A properly conducted Heavy Duty workout involves the utilization of weights that allow for up to ten reps to failure. Such is properly referred to as a high-intensity, low-force program, and is quite safe.
Some are reluctant to embark on a high-intensity program because they mistakenly believe that the last rep of a set carried to a point of momentary muscular failure is dangerous. On the contrary. The last rep of a set carried to failure should be the safest because, by the last rep, the trainee is at his weakest, barely able to generate enough force to complete the rep. It is the first few reps, when the trainee is freshest, and able to exert more force than required to move the weight, that are the most dangerous. If, for instance, you are doing a set of Barbell Curls with 100 pounds for ten reps to failure, no rep requires that you exert much more than 100 pounds of force to complete it. On the first few reps, when the individual is fresh, he usually completes them relatively quickly, exerting well above 100 pounds of force. On the last rep, he is barely able to move the weight at all, exerting just enough force to complete it - maybe 101.1 pounds. Therefore, the last rep is the safest so long as proper technique is employed.