Norihiro Sadato and his colleagues have done an experiment which show that people doing exercises appear to perform better when another person compliments them. Here is a description of their experiment:
Forty-eight adults recruited for the study were asked to learn and perform a specific finger pattern (pushing keys on a keyboard in a particular sequence as fast as possible in 30 seconds). Once participants had learned the finger exercise, they were separated into three groups. One group included an evaluator who would compliment participants individually, another group involved individuals who would watch another participant receive a compliment, and the third group involved individuals who evaluated their own performance on a graph. When the participants were asked to repeat the finger exercise the next day, the group of participants who received direct compliments from an evaluator performed better than participants from the other groups. It indicates that receiving a compliment after exercising stimulates the individual to perform better afterwards.
Read more, here.