Why pushing harder can briefly make thinking worse
The most direct comparison comes from a study that put people through a single bout of high-intensity versus moderate-intensity resistance exercise and then tested cognition. High intensity came out worse [1]. That is a within-study contrast between two doses, which makes it stronger evidence than simply noting that hard exercise feels difficult.
A plausible explanation is that very hard exercise competes for the same resources the brain needs to think clearly. The resistance-exercise study also measured oxygenation in the prefrontal cortex, the front part of the brain involved in focus and control [1]. The finding is consistent with the idea that at extreme intensity the body's demands briefly outweigh what the brain gets, but the study does not establish that as the cause.
The same hard session can look bad now and good later
Timing matters as much as intensity. In children, maximal-intensity exercise was associated with both transient impairments and improvements in memory and attention, depending on when performance was measured [5]. So a single test right after the session can catch the dip, while a later test may catch the rebound.
This is why "does it plateau or decline" has no single answer. The question is really about the shape of the curve over minutes and hours, not just the height of the dose.
High intensity is not automatically harmful
Very hard exercise is not uniformly bad for cognition. In people with multiple sclerosis, a high-intensity interval aerobic programme improved verbal memory and executive functions compared with a standard exercise programme [4]. That is a training programme over time, not a single exhausting session, and it involved a clinical group — both of which may change the result.
The contrast is instructive: a hard interval programme can help, while a single maximal effort can hurt. Dose and duration are different things, and the evidence treats them differently.
What researchers are still trying to pin down
One study protocol describes a six-month high-intensity exercise intervention in older adults, comparing high-intensity, moderate-intensity, and control groups with careful monitoring to hit a target exercise dose [3]. That design is aimed at the long-term question — whether sustained high doses help, plateau, or backfire — which short single-session studies cannot answer.
A separate comparison of high-intensity interval exercise and moderate-intensity continuous exercise looked at immediate and sustained effects on cognitive flexibility in young adults, including a protocol targeting 90% of heart-rate reserve [2]. It did not test whether benefits plateau or decline at the very top of the range, so the upper end of the dose curve remains the least settled part of the picture.
Sources used in this answer
Effects of acute high-intensity resistance exercise on cognitive function and oxygenation in prefrontal cortex
A single bout of high-intensity resistance exercise led to worse cognitive performance than moderate-intensity exercise, and prefrontal cortex oxygenation was also measured.
Comparison of the sustainability effects of high-intensity interval exercise and moderate-intensity continuous exercise on cognitive flexibility
High-intensity interval exercise and moderate-intensity continuous exercise were compared for immediate and sustained effects on cognitive flexibility in young adults, but the study did not test whether benefits plateau or decline at very high doses.
Study protocol of the Intense Physical Activity and Cognition study: the effect of high‐intensity exercise training on cognitive function in older adults
A study protocol describes a six-month high-intensity exercise intervention in older adults comparing high-intensity, moderate-intensity, and control groups with rigorous dose monitoring.
High-intensity interval exercise improves cognitive performance and reduces matrix metalloproteinases-2 serum levels in persons with multiple sclerosis: A …
A high-intensity interval aerobic programme improved verbal memory and executive functions in people with multiple sclerosis compared with a standard exercise programme.
The effects of maximal intensity exercise on cognitive performance in children
Maximal-intensity exercise in children was associated with both transient impairments and improvements in memory and attention, depending on when cognition was measured.