Influence of Training Surface and Mechanical Load on the Prevalence of Patellofemoral Pain in Recreational Runners

This variable will be defined according to the International Consensus on Patellofemoral Pain. Participants will be classified as positive cases if they report anterior or retropatellar knee pain in the previous three months, exacerbated by at least one weight-bearing, knee-flexion activity (e.g., running, stair ambulation, slopes, or squatting), with a duration of ≥4 weeks or ≥2 episodes during that period

Functional severity will be assessed using the Spanish-validated version of the Kujala Anterior Knee Pain Scale. This is a condition-specific 13-item questionnaire that evaluates functional limitations, symptoms, and the ability to perform knee-loading activities. Total scores range from 0 to 100, with higher scores indicating better function and, therefore, lower severity.

This variable will capture the type of terrain on which the runner habitually trains (asphalt, trail, and mountain). Participants will report the approximate percentage of total kilometers completed on asphalt, trail, and mountain terrain over the previous three months.

This variable will be recorded as an average value (meters per week, or normalized as meters per 10 km), given that eccentric loading and patellofemoral joint stress increase substantially during prolonged downhill running.

If mountain terrain is predominant, an additional item will be included to capture the percentage of kilometers run on downhills with a gradient ≥6%.

Training intensity will be assessed using Foster's session Rating of Perceived Exertion (sRPE), based on the Borg CR-10 scale (0-10). Participants will report their mean perceived exertion for each session type (e.g., easy runs, intervals, hill sessions) over the previous four weeks. Weekly internal load will be calculated by multiplying the sRPE value by session duration (minutes).

This variable will include weekly running distance (kilometers), training frequency (number of sessions per week), and session duration (minutes per session).

This variable will be recorded as minutes per kilometer (min/km).

This potential confounding or moderating variable will be measured in years.

This potential confounding or moderating variable will be recorded as a dichotomous variable (female/male).

This potential confounding or moderating variable will be calculated as kg/m².

This potential confounding or moderating variable will be assessed as a dichotomous variable by asking: "Have you experienced knee pain or injury in the past 24 months?" (yes/no).

This potential confounding or moderating variable will be measured as the number of years of running.

This potential confounding or moderating variable will be recorded as the number of training sessions per week.

This potential confounding or moderating variable will be measured as the total elevation gain per week.

This potential confounding or moderating variable will be measured as the number of days per week dedicated to lower-limb and core strength exercises.

This potential confounding or moderating variable will be assessed as a dichotomous variable indicating the use of two or more pairs of running shoes in rotation (yes/no).