Well based on some research ive done, as well as personal experience. I dont think that going to failure is really essential for progress. Ive worked out alone many times and i still have great chest workouts doing 9 reps alone. When i know i couldve squeezed out one more with a partner, but didnt just to be safe.
Stopping one short of failure on a barbell or dumbell in my opinion will give you better results than going to failure on a machine. Its simply a better tool.
Also dumbells are safer than barbells if your really concerned. Cuz you could throw the dumbells away from your body if you fail at a rep, as opposed to a bar which is usually right on top of you in most exercises.
Btw i worked out alone in my garage for 2 years. So i know this scenario well.
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