Metabolic equivalents (METs) classify exercise intensity by energy cost: light is about 1.6–2.9 METs, moderate is 3–5.9 METs, and vigorous is 6 METs and above. A brisk walk at 4 mph is roughly 5.0 METs, which falls squarely in the moderate range. This level means you’re elevating heart rate and breathing enough to notice effort, but you can still talk in phrases (not sing). Near-maximal would imply a much higher effort close to your max, which isn’t indicated by 5.0 METs. So the correct classification is moderate.

Short, all-out efforts with brief, near-full recoveries define Sprint Interval Training. The 3 x 20 protocol fits this pattern: three 20-second maximal sprints with enough rest to recover between each push. This structure aims to tax the anaerobic energy systems quickly and repeatedly, boosting sprint speed and anaerobic capacity. If we contrast with the other options, you can see why this fits SIT best. High-intensity interval training often uses longer work bouts at high but not necessarily maximal effort, with varied rest and more flexible pacing. Four-minute work intervals, as in a 4 x 4 protocol, create a different endurance stimulus and are not the very short, all-out sprints that define SIT. High Intensity Functional Training is a broader method emphasizing functional moves and varied circuits rather than a defined short-sprint interval pattern. So the specific three 20-second, maximal-effort sprints with recovery align most closely with Sprint Interval Training.

Bone-strengthening activities for children and adolescents should provide regular, substantial forces on the bones to stimulate growth and mineral accrual, while staying safe and age-appropriate. The best choice matches three key ideas: perform bone-loading activities on at least three days per week, use moderate to high loading that meaningfully stresses the bones, and accumulate a meaningful amount of time with a variety of weight-bearing activities that challenge the skeletal system. This option fits because it prescribes at least three days per week, involves moderate to high bone loading, and recommends 60 minutes or more per day with activities such as running, jump rope, basketball, tennis, resistance training, and playful weight-bearing moves like hopscotch. It combines enough frequency, intensity, and duration to promote bone health during growth, and it includes a range of activities to safely load the bones in different ways. The other choices fall short in either loading intensity, frequency, duration, or variety: one has only light loading and limited days; another emphasizes a single high-impact activity for a short duration; and the last suggests very high loading every day, which can increase injury risk for youth and isn’t typically recommended as a daily routine.

Injury prevention in physical activity programs relies on proactive, multi-faceted strategies that prepare the body, manage how demands increase, protect vulnerable areas, and teach safe practices. A proper warm-up increases blood flow and flexibility, helping muscles and joints handle activity and reducing strain. Gradual progression ensures the body adapts to higher demands over time instead of jumping into intense work that can cause injuries. Using protective equipment when needed, maintaining a safe environment, and delivering education on safe exercise techniques all support safer participation and proper form. This combination targets prevention before problems arise, rather than reacting after an injury. Options that focus only on treatment after injury, suddenly increasing intensity without protection, or ignoring safety guidelines do not provide the same protective effect.