Why are female athletes so susceptible to knee injury?

But research has shown that strength training has an astonishingly protective effect. Owen Anderson describes the curious vulnerability of the female knee.It's sad but true: female athletes who take part in sports involving jumping and 'cutting' (football, basketball, volleyball, gymnastics etc) have a risk of knee injury that is 4-6 times higher than for men taking part in the same activities. (1).

In the USA, the knee injury rate among female collegiate athletes runs at a stunning one per 1,000 'athlete-exposures' (an 'athlete-exposure' is simply a workout or competition). With over 100,000 college women taking part in organised sports each year, there are more than 10,000 knee injuries per year - or more than 1,000 per month (given a nine-month school year). To put it another way, if just 50 teams with 20 female athletes each carry out their workouts on a particular day, on average at least one serious knee injury would result (2).

Many of these injuries are devastating - from both a personal and financial standpoint. In the USA alone there are an estimated 2,200 complete ruptures of the knee's key internal supporting structure - the anterior cruciate ligament (ACL) - in female collegiate athletes each year, with the total cost of medical treatment running to millions of pounds. The average cost of care, including ACL reconstruction and rehabilitation, is about £11,000 per patient, which would add up to more than £24 million per year. In addition, a female athlete with an ACL rupture will usually miss an entire sporting season, may lose her athletic scholarship, and is likely to experience significant mental pain and stress(3). Bear in mind that these figures are probably just 'the tip of the iceberg', since they totally omit school athletes. In the US, as in the UK, there are many more participants at secondary school than at college, so the total number and cost of injuries in secondary schools are likely to be significantly greater. In the United States, there are probably about 20,000 serious knee injuries among female high school athletes per year, ratcheting the total cost of care for all female athletes - for knee problems alone - to about £70 million. Injury rates in the UK are likely to be similar, which would imply a total of more than 6,000 serious knee injuries per year, at a cost of £14 million. Secondary school girls have knee surgery five times more often than boys, and knee surgeries make up 70 percent of all athletic-related surgeries for young women(4).

The key strap of tissue

Why do anterior cruciate ligament (ACL) ruptures occur, and why do they happen more often in girls and women? To understand ACL ruptures and their preference for female athletes, you first need to know that the ACL is the key strap of connective tissue which stabilises the knee joint and connects the back of the femur (thigh bone) with the front of the tibia (shin bone). The word 'cruciate' in this key ligament's name means 'cross-shaped' or 'marked with a cross', a seemingly odd designation for a straight strap of connective tissue, which is roughly the size of one's little finger. However, within the knee joint, especially when the tibia is rotated in an internal direction (counterclockwise for the right knee, clockwise for the left knee), the anterior cruciate ligament (ACL) runs over the front of - and is roughly perpendicular to - another key supporting structure within the knee called the posterior cruciate ligament (PCL), creating a 'cross' of connective tissue cords within the knee. An easy way to picture this is to cross your index finger under your middle finger; the middle finger represents the ACL and the index finger is the posterior cruciate.

The cruciates provide support for the knee and guide rotational movements at the knee joint. Basically, the ACL stops hyperextension of the knee, limits excessive forward movement of the tibia during knee flexion, and controls internal rotation of the tibia. It's possible that the ACL also controls external rotation of the tibia, especially since it tends to be wrapped around the inside of the lateral femoral condyle (the bony projection at the outside bottom of your femur). The scenarios leading to injury are varied, but many experts believe that most ACL injuries occur not as a result of collisions but after landing from a jump or prolonged explosive stride(5). This is important because if most severe ACL injuries result from breakdowns in ACL tissue during normal sporting activity, dynamic strengthening of the ACL and its associated tissues should lower the risk of injury considerably.

Although collisions are not as important as other mechanisms of knee and ACL injury, they can create mayhem. For instance, someone might collide with you as you are running along, slamming into your leg near the knee while the foot of that leg is planted on the ground. The sudden movement of the tibia which would result could tear the ACL surprisingly easily (remembering that the ACL attaches to the top-front of the tibia). Alternatively, you might step on someone else's foot while running (or while landing after jumping during a basketball or volleyball game), causing your knee to hyperextend; sudden hyperextension at the knee can easily rip the ACL out of its moorings. Or you might simply attempt to come to a sudden stop, often while twisting your leg at the knee, to get out of the way of another athlete or to react quickly to the flight of a football. The extreme knee flexion and torquing which result can easily damage the ACL.

Incidentally, if you do serious harm to your ACL during activity, you'll often hear a 'pop' when the injury occurs, and swelling will follow almost immediately. Also, the knee itself will tend to be quite unstable, usually 'giving way' during weight-bearing.

Wider female pelvis a factor?

Why are females at greater risk of ACL trouble? The reason is unclear, but some sports medicine experts think it is primarily because of anatomical differences. For one thing, the intercondylar notch - a small chasm at the bottom of the femur through which the ACL passes - tends to be smaller in females than in males(6). Thus it is theoretically possible that during cutting and jumping movements, the narrow female notch may fray and weaken the ACL. Any fraying of the ACL would tend to be more serious in female athletes since the female ACL is usually a smaller structure.

Also, the wider female pelvis tends to exaggerate the angle made at the knee between the femur and tibia when the foot is planted on the ground, increasing inward pressure on the knee and external rotation of the tibia, and thus placing excessive stress on the ACL(7).

Even more interesting is the theory that the ACL is more lax in females than males - and therfore more susceptible to overstretching(8). There are receptors for both oestrogen and progesterone on the ACL and the theory suggests that increases in one or both of these hormones may slacken the ACL, heightening the risk of damage. It is known that a woman's ligaments tend to loosen up as a result of the hormonal changes associated with pregnancy, so this theory is not too far-fetched and, if true, would also suggest that the risk of injury would vary with the menstrual cycle(9).

Studies from the University of Michigan have shown that female athletes have less strength in their leg muscles and slower muscle-reaction times than males, which would increase the risk of ACL trauma. The reaction-time disparity is especially interesting: it is clear that to optimise the chance of keeping your ACL intact, you need both to boost the strength of your hamstrings (to help keep the tibia in place during landings from jumps and sudden stops) and to increase the speed with which your hamstrings react to ACL-stressing movements. If they're slow to react, they may be unable to protect the ACL in time to avoid injury, however strong they are.

Following this research, scientists at the Cincinnati Sportsmedicine Research and Education Foundation and Deaconess Hospital in Ohio detected a significant imbalance between hamstring and quadriceps muscle strength in female athletes before training. Importantly, men had 'knee-flexor moments' (an indicator of hamstring strength) during landing from a jump which were three times higher than for females(10).

The Cincinnati researchers went on to develop a plyometric, stretching, and strength-training programme for female athletes which decreased peak landing forces at the knee by minimising unnecessary side-to-side movements of the knee during landing(10). They showed that the plyometric -strengthening programme increased hamstring muscle strength and power, elevated the hamstring/ quadriceps peak power ratio and fortified hamstring strength during lateral and medial movements of the knee (in addition to better balancing strength in those opposing directions).