By Avery Hinks Despite what the name might suggest, the “History Dependence of Force” has nothing to do with record-keeping or the superpower used by heroes in Star Wars. Rather, the history dependence of force refers to a fundamental property of how muscles function during movement. Due to its perplexing nature and applicability to real life, it is an often-studied topic in our lab. To hold the box in this position, the muscles in your arms (mainly the biceps) are undergoing isometric contractions, which means they are sustaining a load (the box) without changing in length. But what if Bob is not there, and you must do all the box-moving alone? Sad, I know. However, depending on whether you are lifting the box from the floor or lowering it from a shelf, your arm muscles might be able make the box easier to carry. Enter the history dependence of force! The definition of the history dependence of force is in the name: it refers to your muscle’s force-producing ability (or its ability to sustain a load, in this case the box) depending on the recent contraction history of that muscle. This “contraction history” can be distinguished into two categories: 1) muscle shortening, or 2) muscle lengthening. Force depression following muscle shortening and force enhancement following muscle lengthening When a muscle shortens to move a load, that is called a concentric contraction. So, let’s say you pick the box up off the floor. In doing this, you must first shorten your arm muscles before assuming an isometric contraction to carry the box out of your house. That prior muscle shortening will hinder your muscle’s force-producing ability, making it harder for you to sustain the isometric load of the box. This aspect of the history dependence of force is called “residual force depression” and refers to a reduced (or “depressed”) level of isometric force following muscle shortening. In the figure below, you can see that, at the same position following muscle shortening (grey), the level of force the muscle generates is lower than if the muscles are contracting with no prior shortening (black). There’s a flip side, though. What if you lower the box from a high shelf before carrying the box out of your house? Here, your muscles will undergo lengthening while holding the load. We call this an eccentric contraction. When muscle lengthening precedes an isometric contraction, the muscle can produce more force than it can in a constant isometric contraction. This aspect of the history dependence of force is called “residual force enhancement” because the force is, well, enhanced! To emphasize with a comparison to the previous figure, you can see below that when an isometric contraction is preceded by muscle lengthening (grey), the muscle’s force is stronger than if an isometric contraction at the same position is not preceded by lengthening (black). See, you don’t need Bob! Do force enhancement and force depression impact muscle activity and perception of effort?What do these phenomena of muscle really mean for you? Sure, we can see that our muscles are stronger or weaker depending on whether they are first lengthened or shortened, but can we personally notice if the box feels lighter or heavier? Findings from our lab say yes! The electrical activity of a muscle (measured by a system called EMG) provides insight on the neural input to that muscle, or in other words, how hard the muscle is activating. A study from our lab by Paquin and Power investigated what a muscle’s electrical activity looks like during states of residual force depression and residual force enhancement. They found that, indeed, greater muscle activation is observed in a state of residual force depression, while less activation is observed in a state of residual force enhancement. In the graph above, “%MVC” represents the percentage of maximum muscle strength, and “iEMG” represents the muscle’s electrical activity. It is clear that across all levels of strength, activation is greater during residual force depression (FD in the graph) and less during residual force enhancement (RFE) compared to a regular isometric contraction (ISO). Therefore, we can see that your muscle may requires less input from your brain to hold the box after lowering it from a shelf but may require more input from your brain after lifting the box off the floor. On top of this, you may also perceive that the box is heavier or lighter. A study from our lab by Kozlowski and colleagues investigated whether perception of effort from a Rating of Perceived Exertion scale (below on the left) differs between states of residual force depression and residual force enhancement. They found that perceived effort was lower in a state of residual force enhancement and higher in a state of residual force depression. Like before, in the figure on the right above, “%MVC” represents the percentage of maximal strength. In a state of residual force depression (rTD in the graph, the blue triangles) rating of perceived effort is higher at levels of 40-100% of maximum strength compared to a state of residual force enhancement (rTE, the green circles). Altogether, when you precede an isometric contraction with muscle lengthening, you are stronger, require less muscle activity, and perceive the task to be easier. Conversely, if you precede an isometric contraction with muscle shortening, you are weaker, require more muscle activity, and perceive it to be harder! Why do force depression and force enhancement occur? Even though the history dependence of force was discovered in the 1950s, the mechanisms are still under some debate. The most accepted mechanisms stem from the cellular level of muscle in microscopic units called “sarcomeres”. Sarcomeres generate force in muscle by the formation of “cross-bridges” when proteins called myosin and actin bind together. During muscle shortening, actin becomes deformed, which inhibits formation of some cross-bridges. This inhibition of cross-bridge formation reduces force in the muscle following shortening, leading to residual force depression. In addition to myosin and actin, there is another protein in the sarcomere called titin that acts like a spring when a muscle is stretched. When a muscle is activated, the length of this spring protein is reduced, increasing the tensile force it produces when stretched. This increase in tensile force from titin leads to greater force in the muscle following lengthening, and thereby residual force enhancement. The bigger picture With these mechanisms behind the history dependence of force in mind, the results on muscle activity and perceived effort in states of force depression and force enhancement make sense:
Other research from our lab has focused on attempting to modify these properties through training programs. Hypothetically, if we can train a muscle to increase residual force enhancement and decrease residual force depression, we may be able to improve the efficiency of the connection between the brain and muscle during movement. Doing this may have applications in elderly populations, where that connection is often impaired.
That will have to be a topic for another day, though. For now, I hope you’ve learned what the history dependence of force is, and what it might mean for everyday movements.
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September 2023
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