Muscle soreness: one-sided protection

As any avid athlete knows, whenever you perform a novel training stimulus, you’re often rewarded with a lot of muscle soreness and some loss of function, with performance taking a hit in the days that follow. You’ll also be aware that the next time you perform the same activity, you’ll get a little less soreness, and your performance won’t suffer quite as much. The same thing happens again until you can routinely perform this previously novel training stimulus with minimal negative effect. The phenomenon is known as the repeated bout effect. It’s been experienced by nearly all athletes, and it’s been documented extensively in scientific research.

What is the repeated bout effect?

There’s still not very much information about the repeated bout effect and why it occurs, just that it does. While it’s generally assumed that this effect is driven by muscular changes, that might not be entirely the case. There’s plenty of evidence for the cross-education effect with strength training, where training one limb results in improvements in strength in the opposite limb. So is it possible that a similar cross-education effect could occur with the muscle damage and loss of function following intense exercise?

As injuries are a common occurrence in sport, such a finding would be particularly relevant. Athletes are often unable to train an injured limb, leaving the muscles and joints of that limb susceptible to muscle damage and further injury.  Therefore, if the repeated bout effect was present in limbs that don’t get exercised, training the non-injured limb could be a powerful tool for helping athletes recover from injury and return to sport safely. Fortunately, recent research has shed light on this very topic.

New research

In a recently published study, researchers recruited 52 young, healthy men who were aged 20 to 28 years old. The subjects all performed two bouts of 30 maximal isokinetic eccentric contractions performed at a speed of 30 degrees per second. An isokinetic contraction is performed on a specifically designed machine that allows for large forces to be created while keeping the speed of movement the same – see this article. Isokinetic eccentric contractions are used because they are known to create large amounts of muscle damage. The two bouts were performed two weeks apart to allow for a full recovery, while keeping the time period short enough to allow for the repeated bout effect to remain, as it fades after extended periods of time.

The subjects were split into four groups. Half the subjects would perform the tests with using knee flexion (to work the hamstrings) and half the subjects would perform the tests using elbow flexion (to work biceps). This was done to determine if there was a different impact on the lower body versus the upper body. For each group, half the subjects performed both the first and the second test on the same limb. This was done to determine the magnitude of the repeated bout effect that is known to occur when an exercised limb is exposed to the same stimulus.

The other half of the group performed the two tests on opposite limbs. This allowed the researchers to determine if performing a strenuous bout on one side of the body could offer protection to the other side of the body, even though no prior exercise had been performed on that limb. Further, because some of the subjects performed both tests on the same side of the body, researchers could determine what percentage of the full protective was transferred to the non-damaged side. For instance, if the protective effect of the first bout was equivalent for subjects in both groups, this would mean that performing a damaging bout of exercise was equally protective to the damaged and non-damaged limbs.

To determine the physiological impact of the damaging exercise bout, the researchers measured several variables:

• Maximal voluntary contraction torque, which tests the maximal strength of the muscle in an isometric or static position. This is a key indicator of muscle function, or how well the muscular system continues to produce force.
• Range of motion was also measured, again as a reflection of the function of the muscles. To get a more nuanced understanding of muscle function, joint position sense and joint reaction angle were measured. These tests provide information about the ability to know where the joint is in space – an ability that can be altered by muscle damage.
• Subjective muscle soreness - an important measurement because it ultimately reflects how the subjects experience the muscle damage.
• Limb circumference - because damaged tissue tends to swell and an increase in swelling indicates damaged tissue.
• Muscle hardness - again as increased swelling leads to increased hardness, indicating muscle damage.
• Echo intensity scans - to determine shifts in hydration that indicate muscle damage.
• Plasma creatine kinase - a key marker of muscle damage in the blood.

 Maximal voluntary contraction and joint range of motion were measured immediately before, right after, and then once per day for five days following the eccentric exercise bout. The other variables were measured at all time points except for right after the exercise bout. As muscle damage and loss of function progresses following the exercise bout for up to 48 hours, and then recovers over several days, it’s important to take multiple measurements to get a better sense of how muscle function and muscle damage are changing.