The common assumption is that lifting a weight that is “heavy” predisposes a trainee to a musculoskeletal injury. The reality is that the weight is just one part of the equation. During a strength training exercise, the goal should be to minimize the external forces imposed on a joint, bone, or connective tissue. “Force” is a mass (the weight that you are lifting) multiplied by the speed used when lifting the mass. If you lift a relatively light or moderate weight quickly (as most people do), the external force that the joints, bones, and connective tissues are exposed to are positively enormous. If you lift and lower a weight slowly (even a heavy weight), the forces that the body encounters are dramatically reduced.
Take home message: Relatively “heavy” weights are not inherently dangerous. Fast speeds and subsequent high “force” is dangerous.