Abstract
Task failure during sustained exercise is often defined mechanically as the inability to maintain a prescribed force, yet cessation may reflect a developing decision to disengage. This study characterized the temporal dynamics of this process and tested whether imminent task failure could be anticipated from multimodal signals. Twenty-three healthy adults performed a sustained isometric knee extension at 20% maximal voluntary contraction (MVC) until task failure. During exercise, auditory probes required participants to indicate whether, since the previous probe, they had thought about or felt the need to stop. Neuromuscular function was assessed before and immediately after exercise, and seven subjective dimensions were reconstructed using Temporal Experience Tracing. Probe-level data were entered into ridge-penalized within-participant Cox proportional hazards models using physiological-behavioral, subjective, and full feature sets; discrimination was quantified with Harrell’s C-index. Participants reached failure after 549 ± 336 s. The first “yes” response occurred at 45% of individual task duration, showing that disengagement-related thoughts preceded failure. Force output remained stable, whereas knee-extensor EMG increased; MVC force, voluntary activation, and 100-Hz doublet amplitude decreased post-exercise, confirming central and peripheral fatigue. Cox models ranked probes by proximity to failure above chance in all participants. The experiential (C-index = 0.988 ± 0.003) and full models (0.984 ± 0.002) outperformed the physiological-behavioral model (0.882 ± 0.014; p < 0.001) and anticipated failure ~50–54 s before cessation. Perceived physical fatigue, mental fatigue, frustration, reduced motivation, EMG activity, and “yes” responses were the main contributors. Task failure reflects a cumulative, multimodal process rather than physiological failure.