Combined translational and rotational perturbations of standing balance reveal contributions of reduced reciprocal inhibition to balance impairments in children with cerebral palsy

Balance impairments are common in cerebral palsy. When balance is perturbed by backward support surface translations, children with cerebral palsy have increased co-activation of the plantar flexors and tibialis anterior muscle as compared to typically developing children. However, it is unclear whether increased muscle co-activation is a compensation strategy to improve balance control or is a consequence of reduced reciprocal inhibition. During translational perturbations, increased joint stiffness due to co-activation might aid balance control by resisting movement of the body with respect to the feet. In contrast, during rotational perturbations, increased joint stiffness will hinder balance control as it couples body to platform rotation. Therefore, we expect increased muscle co-activation in response to rotational perturbations if co-activation is caused by reduced reciprocal inhibition but not if it is merely a compensation strategy. We perturbed standing balance by combined backward translational and toe-up rotational perturbations in 20 children with cerebral palsy and 20 typically developing children. Perturbations induced forward followed by backward movement of the center of mass. We evaluated reactive muscle activity and the relation between center of mass movement and reactive muscle activity using a linear feedback model based on center of mass kinematics. In typically developing children, perturbations induced plantar flexor balance correcting muscle activity followed by tibialis anterior balance correcting muscle activity, which was driven by center of mass movement. In children with cerebral palsy, the switch from plantar flexor to tibialis anterior activity was less pronounced than in typically developing children due to increased muscle co-activation of the plantar flexors and tibialis anterior throughout the response. Our results thus suggest that a reduction in reciprocal inhibition causes muscle co-activation in reactive standing balance in children with cerebral palsy.

The study is interesting, and the paper generally well written with nice figures.Some approaches in the analyses seem not well motivated to me, and some descriptions incorrect.Therefore, I would like to have these things clarified before publication.Despite the fact that I find the statistical evidence weak, I agree with the conclusion that these experiments provide evidence of reduced reciprocal inhibition.This conclusion is just not very well supported by the tests that were performed.Line 90-95: This explanation of reciprocal inhibition put me completely on the wrong foot.This text explain inhibition from the plantar flexors (SOL/GAS) to the dorsi flexors (TA), but the paper actually investigates the opposite inhibition (decreased activation in SOL/GAS due to TA activation, lines 505-506).Because of this explanation, I was looking for inhibition from plantar to dorsi flexors when analysing the paper, which was not actually the paper's topic.Therefore, I would strongly recommend aligning this explanation with the actual inhibition that is investigated.If my interpretation is incorrect, then please also clarify what the correct interpretation is.

Major Comments
Line 133-135: The first hypothesis includes two things (prolonged activity and increase co-activation).Since two different statistical tests are performed, it would make sense to split this hypothesis into two separate ones, each reflected by one statistical test.
Line 203-204: I did not necessarily see the increase/difference in the last two perturbations.In figure 2, the two darkest blue lines seem to overlap almost completely for platform translation and tilt rotation.Please highlight the difference between the two.Line 206-207/374-375: was an investigation performed into if and how the lack of data on the CP side for large perturbations affected the different outcomes?Line 211-218: Was any filtering applied to the marker data?If not, please specify.
Line 275-278: This mathematical equation does not have a unique solution.I think, based on line 300-301, that there should be an if-statement added to highlight that either pathway is active only when its output is positive.This is described in the text but should also be reflected in the mathematical equation.
Line 268: Was a sensorimotor response model created for each individual perturbation, or one for each child that participated?
Line 321: what is meant with Prime gains?Line 342-343: please explain the difference between the simple and extended model.Line 350-354: please motivate why this approach was used to test (I think) the first part of the first hypothesis: "that children with CP would have prolonged activity in the plantar flexors as well as increased muscle co-activation".According to this paragraph, you test for differences in muscle activity among others, but a difference does not necessarily mean that activity was prolonged in specifically the plantar flexors.
Line 363: I disagree with the choice of not applying an adjustment for simultaneous inference.One cannot assume that tests are independent when they are taken on the same individuals performing the same motion.Therefore, some correction is required.Line 366-369: which hypothesis is tested here?Results: generally, the p-values are quite close to 0.05.Please provide additionally some information related to the power of the tests.

Minor Comments
Line 94-95: should one of the antagonistic be agonistic in this sentence?The sentence does not make sense to me now.
Line 114 + 117: the phrase "also investigate the response to rotational perturbations" in line 114, as well as the wording in line 117, imply that only rotational perturbations are going to be applied.However, the tests are applied to combined rotational and translational perturbations.Line 205: I would recommend to add "the same" before "eight perturbations" to make this statement more specific.

Figure 1 :
Figure 1: I did not initially see the light grey persons that show the expected different response for patients with CP.I would recommend a different, more obvious color.
Please update the wording to reflect what is actually being done.Line 189-190: remove both (figure caption).