Load-bearing exercise creates mechanical strain in bone, and bone responds to that strain by remodeling to become stronger where the stress is greatest. Osteocytes sense the strain and send signals that favor bone formation by osteoblasts, tilting the balance toward new bone mineral deposition and a denser, stronger structure over time. This adaptation aligns with Wolff’s law, which states bone grows and remodels in response to the forces placed on it. Calcium intake helps support bone health, but without the mechanical stimulus, the remodeling response necessary to increase density is limited. So the primary mechanism is the strain-induced remodeling that promotes bone formation in stressed areas.

Focusing on technique with multi-joint movements builds a foundation for safe and effective strength gains. These compound patterns—like squats, deadlifts, presses, and rows—teach coordinated control across multiple joints, recruit large muscle groups, and develop core stability. When a novice prioritizes form, the nervous system learns efficient movement and stabilizer strength, which lowers injury risk and creates a solid base for progression. Once technique is solid, loads and volume can be increased safely. Isolating a single joint doesn’t establish the broad motor control and strength base needed early on, and it can be less functional for real-life tasks. Jumping into a 5–6 day-per-week schedule too soon risks overtraining and form breakdown, which undermines progress and safety. Starting with maximal loads to establish a baseline can compromise technique and injury risk before proper movement patterns are ingrained.

For very short, high-intensity efforts, the muscles rely on the phosphagen energy system because it can produce ATP extremely quickly from stored ATP and phosphocreatine without needing oxygen. This rapid ATP regeneration supports maximal effort right at the start, which is crucial for efforts in the 5–15 second window. The capacity of this system is limited, so its dominance is strongest in that brief burst before other systems take over. Lipid oxidation is too slow for high-intensity bursts, and oxidative phosphorylation depends on oxygen and lags in onset, making them unsuitable for powering such short, intense efforts.

MET represents the energy cost of activity relative to resting metabolic rate, and 1 MET is about 1 kcal/kg/hour. To estimate calories burned, you multiply the activity’s MET value by body weight in kilograms and the duration of the activity in hours. So the correct formula is energy expenditure (kcal) = MET × body weight (kg) × time (hours). This accounts for how heavier individuals burn more energy and how longer activity increases total calories. For example, a 70 kg person doing an activity at 6 MET for 0.5 hours would burn 6 × 70 × 0.5 = 210 kcal. The other options omit either weight or time or mix incompatible units, which prevents accurate kcal calculation. If time is given in minutes, convert to hours first (minutes ÷ 60).

When designing resistance training for youth, safety and gradual development of movement skills are the priorities. The best approach centers on teaching proper technique first, using loads well below maximal effort, and emphasizing multi-joint movements that train multiple muscle groups and mirror real-life tasks. This foundation supports functional strength and reduces injury risk as the neuromuscular system and growth factors are developing. Progression should be age-appropriate and gradual, allowing for adaptations in strength, coordination, and confidence without overloading developing joints or growth plates. Adequate supervision ensures correct technique, safe exercise selection, appropriate spotting when needed, and a positive training environment. Recovery is essential, including rest between sessions and attention to sleep and nutrition, to support repair and continued growth. Choosing maximal loads early tends to increase injury risk and can outpace a young athlete’s maturation. Relying exclusively on single-joint isolation movements misses the value of functional, multi-joint training that better translates to everyday activities and athletic performance. And training without supervision or with ignored recovery creates unsafe conditions and a higher chance of overuse injuries.