Do Humans Try to Minimize Energy Expenditure When Controlling Movements? (Hint: No!)
📝 Weekly paper summary
Stephen A. Antos
Category
Cross-sectional study
Context
Utility theory operates on utility functions that mathematically describe the preferences one makes when coordinating actions. For example, Optimal Control theory (Todorov and Jordan, 2002) assumes that the human motor system attempts to minimize endpoint error using the least demanding movement solution possible. Other theories assume humans try to minimize the torques generated to produce a specific action. The common theme is that individuals' preferred movement strategies minimize metabolic energy expenditure.
The central gap with the assumption that individuals prefer movement strategies that minimize energy expenditure is that all current investigations only consider how people move at the point of maximum utility (usually their preferred step length and frequency). However, it is unclear how general this response is along the spectrum of potential movement solutions. For example, suppose an individual perceives two different stepping patterns as equally preferable. Is the individual's energy expenditure the same between both patterns? If energy expenditure dominates the movement solutions an individual prefers, the answer should be that energy expenditure is the same between two patterns of equal preference. Suppose energy expenditure does not dominate the solutions individuals prefer, and perhaps other factors such as balance and general comfort dominate the movement solutions individuals prefer. In that case, there should be differences between the two patterns.
This study aimed to explore whether individuals demonstrated the same metabolic energy expenditure during two different gait patterns of equal preference.
Correctness
- A challenge with this study is that it assumes peoples' preferences were accurately measured (using a two-alternative forced-choice paradigm). However, all we have is an indirect measurement of peoples' preferences.
- Although the study outlines that metabolic cost doesn't drive movement preferences, it cannot comment on which factors shape them.
- Note that movement preferences (i.e., perception) are treated as distinct from control (action) in this theoretical paradigm, with the authors citing different neural mechanisms responsible for each. However, systems-based motor control theories (such as Ecological Dynamics) posit that perception and action are inseparable. Therefore, while the authors of this paper suggest that we can't make inferences about movement control based on their findings, an Ecological perspective might treat these findings more strongly and suggest that systems don't necessarily organize into (perceived) low-energy states which support previous results that the metabolic cost of the actions doesn't primarily drive emergent actions.
Contributions
- Step length-width preferences did not align with participants' energy expenditures, suggesting that other factors must influence peoples' movement preferences.
- This study supports the growing body of literature suggesting peoples' movement preferences don't always represent the most metabolically efficient solution.
- Since (re)training programs aim to change movement behaviours, understanding the main factors influencing movement preferences can help guide program design and coaching strategies.
🧠 Fun fact of the week
As I was in San Francisco for work last week, I walked up to Pier 39 one morning and saw a bunch of sea lions (which I initially thought were seals, but I later learned they are distinguishable by their long front flippers)! Thus, my fun fact this week is related to these animals: sea lions are the only non-human mammal that can keep a beat on their own!
🎙 Podcast recommendation
🗣 Quote of the week
"There are two types of people who will tell you that you cannot make a difference in this world: those who are afraid to try and those who are afraid you will succeed."
- Ray Goforth