The BEST example of a concentric movement that Ernesto could add to help him build strength and add bulk to his muscles is sit-ups.
In a sit-up, the abdominal muscles contract as Ernesto lifts his upper body off the ground, which is a concentric contraction. This type of movement is effective for strength training, particularly for the core, and can contribute to muscle growth when performed with the appropriate resistance and volume.
The other options (downhill runs, wall sits, and yoga stretches) either focus on different muscle actions (like eccentric or isometric) or overall endurance rather than specifically building strength through concentric movements.