There are multiple factors that impact bone health, including birth weight, nutrition, medications for seizures and/or reflux, genetics, and physical activity. Targeted exercise to improve bone health in childhood can be sustained into adulthood, and childhood is the best time to promote bone health.
Spasticity is usually caused by central nervous system injuries, such as brain injuries or spinal cord injuries. Spasticity is a little different from stiffness.
Exploration for an infant means discovering anything about that environment. If that infant needs an opportunity to be brought to them, that's okay. Let an infant explore through their senses, whether it's touch, or smell, or taste, or sight, or hearing.