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Despite the finding that 40% to 60% of stroke patients suffer from knee joint hyperextension during gait, there is a lack of agreement of the possible causes of this problem. The aim of this study was to determine whether there is a relationship between lower limb muscle weakness and knee joint hyperextension in hemiparetic stroke patients.This is a cross-sectional observational comparison study. Twenty patients (mean age 66 years) who had suffered a single hemiparetic stroke and were ambulant with no major lower limb joint pathology participated. Muscle strength of the hip extensors, hip flexors, hip abductors, knee extensors, knee flexors, ankle plantarflexors and ankle dorsiflexors of both limbs was measured using a hand-held dynamometer. Computerized and visual gait analysis identified subjects with and without knee hyperextension in loading response and midstance. Subjects were categorized as having weakness of a particular muscle group if the difference in strength between the paretic and non-paretic muscle was greater than 50%. The Pearson's chi-squared test was used to evaluate the association between weakness and knee hyperextension.A strong relationship was found between ankle plantarflexor weakness and knee hyperextension during midstance (p = 0.044). No relationship was found between lower limb muscle weakness and knee hyperextension during loading response (p > 0.05). No relationship was found between any other lower limb muscle groups and knee hyperextension in midstance (p > 0.05).Weak ankle plantarflexors, in particular gastrocnemius, may have an important role in the presence of knee hyperextension. The results of this study did not support a role for weak hamstrings or quadriceps in knee hyperextension during gait. Further research is needed to clarify the role of gastrocnemius during the stance phase and to determine if strengthening weak gastrocnemius reduces knee hyperextension. Copyright © 2011 John Wiley & Sons, Ltd.