Comparative studies of these two techniques are limited. In one study, Shapiro and Lehrer observed that both techniques were effective in reducing symptoms and intensity of anxiety and depression. The researchers also observed significant differences between the two techniques on subjective perception of warmth in the limb and depth of breathing. However, no differences were found in the subjects’ heart rate and skin conductance. In a more recent study, Klein-Hebling and Lohaus examined the effects of progressive muscle relaxation and imagination on children aged 9–12 years. While their findings showed that both techniques are effective, the researchers only observed short term effects (after every session) but not medium term effects on relaxation.