This study aims to measure and compare the metabolic response during sub maximal eccentric and concentric isokinetic exercise performed to exhaustion. Fourteen healthy female participants in the early phase of the menstrual cycle will perform concentric exercise with one lower limb and eccentric exercise with the contralateral limb. The assignment of exercise mode to limb and the order of testing will be randomized. The exercise protocol will consist of repeated knee extensor contractions on an isokinetic dynamometer for 10 minutes at 40% of maximal torque, followed by exercise at 70% of maximal torque until exhaustion. Oxygen consumption and respiratory quotient and will be measured continuously throughout exercise using open-circuit spirometry. Muscle oxygenation will be measured also continuously using near infrared spectroscopy. Lactate and pH concentration will be measured at frequent time points during the exercise to exhaustion.

The purpose of this study is to investigate and compare the metabolic responses during sub maximal eccentric and concentric isokinetic exercise performed to exhaustion. Eccentric and concentric muscle actions differ in their physiological and metabolic demands, but their effects on respiratory parameters during fatiguing isokinetic exercise are not fully understood. This study is designed to examine whether these two contraction modes produce distinct metabolic responses under controlled exercise conditions.

Fourteen healthy female participants will be recruited and tested during the early phase of their menstrual cycle in order to reduce the possible influence of hormonal variation on physiological responses. A within-subject design will be used, with each participant completing both exercise conditions. One lower limb will perform concentric exercise and the contralateral lower limb will perform eccentric exercise. The limb allocation and the order of the two exercise conditions will be randomized.

All exercise testing will be conducted on an isokinetic dynamometer. The protocol will involve repeated contractions of the knee extensor muscles. Each trial will begin with 10 minutes of exercise at 40% of maximal torque. Following this initial stage, the intensity will be increased to 70% of maximal torque and maintained until volitional exhaustion is reached.

Oxygen consumption and respiratory quotient will be measured continuously throughout the exercise bout using open-circuit spirometry. Continuous monitoring of gas exchange during exercise will allow evaluation of substrate utilization and metabolic responses associated with each mode of contraction. Muscle oxygenation will be measured also continuously using near infrared spectroscopy. Lactate and pH concentration will be measured at frequent time points during the exercise to exhaustion.

The study is expected to provide further insight into the metabolic and respiratory differences between eccentric and concentric exercise performed under submaximal but fatiguing isokinetic conditions. The findings may have implications for exercise testing, training prescription, and rehabilitation settings in which eccentric and concentric muscle actions are used for different physiological purposes.

Inclusion Criteria:

• Physically active young females
• No experience in activities involving significant resistance exercise components
• No experience in muscle-damaging activities for at least 6-months prior to the study.

Exclusion Criteria:

• Any musculoskeletal injuries in the lower limbs within the past 6 months
• Use of contraceptives
• Irregular menstrual cycles
• Smoking
• Consumption of protein and antioxidant supplements
• Potential inflammatory conditions.

Individual participant data will not be made publicly available.