There is some flexibility to discuss the specific focus of essays within these general areas to reflect the interests of students.  For example, students may focus on a specific populations (e.g., post-natal mothers), or a particular physical activity (e.g., walking).  It is important to discuss your idea with Ailsa to confirm suitability.
Well there are many options 

You need to write a 1500 word essay then a 500word communication to the population 

You can be specific or generalized 

Use the title Exercise, a treatment for depression

Then search for research studies related to this topic 

For instance swimming and depression running and depression

Try to use the more recent studies (past 10 years)

Then compare what they say 

For instance Jones (2013) studied the effect of swimming on a large group of subjects who suffered from depression and found that daily swimming had a statistically positive effect on their sense of well-being. However, Parks (2009) also studied the effect of swimming on subjects with depression and found no correlation between this form of exercise and an improvement in the subjects sense of happiness.

Compare different research 

And create critical arguments/analysis 

So for instance you could look at how running can help people with post-traumatic stress.

My advice would be to first make sure that you can find good literature for the topic you want. 

It might be better to be more general and look at something like how running can help depression