A Hull University team asked 30 subjects to do biceps curls and found their muscles worked more when they focused on what the muscles were doing.
But lower rates of muscle activity were recorded when they simply visualised themselves lifting the weight.
The study is being presented at the British Psychological Society conference in Cardiff on Friday.
The team wired their subjects up to weight machines which monitored levels of electrical activity in their biceps and asked them to think in two different ways while exercising.
They assumed that the more electrical activity that was measured - the more the muscle is doing.
When subjects were asked to focus on what their muscles were doing and how they were working there were significantly higher levels of electrical activity.
But when they were asked to visualise lifting the weight, electrical activity was lower.
Earlier studies have shown that thinking about what muscles are actually doing can make more skilful tasks like throwing a ball more difficult.
Dr David Marchant explains: "When athletes are at the starting line they are primarily focusing on the end of the track rather than what their legs are doing."
If they focused on the actual movement in their legs they probably would not perform as well, he said.
But this research shows it can be helpful to focus directly on the muscles to improve muscle strength.
Dr Marchant added: "Say you have a footballer who's got some sort of muscular injury he wants to repair - ask him to think about what the muscle is actually doing while he is exercising and it is better.
"But if he's going to kick a penalty - don't get him to think about the muscle because it will go wrong. Instead focus on the goal."
Dr Marchant suggest the reason why the penalty kick will fail in this scenario is because the player's muscle is over-activated by the mental focus on it.
He suggested that sports coaches and trainers would benefit from tailoring their instructions depending on what they wanted to achieve.
Dr Jim Golby, expert in sports and mental toughness at the University of Teesside's Social Futures Institute, said the study was an interesting one which tested how mental imaging affected performance.
But he said testing the subjects with 10 repetitions did not really replicate any real life sporting conditions.