Effects of Caffeine on Voluntary Force Estimation During Isometric Exercises

Background: Caffeine is widely used as an ergogenic aid to enhance strength performance; however, its effects on perceptual accuracy during submaximal force regulation remain unclear, particularly in multi-joint isometric tasks. This study examined whether caffeine ingestion influences maximal isometric force production and the accuracy of voluntary submaximal force estimation during complex isometric exercises. Methods: Seventeen recreationally trained participants completed a randomized, double-blind, placebo-controlled crossover study. Participants ingested either caffeine (4 mg·kg^-1 body mass) or a placebo before performing an isometric squat test (ISqT) and an isometric mid-thigh pull test (IMTP). Maximal voluntary contractions were assessed, followed by freely estimated submaximal efforts at 50% and 75% of perceived maximal force. Relative peak force and discrepancies between prescribed and exerted force (estimation error) were analyzed, with discrepancies calculated as the difference between exerted force and the prescribed target intensity. Results: Caffeine ingestion did not significantly affect relative peak force during maximal isometric efforts nor improve the accuracy of voluntary submaximal force estimation. Regardless of supplementation conditions, participants consistently misestimated submaximal efforts, tending to overproduce force, particularly at lower intensities. The IMTP showed a closer approximation to prescribed submaximal targets than the ISqT. Conclusions: Ingesting 4 mg·kg^-1 of caffeine does not enhance maximal isometric force output or perceptual accuracy during voluntary submaximal force regulation in multi-joint isometric tasks. Prescribing isometric intensity based solely on perceived effort may therefore be unreliable under these specific testing conditions, particularly at lower intensities.