The aim of this study is to analyze the effect of a motivational online intervention (MOI) using pedometers to increase physical activity in a sample of sedentary students. It will be compared three conditions: MOI+PEDOMETER condition (access to a MOI and the use of a pedometer of new generation); MOI condition (access to a MOI and the use of a blinded pedometer); and CONTROL condition (the use of a blinded pedometer).

Regular practice of physical activity (PA) has shown benefits in physical and mental health. However, the levels of PA are low in the general population. Pedometers and motivational online interventions (MOI) have been used to increase PA levels in sedentary population.

In spite of finding positive outcomes on PA levels when theoretical constructs derived from theories of behavior change and Internet have been used to guide pedometer-based interventions, few interventions have incorporated all these variables together and have analyzed the short-term or mid-term results.

The main objective of this study is to analyze the effect of a short self-administered Motivational Online Intervention (MOI) using pedometers to increase PA levels (primary outcome) and to change different theoretical constructs related to the PA behavior (positive decisional balance, self-efficacy, processes of change, stage of change, and enjoyment) (secondary outcomes) in a sample of sedentary students after 3-weeks intervention (post) and after 3-months (follow-up). It will be compared three conditions: MOI+PEDOMETER condition (access to a MOI and the use of a pedometer of new generation); MOI condition (access to a MOI and the use of blinded pedometer); and CONTROL condition (the use of a blinded pedometer). Hence, two conditions will include components of the "Transtheoretical Model of Change" in the intervention, such as the facilitation of the decisional balance, the increase of the self-efficacy, and the use of experiential and behavioral processes of change (e.g., consciousness raising, counterconditioning, reinforcement management, stimulus control), but only one of these condition will receive the feedback of the pedometer that will allow self-monitoring the behavior.

Inclusion Criteria:

• to practice less than 30 minutes of moderate-intensity physical activity, three times a week;
• to be sedentary or low active (to walk less than 7500 daily steps during a week
• an age between 18 and 40 years old.

Exclusion Criteria:

• to be already using a pedometer;
• to suffer from a physical and/or a psychological disorder to prevent from practicing physical activity;
• to be already participating in other intervention to practice physical activity; and
• history of drug or alcohol addiction.