External stabilization is reported to improve reliability of hand-held dynamometry, yet this has not been tested in burns. We aimed to assess the reliability of dynamometry using an external system of stabilization in people with moderate burn injury and explore construct validity of strength assessment using dynamometry. Participants were assessed on muscle and grip strength three times on each side. Assessment occurred three times per week for up to 4 weeks. Within-session reliability was assessed using intraclass correlations calculated for withinsession data grouped before surgery, immediately after surgery and in the subacute phase of injury. Minimum detectable differences were also calculated. In the same timeframe categories, construct validity was explored using regression analysis incorporating burn severity and demographic characteristics. Thirty-eight participants with total burn surface area 5% to 40% were recruited. Reliability was determined to be clinically applicable for the assessment method (intraclass correlation coefficient >0.75) at all phases after injury. Muscle strength was associated with sex and burn location during injury and wound healing. Burn size in the immediate period after surgery and age in the subacute phase of injury were also associated with muscle strength assessment results. Hand-held dynamometry is a reliable assessment tool for evaluating within-session muscle strength in the acute and subacute phase of injury in burns up to 40% total burn surface area. External stabilization may assist to eliminate reliability issues related to patient and assessor strength.