BACKGROUND:Plantar heel pain is a common condition, but little is known about the relationship between muscle strength and plantar heel pain. OBJECTIVES:To review the evidence relating to muscle strength in those with and without plantar heel pain. METHODS:We systematically reviewed the literature by searching key databases. Included studies assessed muscle strength (or endurance or size as proxies) in those with and without plantar heel pain. A modified Downs-Black quality index was used to assess study quality and the Grading of Recommendations Assessment, Development and Evaluation (GRADE) tool was used to evaluate the strength of the evidence. Meta-analysis was performed where possible. RESULTS:Seven studies met the eligibility criteria. Hallux plantar flexion, lesser toe plantar flexion, ankle dorsiflexion, ankle inversion, and ankle eversion strength values were reduced in those with heel pain compared to those without; however, there was inconsistency in the findings between studies. No difference was found in calf muscle endurance between those with and without plantar heel pain (standardized mean difference, 0.01; 95% confidence interval: -0.56, 0.59). Generally, foot muscle volume was smaller in people with plantar heel pain compared to those without. The quality of individual studies was generally high (score range, 11-16/17 on the modified Downs-Black quality index); however, the GRADE ratings suggest the strength of this evidence to be very low. CONCLUSION:People with plantar heel pain have reduced strength and volume of the foot muscles, but there is no discernible difference in calf muscle endurance. These findings should be interpreted with respect to the very low GRADE ratings and are likely to change with further research. Accordingly, the role of muscle strength in plantar heel pain is worthy of further investigation. J Orthop Sports Phys Ther 2019;49(12):925-933. Epub 9 Oct 2019. doi:10.2519/jospt.2019.8588.