Too much screen time has been linked to heart disease and cancer in a new study.
Researchers at the University of Glasgow analysed the amount of time 390,089 people spent looking at TV and computer screens during leisure time.
They found that the association between a high level of screen time and poor health was almost twice as strong in those with low fitness levels.
Professor Jason Gill said the findings could affect public health guidance.
Discretionary screen time - watching screens during leisure time - is said to be a contributor to sedentary behaviour which is associated with higher risk of mortality and cardiovascular disease.
Professor Gill, one of the lead authors of the study, said: "Our study shows that the risks associated with sedentary behaviour may not be the same for everyone, with the association between leisure time screen use and adverse health outcomes being strongest in those with low levels of physical activity, fitness or strength.
"This has potential implications for public health guidance as, if the findings are causal, these data suggest that specifically targeting those with low fitness and strength to reduce their sedentary behaviour may be an effective approach."
The researchers analysed the behaviour of 390,089 people from the UK Biobank.
They found that higher levels of screen time were associated with a higher risk of "all-cause mortality" as well as a higher risk of both heart disease and cancer.
The findings were independent of physical activity, grip strength, BMI, smoking, diet and other major confounding factors, including socio-economic status.
Study author Dr Carlos Celis said: "If the discretionary screen time health associations we found in this study are causal, it suggests that people with the lowest levels of strength, fitness and physical activity could potentially gain the greatest benefit from health promotion interventions aimed at reducing sedentary behaviours.
"While fitness testing can be difficult to measure in healthcare and community settings, grip strength is a quick, simple and cheap to measure, so could easily be implemented as a screening tool in a variety of settings."
The study is published in BMC Medicine.