Complex sentence: However, it is also common in older horses whose age leads to loss of muscle tone and stretched ligaments; or it occurs due to overuse or injury to the muscles and ligaments from excess work or loads; or from premature work on an immature animal.
Split into simpler sentences: However, it is also common in older horses whose age leads to loss of muscle tone and stretched ligaments;. It also occurs due to overuse or injury to the muscles and ligaments from excess work or loads, or from premature work placed upon an immature animal.