Background <p>Adults aged ≥ 80 years experience the highest rates of fall-related injury and mortality, yet remain underrepresented in prospective fall research. Although reduced muscle strength is recognised as a risk factor for falls, prospective evidence linking muscle strength to severe fall injuries in the oldest old remains limited.</p> Methods <p>This prospective cohort study included 3059 individuals aged ≥ 80 years from the fourth Trøndelag Health Study (HUNT4) 70 + Survey in Norway. Using sex-specific reference values, handgrip and leg strength were classified as strong, moderately reduced, or reduced. Cox proportional hazards regression was used to examine associations between strength categories and severe fall injuries, ascertained from national health registries.</p> Results <p>Baseline median age was 84 years (interquartile range 82–89; 59% women). Over 3.4 years, 223 individuals experienced either a non-fatal (<i>n</i> = 196) or fatal (<i>n</i> = 27) fall injury. Those with strong versus reduced handgrip and leg strength had a lower risk (adjusted hazard ratios of 0.63 [95% CI 0.43–0.92] and 0.60 [95% CI 0.43–0.86], respectively). In sex-stratified analyses, the association with handgrip strength was evident only in women, whereas strong leg strength was associated with lower risk in both sexes. Specifically, women with handgrip strength ≥ 22.2&#xa0;kg and men and women with leg strength corresponding to completing the five-times-sit-to-stand test in ≤ 13.2 and ≤ 14.3&#xa0;s, respectively, had a markedly lower risk of severe fall injuries.</p> Conclusions <p>Among very old Norwegian adults, greater muscular strength was associated with a lower risk of severe fall injuries. Our findings provide clinically interpretable strength thresholds that may support fall injury risk assessment and targeted prevention strategies among the oldest old.</p>