Primary objective: To investigate the effectiveness of isokinetic strength training of ankle and knee muscles in adults with chronic acquired brain injury (ABI). Research design: Series of single case studies. Methods: Twelve people with ABI participated in a 2.5-week baseline, 12-week intervention and a 4-week follow-up phase. Intervention: Concentric isokinetic exercise, twice a week, for plantarflexors (PFs), dorsiflexors (DFs), knee flexors (KFs) and knee extensors (KEs). Outcomes: Peak torque and power at 60 and 90° s-1, PFs and KFs tone at 60° s-1, gait speed and timed chair rises. Results: For single case analyses strength improvements were noted in 11/12 participants' PFs, 5/12 participants' DFs and 7/12 participants' KEs and KFs. Gait speed improved in 8/12 participants and chair rise time improved in 7/12 participants. PFs tone increased in three participants, KFs tone increased in six participants and three participants reported knee pain. For group analyses, peak torque of PFs and KEs, fast gait speed and timed chair rises demonstrated improvement (p < 0.05). Conclusions: Isokinetic strength training may be effective to improve lower limb muscle strength; however, care needs to be taken in selecting suitable candidates as some individuals reported knee pain with this intensive programme.