Evaluating the human capacity of carrying loads without costs: A scoping review of the Free-Ride phenomenon

Objectives Several researchers have obtained discordant results testing the human capability to transport loads without added locomotion costs. Carrying loads has an extended relevance to human ecology, thus a review of the Free-Ride phenomenon detection according to the existing literature is needed. Method A search was made in November 2021 in SCOPUS, Google Scholar, and Web of Science to identify studies comparing the energy expenditure of loaded and unloaded locomotion. Descriptive percentages were calculated with the data obtained from each study, and a Chi-squared (chi(2)) independency test and a contingency table were applied to observe the relationship between sample characteristics, experimental procedures, and the detection of the Free-Ride. Results A total of 45 studies met the inclusion criteria. Eighty two percent do not detect the Free-Ride phenomenon. The general detection is independent of sex, experience, load position, load size, and speed (p value >.05) although the papers detecting the Free-Ride have some common characteristics. Conclusion The literature does not support a Free-Ride capacity, but future research testing this phenomenon should consider the load size, the load position, the level of expertise, or the speed. As the Free-Ride is not generalizable, it can be hypothesized that other mechanisms may have emerged during human evolution to buffer the energetic demands of load carrying.