The benefits of regular resistance training (RT) are well acknowledged, but it is also well known that tremendous inter-individual variability can be detected in responses to RT. The reason(s) for individual variations in responses to RT is a very complex physiological phenomenon and is still poorly known. The individual variation in trainability suggests genetic diversity but also non-genetic determinants potentially contribute significantly to RT responses.

The investigators hypothesize that in this study, a broad spectrum of adaptive responses to RT are detected. By scrutinizing participants' backgrounds, it can be elucidated why individuals respond differently to regular RT. Furthermore, it is hypothesized that the investigators can identify specific predictive markers for RT responsiveness. That is possible by combining information on training responsiveness with the personal characteristics of the participant.

Healthy young adults aged 18-50 will be recruited to the study in two periods to understand the biological basis of heterogeneity in RT responses by minimizing potential age and health-related physiological confounders. They are premised to respond positively to the study's primary outcome, which is the cross-sectional area (CSA) of the m. vastus lateralis (VL). To collect comprehensive data, the number of participants is maximized within the practical limitations based on our previous and extensive RT studies. Therefore, of the 400 recruited participants, 240 will first undergo 12 weeks of RT (data collection I), after which 160 participants will perform an RT period with a similar design (data collection II). Thus, an RT period will be conducted in two separate periods for both of these subsamples, as 240 is the maximum number of participants that can be supervised with our comprehensive and time-consuming physiological measures within the allocated data collection timeframe. Participants comprise an equal proportion of males and females, and sex differences in RT responsiveness will be investigated as a secondary aim of the study. Combined, data collection I and II are referred to as intervention I.

The study design of a data collection I is a single-arm trial. After assessing eligibility, participants engage in the baseline measurements and the 12-week fully supervised RT intervention. Additionally, we employ a dual-trial -design in data collection II in which the one arm performs a non-RT control period before an RT period, whilst the other arm only performs RT. Moreover, test-retest baseline measurements will be performed in data collection II. The RT protocol is similar for both data collections. The participants will train two times a week, and the program will target all major muscle groups. Each training session includes exercises for the lower (leg press, knee extension) and upper body (bench press, biceps curl, and seated row). The participants will perform \~10 repetitions per set (approximately 60-80% of 1RM) and three sets per exercise twice per week, for the first two weeks as a familiarization. After that, the training load will be at an 8- to 12-RM zone individualized for each participant for three sets per exercise. Loads are increased progressively in each exercise during the RT period using double progression. A questionnaire on perceived exertion after each exercise session (sRPE) will be obtained to evaluate the participant's intrinsic effort in performing exercises. The training is executed in a local university gym with a standardized time of day, and training diaries are used to track the training loads.

The measurements are obtained at baseline and after the intervention in data collection I. Furthermore, mid-measurements are also performed in data collection II. Participants are informed of the study goals and are carefully familiarized with study protocols. All the tests are carried out at the same time of day. The participants are given feedback on their test results during the project. In addition to genetics, environmental factors are essential in explaining individuality in training responses. In this project, the investigators focus on gathering comprehensive data on variables of participants' background, nutrition, health status, and physical activity that can potentially influence the heterogeneity of RT adaptations.