This raises a fundamental and disturbing question: are we as humans meant to run for miles on end every day? And are men and women so anatomically different that what's good for one may not be good for the other, without special training?
The most common concern centres on the question of whether jogging or regular running causes osteoarthritis in the knees. This very question was recently put to the residents of Framingham, in Massachusetts, where Harvard researchers have been follow-ing two generations of its inhabitants, carrying out health studies of every description.
In a 12-year study, the researchers wished to determine whether long-term weight-bearing exercise had any influence on either the developmentor prevention of osteoarthritis of the knee. In 1993-4, they asked a cohort of nearly 1300 members of the Fram-ingham Offspring cohort about their exercise habits, evaluated their knee pain and studied their knees using x-rays. All patients with any evidence of osteoarthritis were excluded.
Nine years later, the participants were reexamined for any evidence of osteoarthritis, or knee joint-space loss. After controlling for a variety of factors, the researchers concluded that exercise neither prevented nor caused knee osteoarthritis.
The risk of osteoarthritis was virtually null among the most vigorous exercisers-those who walked six or more miles per week or engaged in an activity that worked up a sweat three or more times a week. The only risk factor in developing osteoarthritis was being overweight-whether in a jog-ger or not. Nevertheless, the cohort consisted mostly of walkers, rather than runners, so the researchers couldn't specify that running would not cause osteoarthritis with certainty (Arthritis Care Res, 2007; 57: 6-12).
Nevertheless, a Danish study, which examined the hips, knees and anklesof 30 men who'd been serious com-petitive runners in the 1950s, foundno differences in joint alignment, range of motion, complaints of pain, cartilage thickness, grade of degenera-tion or osteophytosis (developmentof bone spurs, or osteophytes, that grow along the edges of bones) between the 90 per cent who were still active runners of 40 years' duration and those who'd given up(Am J Sports Med, 1990; 18: 379-81).
A further small-scale study came to similar conclusions (J Am Med Assoc, 1986; 255: 1152-4).
More recently, a Dutch review of 37 studies also found no strong evidence that regular exercise was associated with the progression of knee osteoarthritis (Arthritis Care Res, 2007; 57: 13-26).
Yet another study examining the insides of the knees of eight seasoned runners using magnetic resonance imaging (MRI) also found no evidence of degenerative disease (Am J Sports Med, 2004; 32: 55-9).
Arthritis risk in women
While there seems to be little evidence of a risk of arthritis in men, the same doesn't hold true for women. A study of 81 elite British female athletes-mostly runners-found that ex-athletes had a greater risk of osteoarthritis in the hips and knees than did non-athletes, particularly where the knee joins the upper leg bone, or femur (Arthritis Rheum, 1996; 39: 988-95). The researchers concluded that weight-bearing sports activity in women is associated with a two- to threefold increased risk of osteoarthritis (espec-ially the presence of osteophytes) in the knees and hips.
However, there was one other important factor: joint alignment.
One health risk for both genders is injury to the ligaments that attach to the knee. Evidence shows that distance runners may risk laxity of the knee due to loosening of the ligaments that attach to and support the knee. A lax knee is, in essence, a wobbly one that is more prone to joint displacement (J Electromyogr Kinesiol, 2004; 14: 475-83).
The primary ligaments involved are the anterior and posterior cruciate ligaments, two finger-sized pieces of tissue that crisscross within the knee joint, and support the joint as well as allow it to rotate comfortably.
At present, the statistics for female athletes is sobering. Women involvedin running and jumping sports are four to eight times more likely to do their knees in then men (Clin J Sport Med, 2004; 14: 281-6). In the US, women collegiate athletes suffer some 10,000 knee injuries a year.
Several theories propose that anat-omy predisposes women to running injuries: the wider pelvis exaggerates the angle of the knee when running and moving, and female hormones-present in knee ligaments-cause them to be naturally more stretchy than those in men. Also, the lesser leg strength of women and slower reaction times increase the risk of injury. With three times the hamstring strength of women, men are built to stride faster. The bony space in the femur through which the anterior cruciate ligament passes is also smaller in women than in men, allowing for less range of motion.
Studies of women taking the Pill, which suppresses menstruation and the normal hormonal cycle, show that they are less likely to develop knee injuries, which suggests that hormones, which make women naturally more flexible, also make ligaments more prone to injury (Clin J Sport Med, 2004; 14: 281-6).
Because ligaments have a high collagen content, making them highly elastic, exhausting the muscles through exercise may stretch the ligaments-possibly permanently.
To test this possibility, researchers devised a rigorous exercise protocol, then examined the flexion/extension power of the leg muscles, and degreeof knee laxity using an arthrometer, before and after exercise. Joint laxity was found in both knees, although this was significant only in the left knee,and not on the right (Am J Sports Med, 1986; 14: 30-4).
Assuming that most of the study participants were right-handed, these results suggest that joggers, both men and women, run a greater risk of developing laxity on their non-domin-ant side. This has been borne out by studies among soccer players, which showed that the knee on the non-dominant side is generally more prone to laxity (Int J Sports Med, 2004; 25: 594-8).
The risks of a loose ligament has nothing to do with weight-bearing. Research comparing the laxity of the knee among runners, basketball players and weightlifters shows that power-lifters who do frequent squats don't put as much pressure on their ligaments as basketball players and runners do, suggesting that it's the constant flexion and extension of the knee-without a chance to relax and recover, such as during constant running-that puts greater stress on the knee (J Electromyogr Kinesiol, 2004; 14: 475-83; Am J Sports Med, 1986; 14: 24-9).
One study of distance runners found an 18-per-cent increase in laxity after exercise, partly caused by a true perma-nent stretching of the ligaments, and partly by a decrease in the resting tone of the muscles (Br J Sports Med, 1989; 23: 165-8).
What does running do to cartilage? Some answers may lie in the research conducted on beagles trained by a special running programme to run beyond their normal activity. After some months, the researchers found that endurance-type running exercise caused a reduction in articulating cartilage, which "indicates either a disorganization or a reorientation of. . . the collagen network" (Ann Rheum Dis, 1996; 55: 253-64). These results may not apply to humans, but they do raise obvious concerns. If dogs, which are born to run, can have their collagen affected by constant running, what does it do in people?
As always, a middle course may be the most sensible. For men, osteoarth-ritis is not a worry, but effects on the ligaments may be a possible concern. Ensure that you carry out some of the suggestions in the box (page 16) before bursting into a run. To avoid exhausted muscles, think twice before punishing them regularly beyond the minimum for cardiovascular training, and consider doing a milder jog.
For women, long, punishing daily jogs may be a case of a workout too far-unless you are prepared to engage in special training to strengthen the muscles supporting the knee.
How to prevent knee injury
- Build up your hamstrings first. Scientists have found that, before training, women have significantly different strengths between the hamstring (back thigh) and quadriceps (front thigh) muscles. So, do squat jumps, ankle jumps, and jump from one leg to the other.
- Do exercises that develop strong calf, thigh and ankle muscles, and those around the knee. These will help to support your knees.
- Work in stretching and strengthening exercises before embarking on a jogging programme. It's vital that you are strong and supple when you begin to run regularly. For strength training, focus on the hip abductor and external rotator muscles.
- Incorporate agility exercises as well, including rapid directional changes, which help to develop quick contractions of muscle pairs that will increase reflex-response time and reduce surprise movements of the joints.
- Exercise on even, flat surfaces or soft surfaces like grass, and replace your running shoes regularly, every three to six months, before they wear out and their shock-absorbing capacity is gone.
- Work on keeping your centre of gravity slightly forward and stay on your toes. If your foot is not aligned naturally, consider wearing inserts in your shoes that maintain the correct alignment.
- Pay attention to odd cracking noises in your knees. If pain, swellingor any changes develop, these could be the first signs of degenerative disease.
- Consider altering your weekly exercise programme so that, besides just running, you introduce other non-impact activities such as swimming, walking or biking.
Checking for knee laxity
- Have yourself tested with an arthometer. This device measures the 'drawer' movement of the lower leg bone against the thigh bone. If it can move forward too much-called the 'anterior drawer sign'-you are likely to have a lax anterior cruciate ligaments.
- Test the range of motion of all of your major joints. This will help to determine the looseness of all your ligaments; General laxity of joints in women has been linked with a tendency to have ligament tears. Test whether you can:
- touch your thumb to your forearm while your wrist is bent
- bend forward and touch your palms to the floor while keeping your knees straight
- hyperextend your knee
- extend your elbow beyond the ordinary range.
A 'yes' to any of the above could indicate a greater tendency to suffer ligament-related injuries (Orthopedics, 2005; 28: 656-60).