Acute Mental Fatigue and Muscle Strength

This study will investigate the acute effects of mental fatigue on lower-extremity muscle strength, interlimb symmetry, and neuromuscular performance in healthy young adults. The study will use a within-subject, repeated-measures design in which participants complete lower-extremity strength testing immediately before and after a standardized computer-based mental fatigue protocol. Each participant's pre-fatigue measurements will serve as the baseline comparison for their post-fatigue measurements.

Participants will complete one experimental session lasting up to approximately two hours at the Florida Institute for Human and Machine Cognition (IHMC) Healthspan, Resilience, and Performance Research Complex. Before testing, participants will undergo eligibility screening, provide written informed consent, and complete familiarization with the strength-testing procedures and computer-based cognitive tasks. Following familiarization, participants will complete a standardized rest period of at least five minutes before baseline measurements are collected.

Lower-extremity muscle strength will be assessed using a Biodex isokinetic dynamometer. Participants will perform knee extension and flexion with both lower extremities at three different movement speeds to assess strength across multiple testing conditions. Peak torque will be used as a primary measure of muscle strength. Testing of both limbs will also allow calculation of a Limb Symmetry Index (LSI) to quantify differences in strength between the lower extremities. Participants will rate their perceived physical exertion following each strength assessment.

Surface electromyography (EMG) will be used to record muscle activity from selected muscles of both thighs during strength testing. Participants may also perform maximal voluntary isometric contractions before the primary strength assessment to assist with normalization and interpretation of EMG signals. EMG measurements will be used to examine whether changes in strength following the mental fatigue protocol are accompanied by changes in muscle activity.

Following the baseline strength assessment, participants will complete approximately 30 minutes of standardized computer-based cognitive tasks designed to require sustained attention, concentration, reaction time, working memory, and mental effort and thereby induce acute mental fatigue. Tasks may include a psychomotor vigilance task, an incongruent Stroop task, and an n-back working-memory task. Reaction time and response accuracy may be recorded during the cognitive tasks.

Subjective mental fatigue and sleepiness will be assessed before and after the mental fatigue protocol. Following the cognitive tasks, participants will also rate the mental effort required to complete the tasks. These measures will be used to characterize the subjective response to the mental fatigue protocol and evaluate whether participants experienced increased mental fatigue following the intervention.

Heart rate and cardiac activity may be continuously monitored using a Polar H10 chest sensor during the experimental session to provide additional information regarding physiological responses during the strength-testing and cognitive-task portions of the protocol. Brain electrical activity may also be recorded using noninvasive electroencephalography (EEG) during the cognitive tasks in a subset of participants.

Immediately following completion of the mental fatigue protocol and post-task questionnaires, participants will repeat the same bilateral Biodex strength assessment performed at baseline. Testing procedures, positioning, movement speeds, instructions, and outcome measurements will be kept consistent between the pre- and post-fatigue assessments. Surface EMG will again be recorded during strength testing, and participants will rate their perceived physical exertion.

The primary analyses will compare pre- and post-fatigue measures of lower-extremity muscle strength and limb symmetry to determine whether acute mental fatigue is associated with changes in peak torque or LSI. Additional analyses will examine subjective mental fatigue, sleepiness, mental effort, perceived physical exertion, muscle activity, cognitive-task performance, and physiological responses. The findings will help determine whether acute mental fatigue should be considered when measuring and interpreting lower-extremity strength and limb symmetry in research, rehabilitation, and human performance settings.