Differences in human antagonistic ankle dorsiflexor coactivation between legs; Can they explain the moment deficit in the weaker plantarflexor leg?

The present study examined the hypothesis that the antagonistic ankle dorsi flexor coactivation level during maximum isometric voluntary plantarflexion (MVC) is a function of ankle angle. Six male subjects generated plantarfle xion and dorsiflexion MVC trials at ankle angles of -15 deg (dorsiflexed di rection), 0 deg (neutral position), +15 deg (plantarflexed direction) and 30 deg having the knee flexed at an angle of 90 deg. In all contractions su rface EMG measurements were taken from tibialis anterior and soleus which w ere considered representative muscles of all dorsiflexors and plantarflexor s, respectively. Antagonistic dorsiflexor coactivation was expressed as nor malized EMG and moment. Calculations of the antagonistic dorsiflexor moment were based on the tibialis anterior EMG-dorsiflexor moment relationship fr om contractions at 50, 40, 30, 20 and 10 % of the dorsiflexion MVC moment. In both legs dorsiflexor coactivation level followed an open U-shaped patte rn as a function of ankle angle. Differences of 9 and 14 % (P < 0.05) were found in the measured net plantarflexion MVC moment between legs at ankle a ngles of -15 and +30 deg, respectively. No difference (P > 0.05) was found in the calf circumference between legs. Differences were found in the antag onistic dorsiflexor coactivation between legs at ankle angles of -15 and +3 0 deg. In the weaker leg the antagonistic EMG measurements were higher by 1 00 and 45 % (P < 0.01) and the estimated antagonistic moments were higher b y 70 and 43 % (P < 0.01) compared with the weaker leg at -15 and +30 deg, r espectively. This finding was associated with a decreased range of motion ( ROM) in the weaker leg (14 %, P < 0.01), such that no difference (P < 0.05) was found in dorsiflexor antagonistic coactivation between legs at end-ran ge ankle angles. The findings of the study (i) have to be taken into consid eration when estimating musculoskeletal loads in the lower extremity, (ii) imply that stretching training can result in a stronger plantarflexion at e nd-range ankle angles through inhibition of the dorsiflexors, and (iii) imp ly a neural drive inadequacy during a plantarflexion MVC at end-range angle s.