By Avery Hinks An eccentric muscle contraction is when a muscle stretches while producing force. That can be hard to picture, so think of an eccentric contraction as the lowering phase after lifting something. Whether you are lifting weights at the gym, lifting grocery bags out of your car, or lifting your toddler who is tired and doesn’t want to walk anymore, there will come a time when you need to lower what you’ve lifted. You don’t want to simply let what you’ve lifted drop to the ground (especially if it’s your toddler), so you control the lowering motion. To have this control, your muscles are contracting while stretching. This is an eccentric contraction! Eccentric contractions are much more than just controlling the lowering of a weight, however. They also demonstrate one of the best examples of the extreme and rapid adaptability of the body’s muscular system. Muscle damage induced by eccentric exerciseWhat if instead of one eccentric contraction, you do 100 or more eccentric contractions? In the lab, we have equipment that allows us to isolate such exercise, but in the real world this might occur in a long downhill hike or a very intense gym session. A muscle’s strength comes from force produced by the smallest structural units within the muscle, called “sarcomeres.” When your muscle stretches while contracting, sarcomeres stretch too. If sarcomeres repeatedly stretch while producing force, they break apart. This damage has profound effects on the muscle as a whole. These effects include muscle weakness that can last a whole week, and an inflammatory response that causes intense muscle soreness. The graphs below are classic representations of these impairments. The top graph shows the strength generated in a maximal-effort contraction in arm muscles. Immediately following intense eccentric exercise (“Post” on the graph), strength was reduced by 40%, and remained below the pre-exercise strength value for at least 5 days. As shown by the bottom graph, alongside this prolonged loss of strength, the muscle also experienced soreness 1 to 4 days following the exercise. While uncomfortable, these effects are not necessarily bad. The muscle is a remarkable thing, and can not only recover, but recover stronger! The “Repeated Bout Effect” of eccentric exerciseAs described above, following 1 bout of intense eccentric exercise, the muscle experiences damage and weakness. The response is different, though, following a second, identical bout of eccentric exercise performed 4 weeks later. After the initial exercise bout, adaptations occur in the muscle that protect it from damage during the second bout. Hence the name for this phenomenon: “the Repeated Bout Effect”. The adaptations are widespread, occurring in the brain, the inflammatory system, the connective tissue surrounding the muscle, and the muscle itself. But what does this really look like? The graphs below are taken from a study in our lab by Hinks and colleagues. Graph A shows the blood levels of an enzyme called Creatine Kinase. Creatine Kinase is an enzyme involved in muscle activity. When sarcomeres are damaged during eccentric exercise, the enzyme leaks out of the muscle fibre cell, eventually ending up in the blood. You can see that following the first bout (black in the graph), a large Creatine Kinase spike occurred. Following the second bout (red), however, Creatine Kinase almost didn’t increase at all, because there was less damage to the muscle’s sarcomeres. The other two graphs show the measures discussed earlier: muscle soreness (B) and maximum strength (C). Graph B shows a clear reduction in soreness in the second (red) compared to the first (black) bout, indicating a reduced inflammatory response. Most importantly, C shows that the muscle experienced less of an impairment in strength immediately following exercise (the “0min” time point on the graph). On top of that, the muscle’s strength recovered earlier, returning to normal the next day (“24Hr”)! Do the benefits of the repeated bout effect apply to everyday life?The measure of maximum strength discussed so far has limited applicability to everyday movements. Strength was assessed in “isometric” contractions, which entails pulling against a fixed object. While isometric contractions are a valid measure of someone’s strength, it does not tell us much about how a muscle performs when moving through a range of motion. The study form our lab by Hinks and colleagues set out to fill this gap. To do this, they assessed the arm muscles’ “dynamic” performance following an initial and repeated bout of eccentric exercise. The distinction between this “dynamic” performance and the previous “isometric” performance is that now the participants are pulling against an object that can move. Think of it like the action of lifting a weight as fast and hard as you can. We assessed three important measures of dynamic performance: peak power, rate of force development, and rate of velocity development. I’ll go through them one by one. When measuring dynamic performance in muscle, two factors must be considered: force (or strength) and velocity (or speed). These factors must be considered together because a muscle’s level of force affects how fast it can contract. As a muscle contracts at increasing speed, the amount of force it can produce decreases, and vice versa. Measuring a muscle’s “power” allows us to assess the best of both these factors, because power is equal to force times velocity. Power is especially important in athletes looking to maximize their dynamic performance, such as sprinters and cyclists. The relationships between force, velocity, and power are best demonstrated by the graph below. You can see that force (solid line) decreases with increasing velocity, and peak power (dotted line) occurs at moderate levels of both force and velocity. We can also assess a muscle’s dynamic performance by looking at the rates at which it develops force and velocity. These are called “rate of force development” and “rate of velocity development”, respectively. Rate of force development is exactly as it sounds: a person who reaches maximum force faster will have a greater rate of force development. Rate of velocity development can be more difficult to picture, but it is interchangeable with “acceleration” (yes, like in cars), and describes how fast a person can increase their velocity. Rate of torque development and rate of velocity development may seem trivial, but they have important applications to everyday life. A prominent example can be found in elderly individuals experiencing falls. If the muscles can react quickly (rate of velocity development) and quickly reach maximum strength (rate of torque development), a fall may be prevented. We found that the Repeated Bout Effect indeed protects these measures of dynamic performance from muscle damage! In the graphs below, you can see that peak power (A), rate of velocity development (RVD; B), and rate of force development (RTD; C) all experienced less impairment following the second (red) bout of eccentric exercise, and recovered sooner. What’s the bigger picture?As discussed above, measures of dynamic muscle performance have important real-world applications, such as sport performance and prevention of falls in elderly individuals. Therefore, whether the Repeated Bout Effect’s ability to protect from muscle damage and weakness reduces impairment of these dynamic measures is important—and now we know! This research does not tell the whole story, though. These findings were only determined in arm muscles, while many of the real-world applications discussed apply primarily to leg muscles. Leg muscles do not often display a Repeated Bout Effect as strongly as arm muscles. Therefore, it would be interesting to see whether these dynamic measures are protected in leg muscles as well. That’s all for now! I hope you’ve learned that the next time your muscles feel sore after a hard workout, that is just your muscles adapting to optimize your performance.
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AuthorAvery Hinks Archives
September 2023
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