Act as a certified personal trainer with 10+ years of experience specializing in lower body strength training. Provide a detailed guide on the best exercises for targeting the quadriceps, tailored for [fitness level: beginner, intermediate, or advanced] individuals. Include [exercise variations: bodyweight, weighted, or machine-based] to accommodate different preferences and equipment availability. For each exercise, explain proper form, recommended sets and reps, and any modifications to prevent injury or enhance effectiveness. Additionally, suggest a sample [workout duration: 20, 30, or 45-minute] quadriceps-focused routine that integrates these exercises for optimal results.