It's so refreshing to find sustainably produced exercise equipment, especially with rampant overuse of plastics. These little balls are firm with just the right amount of give for working with resistant body tissues. I've had chronic pain and tightness in my calves and hips, and these little guys do the work that stretching can't! Highly recommend. The little ball is also genius and I've used it on my jaw, brow, and neck with positive results:)