Why is this review important?
Dysmenorrhoea is the pain related to menstrual cycle.
It is commonly divided into two categories based on pathophysiology. Primary dysmenorrhoea is menstrual pain without organic disease, and secondary dysmenorrhoea is menstrual pain associated with an identifiable disease.
It is the most common gynaecological condition regardless of age or nationality where the prevalence varies from 45% to 95% and reason for absenteeism from work or school in 5% to 14% women. Despite affecting quality of life and wellbeing, rather than seeking formal treatment it is either underdiagnosed, self-helped or with over-the-counter-medication.
Intervention needs to be affordable in time and cost, so exercise may provide an affordable alternative or adjunct to medication (analgesics). The studies show either no effect of an exercise, positive effect and/or negative effect associated with anxiety (negative emotional association). 
Such far-reaching implication of dysmenorrhoea for women, needs an evidence-based multidimensional management. In concordance, this review targets conclusive-information from the interventional studies on exercises.
What this review talks about?
The review objective is to evaluate the effectiveness and safety of exercise for women with primary dysmenorrhoea. Randomised controlled trials (RCTs) studied the effect of exercise when compared with no-treatment or use of non-steroidal anti-inflammatory drugs (NSAIDs).
Reviewers defined the exercise as “physical activity characterized by using planned-structured repetitive movements to increase or maintain the fitness” incorporating high-intensity (aerobics, resistance training) and lower-intensity (tai chi, yoga) exercises.
Which studies were included in the review?
Authors studied twelve RCTs (854 women participants) for qualitative synthesis but excluded two studies from quantitative analysis so the analysis included 10 trials with 754 women. These studies were published till August 2019. The women participants had primary dysmenorrhoea and their mean age ranged between 20 to 29 years. None of the studies reported body mass index and how secondary dysmenorrhoea or other pathologies were ruled out. 
The outcome was reported as changes in pain at the end of the intervention. Pain intensity was measured in ten studies on a visual analogue scale (VAS), one study used the numeric rating scale (NRS) and another one used the McGill Pain Questionaaire (MPQ). Lower scores indicate less intense menstrual pain. Adverse events were reported only in one study and one of the studies also reported absence from work or school. None of the studies reported on daily-life-activities restriction.
Key results of this review
Exercise may provide a large reduction in menstrual pain intensity compared to no treatment. This improvement corresponds to a 25 mm reduction on a 100 mm VAS. There is no minimum clinically important difference (MCID) for menstrual pain intensity related to primary dysmenorrhoea. The difference between exercise and no treatment is approximately 2.5 times the MCID for pelvic pain in endometriosis (is 10 mm on a 100 mm VAS) and is likely to be very clinically significant for women.
Most studies asked women to exercise at least three times per week, for about 45 to 60 minutes of exercise each time. It is unclear if exercising less frequently, or for a shorter duration would have the same results. Exercise was performed regularly throughout the month, with some studies asking women not to perform exercise during the periods itself. 
The quality of the evidence was low to very low. The main limitations were imprecision due to small sample sizes (too few women in the study), inconsistency (studies gave very different results) and risk of bias related to blinding (where researchers or participants knew what treatment they were getting). The evidence for the safety of exercise, effect on overall menstrual symptoms or overall quality of life, were not well reported. No studies reported on rates of being absent from work or school or on restrictions of daily life activities.
Authors’ conclusions
The current low-quality evidence suggests that exercise, performed for about 45 to 60 minutes each time, three times per week or more, regardless of intensity, may provide a clinically significant reduction in menstrual pain intensity of around 25 mm on a 100 mm VAS. All studies used exercise regularly throughout the month, with some studies asking women not to exercise during menstruation. Given the overall health benefits of exercise, and the relatively low risk of side effects reported in the general population, women may consider using exercise, either alone or in conjunction with other modalities, such as NSAIDs, to manage menstrual pain. It is unclear if the benefits of exercise persist after regular exercise has stopped or if they are similar in women over the age of 25. Further research is required, using validated outcome measures, adequate blinding and suitable comparator groups reflecting current best practice or accounting for the extra attention given during exercise.
What next?The studies on exercise for dysmenorrhoea have not generally been undertaken to a high methodological standard. Exercise has any effect on the outcomes, the evidence was of very low quality and insufficient evidence to determine whether exercise reduced menstrual pain intensity compared to NSAIDs like ibuprofen, paracetamol or whether there was any difference in adverse events between exercise and no treatment. Not enough evidence to conclude absence from work or school. No studies reported on quality of life or restriction of daily life activities.
So the future research requires to include three-armed trials comparing exercise, current gold standard treatment (NSAID or the oral contraceptive pill), and no exercise or an attention control. Issues related to blinding may be difficult to control, but clearer reporting of randomisation and allocation concealment may strengthen the evidence. Considering the likelihood that primary dysmenorrhoea does not resolve after adolescence, future research should also include women throughout their reproductive lifespan. More comprehensive reporting in future studies of outcome, safety, adverse-effects, absence from work/school, long-term effects and rescue-medication use will provide greater understanding of exercise benefits and the potential risks that is most important for the patients, clinicians and policy makers.

Comment by Sunita Gudwani