WE ALL KNOW the feeling. The first day after a long hike in hilly country, your legs feel weak and jelly- like. You go to sleep exhausted, but happy. Then the gremlins go to work. When you get up next morning, your leg muscles are so stiff and painful that you're reduced to a hobble.
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But as you sit there regretting that you raced up those inclines and that you need to do more exercise, don't
feel too bad. If two Australian researchers are correct, it's going downhill, not uphill, that does the damage.
And unfit people are no worse off, says David Morgan, research director of the Centre for Biomedical
Engineering at Monash University in Melbourne. "Fit cyclists are more vulnerable than couch potatoes."
For most of us, the impact of this type of damage is just a few days' soreness. But for athletes, the sudden
loss of muscle power can mean the difference between winning and losing.
It's long been known that some types of exercise lead to muscle soreness. The damage was thought to be caused by electrical disturbance or a chemical problem, such as an imbalance of calcium. But no one theory accounted for all the damage. Then, in 1990, Morgan cast his engineer's eye over the problem while on sabbatical at Harvard Medical School, near Boston, Massachusetts. The theory he came up with has now been rigorously tested by Uwe Proske, professor of physiology at Monash. The researchers' ideas are now being included in a series of exercises to protect athletes against injury. |
Muscles are designed to contract. When the brain sends an impulse to a leg muscle, for instance, it shortens. Muscles or muscle groups that control joints are also designed to work in opposing pairs. So when the quadriceps at the front of the thigh contracts, the leg below the knee extends. Bending the knee is then a matter of contracting the opposing muscles that run down the back of the thigh, such as the hamstring muscles. When one group contracts, the opposing muscles relax so they can be pulled back into their elongated form.
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But there are some forms of exercise - known as eccentric exercise - in which muscles are stretched as they contract. In other words, instead of initiating an action, the contracting muscle acts as a moo brake to restrain an action. Walking or running downhill, for example, stretches the quadriceps and the calf muscles.
The point is that muscles are not regularly called upon to cope with long bouts of eccentric exercise. When they are, the result is damage, weakness and pain. "The muscles always recover," says Proske. "That's why this form of soreness is often not seen as a true injury. But the muscle is weakened and cannot develop its usual force ... The weakness also makes the muscle more prone to injury."
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Morgan's theory stems from a detailed understanding of the structure and properties of muscle. Muscle fibres are made chiefly of two proteins, actin and myosin, which interlock within repeating units called sarcomeres. The interlocking is done by myosin cross bridges, which can detach and reattach themselves to actin. The fibre contracts when these cross bridges pull the myosin past the actin like oars pulling a boat through water (see "Roots of Fatigue", Inside Science, New Scientist, 21 May 1994). The force of contraction exerted by a muscle fibre increases as it gets longer - but only up to a certain length. Beyond this 'yield point', the force it can exert falls off. Morgan's own research showed that sarcomeres along a fibre are not uniform in length. Even at rest, some sarcomeres are more stretched than others. "While talking to some people in Boston about eccentric exercise and thinking about my own work," he says, "I suddenly recognised that the two might be linked." |
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When a fibre is stretched during eccentric exercise, he reasoned, the longer sarcomeres would reach the yield point first, creating a vicious circle. These sarcomeres would be able to exert less and less force as they got longer, and so would become easier and easier to stretch. They would also stretch in preference to shorter sarcomeres, which would exert a stronger force.
The fibre would then begin to behave like a bendy straw with a concertina section. When pulled from the ends, the folds - of the concertina "pop' one after another. In a muscle fibre, the elongated sarcomeres would also pop one at a time. To test the idea, Morgan and Proske stimulated stretched muscle fibres from toads and looked with an electron microscope for signs of damage. Not only did they find popped sarcomeres, but there were about as many as they had predicted.
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In the lab, Morgan and postgraduate student Jason Talbot found what happens when a fibre with "popped" sarcomeres relaxes. Usually, the filaments fit together again so that everything returns to normal. But sometimes the filaments remain disrupted. In these cases, the damage can spread to adjacent sarcomeres during the next eccentric action. Eventually, the whole muscle fibre can die, says Morgan, leading to a weakening of the whole muscle and pain as the body's immune cells secrete enzymes to clean up the mess. 'This work provides the first comprehensive, plausible explanation at the molecular level as to why eccentric exercise makes you sore," says Proske.
The researchers found that the body responds to this damage by building a new muscle fibre that is packed with more sarcomeres than before. Rats that ran downhill developed 15 per cent more sarcomeres in their thigh muscles than rats that ran uphill or did not exercise. After this remodelling, each sarcomere does not have to stretch so far as before, so fewer if any reach the yield point.
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Proske and his group also tested humans, walking them backwards down an inclined treadmill to exercise their calf muscles eccentrically. The induced soreness took about a week to recover, and just one episode was enough to condition the body. 'What we began to realise," says Proske, "is that muscles are continually fine-tuning their length depending on the individual's lifestyle." |
They found that a single bout of eccentric exercise every week kept the muscles permanently adapted. This fact is at the heart of exercises being developed for athletes by Camilla Brockett, a postgraduate physiology student at Monash. In one of them, for footballers, a player kneels on the floor and leans forward while another player sits on his or her feet. This strengthens the hamstring muscles, which are prone to damage in football because in kicking a ball players need to simultaneously extend and contract their hamstrings.
Eccentric exercise has not been included in standard training programmes, says Proske. But including it could make all the difference. "Why and when you become sore as a result of unaccustomed exercise is not a matter of fitness, but improper training."
Further reading. "Changes in the mechanical properties of human and amphibian muscle after eccentric exercise", by C. Jones and others, European Journal of Applied Physiology, vol 76, p21.
Cool Running 21.01.00.
Tim Thwaites is a science writer in Melbourne. This article first appeared in the New Scientist http://www.newscientist.com/. Reproduced here with the permission of the copyright holder. Photos by Cool Running.
