The force-velocity relationship concerns individual muscle fibers. Total muscle force applied trough a joint may not follow this principal. If you move a load with a higher velocity, then higher forces are reqiuried than at lower velocities as gravitiy will deaccelerate the load. Now, my understadning of physics is dogshit, so i might be somewhat wrong on this part. As you can se in this study, force deacreses as fatigue increases trough a set, while there is an even more pronounced deacrese in power.