In two preregistered experiments, we investigated whether covert language is involved in sustained physical efforts, specifically if people are less able to push themselves physically when distracted from using inner speech. In both experiments, participants performed 12 cycling trials (Experiment 1: N = 49; Experiment 2: N = 50), each lasting 1 min where participants were required to cycle as fast as possible while simultaneously engaging in either a visuospatial task, a verbal task or no interference. Experiment 1: Participants performed worse in the verbal interference condition compared with the control condition (d = 0.29) and verbal interference performance was numerically but not significantly worse than visuospatial interference (d = 0.22). Experiment 2: A more demanding interference task yielded significant slower cycling with verbal interference compared to both control (d = 1) and visuospatial interference (d = 0.43). These results indicate that inner speech plays a causal role in control of sustained physical efforts.