I've recently started working out at a gym, and I've been critiqued by a friend with whom I work out that I don't do the exercises as they are supposed to be done. I.e. when doing bicep curls I tend stop lowering the weight at approximately 120-130 degrees rathter than 150-160 degrees. The reason for this is that I don't have any force at all, even for really low weights, if I extend the arm further. My question is how this affects my strength gain? Do these additional 30 degrees make a difference?
Most personal trainers will tell you to work a muscle through its full range of motion (ROM). The goal is to ensure that the muscle is under a constant and consistent load during each repetition of the exercise. The load on the muscle tends to be greater for full range of motion even if you lift less weight. A relatively recent (January 2014) study by the Journal of Strength and Conditioning Research found that
Greater morphological and architectural RT adaptations in the LR (owing to higher mechanical stress) result in a more significant increase in strength compared with that of the SR. The practical implications for this body of work follow that LR should be observed in RT where increased muscle strength and size are the objective, because we demonstrate here that ROM should not be compromised for greater external loading.
What they found was that during resistance training (RT), the group that exercised with full range of motion (LR) actually outperformed the other test group that used shorter range of motion (SR). There was even some discussion that ROM could also provide a reduction in fat stores.
There are lots of factors, however, that can cause you to lose range of motion. Some include age and previous injury. If you’re concerned about your strength gains, I would recommend you look at obtaining ROM (Range Of Motion) exercises from a trainer. They would likely be simple stretches to improve ROM.