Examining the Alignment between Subjective Effort and Objective Force Production

Abstract

Ratings of Perceived Exertion (RPE) are frequently used to prescribe exercise intensity. A central assumption of using RPE scales is that the subjective perception of effort maps onto objective performance in a consistent way. However, the degree and shape of how RPE aligns with objective performance is not fully understood. Here we investigate that degree and shape of alignment, as well as how time (i.e., how often has an effort to be performed) and mental effort (i.e., if one has to invest mental effort as well as physical effort) covary with the alignment. In a randomized within-subjects experiment, we used a grip-to-scale method that asked participants (N= 43) to repeatedly squeeze a handgrip transducer with four to-be-produced RPE target levels relative to their maximum strength (representing 20%, 40%, 60% or 80%). Across all levels, participants applied less force than the equivalent fraction of their maximum voluntary contraction strength, and RPE and the resulting objective force were not linearly aligned. In addition, at constant RPE-levels, participants produced less force over time and this effect was moderated by the to-be-produced RPE-levels. Lastly, anticipating mental effort after the physical effort altered the alignment as a function of the to-be-produced RPE-level and experimental duration. Taken together, our results indicate that perceived effort and objective performance do not straightforwardly map onto one another, and several factors can affect the degree and shape of how RPE and performance align. These findings have implications for scientific and applied contexts were RPE is used to prescribe exercise intensity.

The Relationship Between Prescribed Ratings of Perceived Exertion and Force Production in Repeated Isometric Contractions

“…Consistent with this, participants found producing moderate RPE levels to be the most challenging, and they produced relatively lower forces at these RPE levels. This tentative interpretation is consistent with recent work, arguing that effort costs are integrated across domains and that the specific costs vary as a function of the required effort level (Rewitz et al, 2023). In summary, there appear to be various variables that influence the relationship between RPE and force production, and more research is needed to clarify the impact of these variables.…”

“…As we have shown, force output prescribed by RPE does not necessarily follow an exclusively linear trend but can as well be described using quadratic and cubic functions. This means that although higher effort ratings generally result in higher force output, there is no equidistance between subjective increases in effort and objective increases in force (Rewitz et al, 2023). Interestingly, when participants had access to visual feedback, the force output was only described by a linear function.…”

“…Considerably fewer studies have used the prescription approach, and of those, even fewer focused on physical tasks that are not aerobically based (i.e., resistance exercises). Research on the link between RPE and resistance-based tasks has yielded heterogenous results, as both linear and non-linear associations have been reported (Cochran et al, 2007;Li & Yu, 2011;Morrin et al, 2018;Pincivero, 2011;Rewitz et al, 2023). To illustrate, in some studies, a linear increase in RPE mapped onto a linear increase in the produced force (Li & Yu, 2011;Morrin et al, 2018), whereas in other studies, a linear increase in RPE led to both linear and quadratic increases in the produced force (Cochran et al, 2007;Rewitz et al, 2023;Pincivero, 2011).…”

“…Research on the link between RPE and resistance-based tasks has yielded heterogenous results, as both linear and non-linear associations have been reported (Cochran et al, 2007;Li & Yu, 2011;Morrin et al, 2018;Pincivero, 2011;Rewitz et al, 2023). To illustrate, in some studies, a linear increase in RPE mapped onto a linear increase in the produced force (Li & Yu, 2011;Morrin et al, 2018), whereas in other studies, a linear increase in RPE led to both linear and quadratic increases in the produced force (Cochran et al, 2007;Rewitz et al, 2023;Pincivero, 2011). The inconsistencies between studies are potentially due to differences in the implemented tasks (e.g., handgrip squeeze vs. knee extension), contraction and rest durations (Farah et al, 2012;Diniz et al, 2014), and the number of prescribed RPE levels, which ranged from four to ten in the aforementioned studies.…”