By Avery Hinks Muscles can perform 3 types of contractions: isometric, concentric, and eccentric. In isometric contractions, muscles contract without moving the joint, like when planking. In concentric contractions muscles shorten while contracting, like when lifting a box off the floor. In eccentric contractions, muscles lengthen while contracting, like when lowering a box from a high shelf. These different contraction types contribute to the properties of muscle called residual force depression and residual force enhancement. What are residual force depression and residual force enhancement?In a previous knowledge translation, I outlined the mechanisms and relevance of residual force depression and residual force enhancement. Following a concentric contraction, muscles are weaker, require more activation from the brain, and feel more effortful compared to an isometric contraction at the same muscle length. This is called residual force depression. Conversely, following an eccentric contraction, muscles are stronger, require less activation, and feel less effortful. This is called residual force enhancement. Essentially, residual force depression makes your muscles less efficient while residual force enhancement makes your muscles more efficient. The mechanisms behind these phenomena come from the cellular level of muscle, in tiny force-producing structures called sarcomeres. Therefore, force depression and force enhancement are as fundamental to muscle as more basic properties like strength and speed. Because they are fundamental, there has been interest in recent years regarding whether force depression and force enhancement are modifiable with training. Hypothetically, if we can train a muscle to reduce force depression and increase force enhancement, we can make the muscle more efficient! This may be applicable in sport settings or in elderly populations, where small alterations to the efficiency of movement make a big difference. How can we target residual force depression and residual force enhancement in training?Based on what we know mechanistically about residual force depression and enhancement, we can point to two specific ways to target them in training. First, there is a structural component. Muscle structure is arranged in a hierarchy, with many sarcomere units comprising a muscle fibre, many muscle fibres comprising what’s called a muscle “fascicle”, and many muscle fascicles comprising a whole muscle. Training—particularly resistance training—can have profound effects on muscle structure across this hierarchy. Because the mechanisms behind residual force depression and enhancement occur in the sarcomere, it is reasonable to believe that adaptations in muscle structure following training may modify residual force depression and enhancement. The second target for modifying residual force depression and enhancement is the neural component. As mentioned, residual force depression and residual force enhancement are associated with greater and less required activation from the brain, respectively. On top of this, “neural inhibition”, which is when the brain inhibits a muscle’s activity, occurs during residual force enhancement-type contractions. Partly due to this neural inhibition, ~20% of people show no force enhancement at all! We call these people “non-responders.” Profound neural adaptations occur following training as well. Altogether, we may be able to reduce force depression and improve force enhancement through structural adaptations, neural adaptations, or both. So…can we modify residual force depression and residual force enhancement through training?Unfortunately, not as easily as we would expect based on everything described above. The first investigation into this question came from outside our lab, in a study by Siebert and colleagues. They compared residual force enhancement during leg extension movements (pushing at the hip and knee) between weightlifters and a non-weightlifting reference group. They observed no statistically significant differences in force enhancement between weightlifters and non-weightlifters. However, as shown by the graph below, when holding the isometric contraction for longer (“t3” in the graph), weightlifters began to show greater force enhancement than the reference group. Therefore, weightlifting showed potential to improve force enhancement, however, not enough to produce a very noticeable difference compared to non-weightlifters. Since this initial investigation, our lab has conducted three training studies to obtain a closer look. Each of our first two studies employed two different training programs: one emphasizing eccentric contractions, and one emphasizing concentric contractions. The distinction between these two training types is eccentric training can increase muscle size by making the muscle longer, while concentric training can increase muscle size by making it wider. Therefore, the purpose of these training studies was to target the structural component of force depression and force enhancement. In our first eccentric vs. concentric training study, Chen and Power recruited human participants and submitted them to 4 weeks of training. They observed a 36% decrease in residual force enhancement following eccentric training and an 89% increase following concentric training. So, good, right? Not quite. It appeared that neural adaptations, rather than the targeted structural adaptations, contributed to these alterations to residual force enhancement, mainly by reducing the number of “non-responders” following concentric training. Additionally, Chen and Power did not see any alterations to residual force depression following either concentric of eccentric training. This suggests force depression is not modifiable by either structural or neural adaptations to training. Chen and colleagues then went on to perform an eccentric vs. concentric training study in rats. They did this by employing downhill running (which emphasizes eccentric contractions) and uphill running (which emphasizes concentric contractions) training. Testing rats allows us to assess force depression and enhancement in a single dissected muscle, thereby removing the neural components and isolating the structural components of muscle function. Consistent with results from the previous human eccentric vs. concentric training study, neither downhill nor uphill running training altered force enhancement or force depression in the muscles of these rats. Additionally, an investigation by Mashouri and colleagues in these same rats showed the same lack of effects on force depression at an even smaller scale, in single muscle fibres. Despite these findings showing little influence of muscle structural adaptations on residual force depression and residual force enhancement, we attempted once more to modify these properties through training. In humans, Hinks and colleagues employed isometric resistance training at long and short muscle lengths. Like eccentric vs. concentric training, these training programs are intended to induce different adaptations in muscle structure. Isometric training at a long muscle length may increase muscle size primarily by elongating the muscle, while isometric training at a short muscle length may increase muscle size primarily by widening the muscle. While we observed the intended adaptations in muscle structure, these did not appear to have any impact on residual force depression or enhancement, as neither changed with training. There was also considerable variability between participants, particularly in how some force enhancement “non-responders” showed force enhancement following training, while others only became “non-responders” after training (as shown in “B” in the graph below). Current consensus on residual force depression and enhancement with training Currently, the only direct evidence that training may improve force enhancement is following concentric resistance training, due to neural adaptations. Structural adaptations do not appear to alter either force enhancement or force depression, and force depression does not appear to be modifiable with training. In the human studies, however, we must consider whether measurements of residual force enhancement and residual force depression are reliable from day to day. If they are not reliable, that would, of course, impact the accuracy of our human training study findings. This is a question a study from our lab is currently looking into, and will have to wait for another day!
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September 2023
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