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 exercise, the goal should be to minimize the external forces imposed on a joint, bone, or connective tissue. "Force" is a mass (the weight) multiplied by the speed used when lifting the mass. If you lift a relatively light or moderate weight quickly/explosively (as many people do), the external force that the joints, bones and connective tissues are exposed to are positively enormous (100lbs lifted explosively can translate into 800lbs of force on the body). If you lift and lower the weight slowly (4-1-4), even a heavy weight, the forces the body encounters are dramatically reduced and for the most part very safe.
Take home message: Relatively "heavy" weights are not inherently dangerous. Fast lifting speeds and the subsequent high "force" is dangerous.
