With losing weight and getting fit common resolutions at this time of the year, many people may be turning to exercise DVDs for help. However, a new study has found that some of these DVDs may be psychologically harmful.

US researchers studied 10 popular instructor-led commercial exercise DVDs, evaluating both the motivational language and imagery used.

They found that most of the instructors and models in the DVDs were slim, white females who were often wearing revealing clothing. The researchers believe that this sends a subtle message about what fit people should look like.

"This perpetuates objectification of the female body in particular and emphasises physical appearance as opposed to improved health," commented Prof Brad Cardinal of Oregon State University.

The study also found that while around 25% of the language used in the DVDs was motivational, one in seven of these motivational sentences could actually be considered negative.

Such negative statements included ‘you better be sweating' and ‘say hello to your sexy six-pack'.

The researchers said that these types of statements encourage comparisons and focus on outcomes. Furthermore, they do not take into account people's differences in health and fitness.

The study noted that while these exercise DVDs are usually marketed to novice exercisers, the movement skills of those in the DVDs tended to be designed for intermediate, and even advanced, levels of fitness. As a result, the instructor's message sometimes taunted people to keep up and this could lead to adverse outcomes, such as injuries.

"These findings raise concerns about the value of exercise DVDs in helping people develop and commit to a workout programme. There are a lot of exaggerated claims through the imagery and language of 'do this and you'll look like me'.

"You're inviting into your home these images and messages that could make you feel bad about yourself, and ultimately hinder your efforts to improve your health. If the experience is not positive, the likelihood the person is going to continue with an exercise programme diminishes," Prof Cardinal said.

He urged people to be cautious when purchasing these DVDs and noted that many of the instructors have little or no credentials in fitness instruction.

"We don't think the videos are very psychologically safe. Remember that we all have different body shapes and styles, and our bodies may respond differently to the exercises being shown. Don't expect to get the same results as what you see on the screen or compare yourself to others," he added.

Details of these findings are published in the Sociology of Sport Journal.