Using metabolic energy to quantify the subjective value of physical effort

Abstract

Economists have known for centuries that to understand an individual's decisions, we must consider not only the objective value of the goal at stake, but its subjective value as well. However, achieving that goal ultimately requires expenditure of effort. Surprisingly, despite the ubiquitous role of effort in decision-making and movement, we currently do not understand how effort is subjectively valued in daily movements. Part of the difficulty arises from the lack of an objective measure of effort. Here, we use a physiological approach to address this knowledge gap. We quantified objective effort costs by measuring metabolic cost via expired gas analysis as participants performed a reaching task against increasing resistance. We then used neuroeconomic methods to quantify each individual's subjective valuation of effort. Rather than the diminishing sensitivity observed in reward valuation, effort was valued objectively, on average. This is significantly less than the near-quadratic sensitivity to effort observed previously in force-based motor tasks. Moreover, there was significant inter-individual variability with many participants undervaluing or overvaluing effort. These findings demonstrate that in contrast with monetary decisions in which subjective value exhibits diminishing marginal returns, effort costs are valued more objectively in low-effort reaching movements common in daily life.

Metabolic costs of walking and arm reaching in persons with mild multiple sclerosis

“…Similar to walking, reaching metabolic costs vary with factors such as speed, mass, distance, and age (47, 71). We appear to be the first to measure the metabolic costs of arm reaching in pwMS and demonstrate that across a range of feasible reach speeds, the costs were not related to MS disease state.…”

“…While not the primary focus of the study, a second limitation is that we did not measure preferred walking or reaching speeds. Both preferred walking and reaching speeds are often selected to minimize the cost of transport (J kg −1 m −1 ) of the movement (33, 47, 71). Recent evidence suggests that pwMS minimize walking cost of transport when selecting preferred speeds (45, 74), but that the costs tend to be more expensive than controls.…”

“…Walking and reaching metabolic data were analyzed using linear mixed effects regression models (LMER) predicting metabolic power. Due to the established nonlinear relationship between movement speed and metabolic power in both walking and reaching (33, 47), power was log-transformed to best satisfy the standard regression assumptions of linearity, homoscedasticity, and normality. Fixed effects were the movement speed, a binary MS indicator (pwMS = 1), and their interaction.…”

“…A movement that is less metabolically costly than walking is seated arm reaching (32, 47), but arm reaching energetics have never been explored for pwMS despite large percentages of individuals reporting upper limb impairment (48). Accordingly, the purpose of our study was to investigate whether the metabolic costs of both walking and seated arm reaching are higher for persons with mild MS as compared to healthy age- and sex-matched control participants (HCs).…”

Learning vs. minding: How subjective costs can mask motor learning

“…Our model assumes that objective effort is similar in younger and older adults, which is consistent with measured metabolic power in Huang and Ahmed 2014 [29]; thus, our results suggest that it is the subjective weighting of effort that differs. [48]. An alternative explanation is that the objective effort costs are higher in older adults compared to younger adults.…”