar fasciitis, and lower extremity stress fractures. Both men and women can benefit from the same preventative measures like adequate stretching, appropriate warm-up and cool-down, sport-specific strengthening and conditioning exercises. Treatment options are also generally applicable to both men and women, such as relative rest, icing, anti-inflammatories, and physical therapy [2]. However, to tailor the most effective training regimen for the female athlete it is important to consider sex-specific susceptibilities to injury. By exploring the biomechanical, neuromuscular and cellular mechanisms of injury risk, it is possible to develop and implement appropriate preventative and treatment options tailored specifically to the female population.

1. Bonaiuti D, Shea B, Iovine R, Negrini S, Robinson V, Kemper HC, Wells G, Tugwell P, Cranney A. Cochrane Review on exercise for preventing and treating osteoporosis in postmenopausal women. Eura Medicophys. 2004;40(3):199-209.

2. Cosca DD, Navazio F. Common problems in endurance athletes. Am Fam Physician. 2007;76(2):237-44.

11. Mudd LM, Fornetti W, Pivarnik JM. Bone mineral density in collegiate female athletes: comparisons among sports. J Athl Train. 2007;42(3):403-8.

16. Reid IR. Relationships among body mass, its components, and bone. Bone. 2002;31(5):547-55.
 

22. Villareal DT, Fontana L, Weiss EP, Racette SB, Steger-May K, Schechtman KB, Klein S, Holloszy JO. Bone mineral density response to caloric restriction-induced weight loss or exercise-induced weight loss: a randomized controlled trial. Arch Intern Med. 2006;166(22):2502-10.
 

23. Warden SJ, Creaby MW, Bryant AL, Crossley KM. Stress fracture risk factors in female football players and their clinical implications. Br J Sports Med. 2007 41: i38-i43.

Injury Risks for the Female Athlete - Part 2

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 [5]. 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 [12]. 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 [7].

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 [10], 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 [8]. 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 [8], which may independently reduce their likelihood of injury. In studies comparing the cartilage in the knee, men had collagen of greater thickness [4]. 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 [23]. 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% [1]. 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 [18]. 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 [3]. 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 [18].

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 [17]. 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 [17]. 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 [23].

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 [13]. 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.


1. Bonaiuti D, Shea B, Iovine R, Negrini S, Robinson V, Kemper HC, Wells G, Tugwell P, Cranney A. Cochrane Review on exercise for preventing and treating osteoporosis in postmenopausal women. Eura Medicophys. 2004;40(3):199-209.

3. DeBar LL, Ritenbaugh C, Aickin M, Orwoll E, Elliot D, Dickerson J, Vuckovic N, Stevens VJ, Moe E, Irving LM. Youth: a health plan-based lifestyle intervention increases bone mineral density in adolescent girls. Arch Pediatr Adolesc Med. 2006; 160(12):1269-76.

4. Ding C, Cicuttini F, Scott F, Glisson M, Jones G. Sex differences in knee cartilage volume in adults: role of body and bone size, age and physical activity. Rheumatology (Oxford). 2003;42(11):1317-23.

5. Huston LJ, Wojtys EM. Neuromuscular performance characteristics in elite female athletes. Am J Sports Med. 1996;24(4):427-36.

7. Landry SC, McKean KA, Hubley-Kozey CL, Stanish WD, Deluzio KJ. Neuromuscular and lower limb biomechanical differences exist between male and female elite adolescent soccer players during an unanticipated side-cut maneuver. Am J Sports Med. 2007;35(11):1888-900.

8. Magnusson SP, Hansen M, Langberg H, Miller B, Haraldsson B, Westh EK, Koskinen S, Aagaard P, Kjaer M. The adaptability of tendon to loading differs in men and women. Int J Exp Pathol. 2007;88(4):237-40.

10. Miller BF, Hansen M, Olesen JL, Schwarz P, Babraj JA, Smith K, Rennie MJ, Kjaer M. Tendon collagen synthesis at rest and after exercise in women. J Appl Physiol. 2007; 102(2):541-6.

12. Myer GD, Ford KR, Hewett TE. The effects of gender on quadriceps muscle activation strategies during a maneuver that mimics a high ACL injury risk position. J Electromyogr Kinesiol. 2005;15(2):181-9.

13. Nichols JF, Rauh MJ, Barrack MT, Barkai HS. Bone mineral density in female high school athletes: interactions of menstrual function and type of mechanical loading. Bone. 2007;41(3):371-7.

17. Rittweger J. Can exercise prevent osteoporosis? J Musculoskelet Neuronal Interact. 2006;6(2):162-6.

18. Ruffing JA, Nieves JW, Zion M, Tendy S, Garrett P, Lindsay R, Cosman F. The influence of lifestyle, menstrual function and oral contraceptive use on bone mass and size in female military cadets. Nutr Metab (Lond). 2007;4:17.

23. Warden SJ, Creaby MW, Bryant AL, Crossley KM. Stress fracture risk factors in female football players and their clinical implications. Br J Sports Med. 2007 41: i38-i43.

Can't sleep? There's a mutation for that.

Forget caffeine, stress, or exercise–I have a new reason to blame for my odd sleeping habits: my genes! Or rather, a mutation in one of them. Here's a part of the recent NY Times article on this discovery:

The scientists were searching the samples for variations in several genes thought to be related to the sleep cycle. In what amounts to finding a needle in a haystack, they spotted two DNA samples with abnormal copies of a gene called DEC2, which is known to affect circadian rhythms. They then worked back to find out who provided the samples and found a mother and daughter who were naturally short sleepers. The women routinely function on about 6 hours of sleep a night; the average person needs 8 to 8.5 hours of sleep....

What distinguishes the two women in the study and other naturally short sleepers is that they go to bed at a normal time and wake up early without an alarm. The two women, one in her 70s and the other in her 40s, go to bed around 10 or 10:30 at night and wake up alert and energized around 4 or 4:30 in the morning, Dr. Fu said.

“When they wake up in morning, they feel they have slept enough,” Dr. Fu said. “They want to get up and do things. They arrange all their major tasks in their morning.”

Sound familiar? Does to me. I'll keep an eye out for more info on this DEC2, which is apparently interrupting my sleep.

Injury Risks for the Female Athlete - Part 1 (the most running-related post yet!)

I've been working on an article for the ACSM Health & Fitness Journal about injury risk to the female athlete. The first submission got sent back with a bunch of comments, so I figured as I'm revising I'll post it in a few installments. To the runners/active women out there, it'll hopefully be of some use.

Injury Risks for the Female Athlete

While there are more and more studies showing differences between men and women’s physiology and specifically their response to exercise, historically nearly all studies have been done on men. Thus, most of the data available to the public (in scientific journals, textbooks, and encyclopedias), while providing a great deal of insight into the physiology of exercise, disregards large portions of the population and is severely limited in scope. Because of women’s unique set of physiological responses and health concerns, it is important to consider women as a specific sub-population in the study of exercise and athletics. This article will focus on the topic of injury risk for women, reviewing the current literature on this subject to better understand the special concerns of the female athlete.

The increasing number of women participating in sports also means that more women are likely to sustain injury. While the timing, location, and nature of an injury may vary from person to person, there are specific injury risks for the female athlete. In particular, women are more likely than men to sustain musculoskeletal injuries during physical activity [10], as well as lower-extremity injuries in general [21]. By far the most documented injury in female athletes is the anterior cruciate ligament (ACL) tear. Studies have reported the occurrence of ACL tears in women as up to nine times greater than in men [15]. In soccer and basketball in particular, women are three times more likely to tear their ACL than males [15]. Suggested reasons for greater injury incidence in women have ranged from biomechanics to coordination and fatigue to ligament and tendon properties. To further understand the sex differences associated with injuries and take steps to prevent them, it is crucial to examine these risk factors.

Biomechanical differences are perhaps the most noticeable factor that can predispose a woman to injury. Gait studies have identified particular differences in the up and down motion of the pelvis (or pelvic obliquity) and vertical motion of the whole body. Women generally have greater pelvic obliquity, which translates into less vertical motion [20]. This is a more biomechanically efficient gait, because less energy is expended lifting the body up and down with each stride. However, the greater pelvic motion also causes movement of the lower spine, which has been associated with acute and chronic back pain as well as disc damage. Thus there may be tradeoff between gait efficiency and injury risk – what serves as an advantage for women in conserving energy may promote the development of low back pain.

Studies on the biomechanics of landing from a jump have demonstrated several differences in men and women. Women land with their knees less flexed and turned slightly more inwards than men [14]. The inward turning of the knee is called knee valgus. Both knee flexion and valgus angle have been associated with knee injury and ligament damage. While landing with knees less flexed (and legs more extended) helps decelerate the body from a fall and can absorb more impact from the landing, it puts much more strain on the ACL. Even slight increases in valgus angle (as little as 2 degrees) can increase the force on the ACL by threefold and potentially cause injury. Women have an average of 4.5 degrees greater knee valgus than men during jump landings [14]. This biomechanical difference has important implications for females participating in sports that require jump landings, such as volleyball, basketball, and track and field. Awareness of women’s higher susceptibility to ligament injury may encourage injury prevention and emphasis on correct landing techniques.

Another explanation for increased injury risk in women is neuromuscular fatigue. There is a significant link between fatigue and injury, for example game-related injuries occur much more often at the beginning or end of a season [9]. Injury may occur due to vigorous pre-season training or the accumulated strain of many competitions at the peak of the season. Other studies have reported a higher incidence of knee injuries during the last 15-30 minutes of soccer or rugby matches, which corresponds to the time at which athletes are physically exhausted from the game. Neuromuscular control of the legs is important during maneuvers like landing from a jump or moving from side-to-side, and lack of control is likely to cause injury [9]. High intensity sports that incorporate quick movements and place a high load on the joints (like basketball, soccer, and football) require sustained effort that can fatigue an athlete. The accumulation of fatigue lowers the force-generating capacity of the muscle, affects motor control, and slows reaction times [6]. These deficiencies may change how an athlete performs landing and side-to-side movements, which may lead to injury. Several studies have indicated that women show a greater performance change with fatigue than men, such as a reduced capacity to control the knee and hip joints [6]. These abnormal movements may increase female athlete’s risk for injury, especially of the ACL.


6. Kernozek TW, Torry MR, Iwasaki M. Gender Differences in Lower Extremity Landing Mechanics Caused by Neuromuscular Fatigue. Am J Sports Med. 2008;36(3):554-65.

9. McLean SG, Felin RE, Suedekum N, Calabrese G, Passerallo A, Joy S. Impact of fatigue on gender-based high-risk landing strategies. Med Sci Sports Exerc. 2007;39(3):502-14.

10. Miller BF, Hansen M, Olesen JL, Schwarz P, Babraj JA, Smith K, Rennie MJ, Kjaer M. Tendon collagen synthesis at rest and after exercise in women. J Appl Physiol. 2007; 102(2):541-6.

14. Pappas E, Hagins M, Sheikhzadeh A, Nordin M, Rose D. Biomechanical differences between unilateral and bilateral landings from a jump: gender differences. Clin J Sport Med. 2007;17(4):263-8.

15. Prodromos CC, Han Y, Rogowski J, Joyce B, Shi K. A meta-analysis of the incidence of anterior cruciate ligament tears as a function of gender, sport, and a knee injury-reduction regimen. Arthroscopy. 2007;23(12):1320-1325.

20. Smith LK, Lelas JL, Kerrigan DC. Gender differences in pelvic motions and center of mass displacement during walking: stereotypes quantified. J Womens Health Gend Based Med. 2002;11(5):453-8.21.

21. van Gent RN, Siem D, van Middelkoop M, van Os AG, Bierma-Zeinstra SM, Koes BW. Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br J Sports Med. 2007;41(8):469-80.