Intrinsic factors might be considered modifiable and non-modifiable.
If the injury studies can be considered collectively, the injury rate jumps dramatically at age 14 then again around age 23-25. But this is only in males. The same pattern doesn't show up in females. Of course there are studies that show younger athletes at a greater risk of injury as well as a number of studies showing age as no factor in injury. But ask the doctors whom they are operating on. ACL surgery in girls is mostly non-existent under 14, and then the number jumps rapidly staying relatively constant until their early 20's.
The rate of injuries in females is substantially greater than males. The most visible injury is an ACL tear where females have 3-8 times the risk of an injury than males. A similar risk of ankle injury has not been demonstrated. Of course there are projects that show no difference between males and females, but the majority of studies comparing males and females show females to be at a higher risk.
Menstrual cycle phase
This factor is difficult to study due to variations in menstrual cycle length. Of the few studies that have looked at this factor have shown that more injuries (especially ACL injuries) occur during the ovulatory (middle) phase of menstruation whether on or off the oral contraceptive pill. Other studies have suggested that more injuries occur at or the onset of menstruation. More controlled research has to be done to determine if this really is a factor, and if so, just what mechanism is at work.
The basic build of the athlete has been examined in a variety of ways. The most common variable is the body mass index [BMI=wt (kg)/ht (m)2]. Male athletes with low or high BMI and short women are at the greatest risk of injury. Males at puberty are interesting. As so many boys increase height faster than they increase muscle mass, the tall underdeveloped boy is at a greater risk for injury than the boy who hasn't hit that 'growth spurt' or those who are past their spurt. Usually, the taller male and the shorter female are at the greatest risk. With the variety of methods of reporting body size, it is not surprising that there are about as many studies saying body size has an effect as there are studies showing no effect.
Regardless of what you think about soccer being a game where you need to be able to use both feet, even the elite have a preferred foot and will set up that foot every chance they get. Another problem is the concept of dominant leg. We all know our dominant hand, but for the feet, is the dominant leg the preferred kicking leg or the take-off leg when jumping? It is not always the same leg. Thus, this as a factor is not very clear. In some studies, the dominant leg is the one most injured. In others, it isn't. Some show more sprains in the dominant leg, but not for muscle strains. Obviously, this as a risk factor is still uncertain.
Much is made of the Q-angle at the knee. This is the angle at the lower leg to the upper leg. The thought is that the wider pelvis of females makes this angle substantially different from males. Despite its ease of measurement and logical involvement in knee injuries, the Q-angle has not been shown to be a consistent predictor of knee injury. Another bony dimension is the intercondylar notch of the femur. The knee end of the femur has a notch in it that some say is proportional to the size of the ACL. A narrow notch means a small ACL and prone to injury. One camp says that small ACL's are injured regardless of sex. There are other anatomical indexes under study, some of which seem to be predictive and other not.
Foot arch height, width, and length have been implicated in leg and low back injuries. Other dimensions (forefoot, midfoot, hindfoot) have been looked at with varying degrees of success. A main reason for discrepancy between studies may well be the method of measuring foot dimensions; a very tricky procedure.
There is no doubt that one of the best ways to prevent injury is to improve fitness. The factors of fitness most often discussed are aerobic fitness and strength. An improvement in aerobic fitness improves the player's ability to perform late in the match. Nearly 25% of injuries in soccer happen in the last 15 minutes of the game. Improving endurance means they tire less at the end of the game. Strength improvement has been used to control body movement so that any movement done by a stronger muscle meaning the muscle isn't working as hard to do the same work. The best way to reduce the risk of injury is to improve fitness.
It is a common practice to stretch to improve flexibility and reduce the incidence of muscle strains. A common misconception is that stretching is warm-up. It isn't. Stretching is a part of warm-up, or more properly, cool-down. The problem is that the data to support the practice is contradictory. Not too mention that athletes with ACL injuries are usually among the most flexible of players. Might it be possible to have too much flexibility? That is another unknown. Imbalances of muscle tightness though have been shown to be a predictor or muscle injury. Joint laxity, or loose joints, is another variable that predicts injury. General range of motion has not been shown consistently to be a factor in injury.
Prior injury and inadequate rehabilitation
Without question, the best predictor of injury is an incompletely rehabilitated prior injury. Returning to play too soon is a ticket to another injury. Many studies have shown that a major injury is usually preceded by a recent minor injury. And the second injury may or may not be to the same location. Return from an ankle sprain too soon and another ankle injury or something else, like a knee injury, is likely. Coming back a couple weeks too early may be leading to many months out from a more major injury. Wait until a health care professional release the injured athlete to return to competition.
Injury prevention is quite a topic, isn't it? Three factors that have been consistently shown to help in prevent injuries: don't return from an injury too soon, improve fitness, and correct imbalances in muscle tightness.
But don't forget other things that you can influence such as choice of shoes, ankle support, field maintenance, and improved skill. We will never eliminate injuries, but at least we should all try to do what we can.
Copyright 2003 Donald T. Kirkendall
Donald Kirkendall has a Ph.D. in Exercise Physiology, and is on the faculty in the Department of Orthopaedics at the University of North Carolina. He is a Fellow in the American College of Sports Medicine. He has coached soccer for ages U10 through college, and is on the USSF Medical Advisory Committee. He's edited seven books in exercise science and sports medicine, and has published numerous articles on soccer and sports sciences.