Peer review of: Plasticity of muscle synergies through fractionation and merging during development and (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
This is a very well written manuscript tackling the issue of development and training on muscle 
synergies. The authors considered the effect of development and training on synergies by studying 
the synergies for running in preschoolers and adults from sedentary subjects and marathoners, 
with cross-sectional and longitudinal comparisons. The experimental work has been carefully 
performed and described. However there are problems with the analysis and interpretation of the 
results. The authors should address the following issues in order to convince the reader that what 
they report is correct and complete. 
 
Major comments 
 
1) An important point of this work is the cross-sectional comparison of the muscle synergies. Since 
muscle synergies generally depend on locomotor speed, a potential caveat is represented by the 
different speeds of the different groups. Children ran over-ground at self-selected speeds, but I 
couldn’t find a specification of their speeds. Absolute speed values should be normalized to take 
into account the different size of participants, especially for children. The reported mean speed 
values for the adults were: sedentary: 6.2 ± 0.9 km/h; novice: 6.8 ± 0.9 km/h; experienced: 7.5 
± 1.3 km/h; elite: 12 km/h. The authors should assess the potential effect of the different speeds 
on muscle synergies. 
 
2) Related to the above point, in the manuscript I couldn’t find any comparison between the 
present results on untrained adults and the results obtained in previous work. There have been 
several publications on this topic (none of which I found cited here). In particular, Yokoyama et al. 
(2016; 2017) showed that human locomotor networks may have speed dependency, consistent 
with speed control mechanisms observed in insects and in vertebrates. Interestingly, they 
observed that the number of extracted locomotor modules (using NMF) increases with increasing 
speed. In 2016, Yokoyama et al. also compared different extracted sets of modules between non-
runners and runners (>5 years of experience). They suggested that “the acquisition of novel 
locomotor movement following long-term training is achieved via the reorganization of locomotor 
networks consisting of existing locomotor modules”. Since this is a key argument of the present 
manuscript, I think that Yokoyama’s results should be compared and discussed in the present 
manuscript. 
 
3) Another major concern is about the double integration of vertical ground reaction force (vGRF) 
used to assess the biomechanical performance. The authors apparently realized the vGRFs’ 
integration only during the stance phase. The integration constant was thus selected to fix the 
position of the COM at the same position at the begin (landing) and end (take-off) of the stance 
phase (according to Fig. 1). Even if the Blickhan‘s spring-mass model (1989) assumes the same 
height and velocity of the center of mass at landing and take-off, these authors pointed out that 
‘animals in general do not have a similar takeoff and landing velocity as assumed in the model. 
They take off with straightened legs and land with bent leg, and the leg has to be stiffer during 
landing than during takeoff’. The deviation from the symmetrical model has been documented (see 
for example Cavagna 2006 or Maykranz and Seyfarth, 2014). Conversely, using kinematics data 
Dalleau et al. (1998) observed that the height of the COM was about the same at landing and 
take-off. This symmetry is most likely due to the fact that their subjects ran at a speed (18,3 
km/h) much greater than in the present study (between 6 and 12 km/h): the speed tends to 
reduce the asymmetry between landing and take-off(Cavagna, 2006), as the role of the tendon 
becomes privileged. 
In order to avoid the biased hysteresis, the integration constant must be set on the assumption 
that the average velocity of the center of mass is nil over at least one stride, as it has been done 
in the force-platform experiment reviewed by McMahon and Cheng (1990). This modification may 
change completely the authors’ estimation of run efficiency and in turn the correlation with the 

presence or absence of specific muscle-synergy merging patterns. 
 
Minor comments: 
 
- Inconsistent use of km/h or m/s (see for example fig S4 and Methods section) 
- Change ‘toddlers’ (in Fig 5) in ‘preschoolers’ (as in the other figures) 
- Why the running efficiency was not evaluated in preschoolers? 
 
References 
 
T. A. McMahon, G. C. Cheng, The mechanics of running: how does stiffness couple with speed? J. 
Biomech. 23 Suppl 1, 65–78 (1990). 
G. Dalleau, A. Belli, M. Bourdin, J. R. Lacour, The spring-mass model and the energy cost of 
treadmill running. Eur. J. Appl. Physiol. Occup. Physiol. 77, 257–263 (1998). 
R. Blickhan, The spring-mass model for running and hopping. J. Biomech., 22, 1217–1227 (1989). 
D. Maykranz, A. Seyfarth, Compliant ankle function results in landing-take off asymmetry in 
legged locomotion. J. Theor. Biol. 349, 44–49 (2014) 
G.A Cavagna, The landing-take-off asymmetry in human running. J. Exp. Biol. 209, 4051–4060 
(2006) 
H. Yokoyama, T. Ogawa, N. Kawashima, M. Shinya, K. Nakazawa. Distinct sets of locomotor 
modules control the speed and modes of human locomotion. Sci Rep 6: 36275, (2016). 
H. Yokoyama, T. Ogawa, M. Shinya, N. Kawashima, K. Nakazawa Speed dependency in -
motoneuron activity and locomotor modules in human locomotion: indirect evidence for 
phylogenetically conserved spinal circuits. Proc Biol Sci 284: 20170290, (2017). 
D. J. Clark, L. H. Ting, F. E. Zajac, R. R. Neptune, S. A. Kautz, Merging of healthy motor modules 
predicts reduced locomotor performance and muscle coordination complexity post- stroke. J. 
Neurophysiol. 103, 844–857 (2010). 
 
 
 
Reviewer #3: 
Remarks to the Author: 
Nature Communications 
NCOMMS-19-428952 
Reshaping muscle synergies through fractionation and merging during development and training of 
human runners 
by Vincent C. K. Cheung, Ben M. F. Cheung, Janet H. Zhang, Zoe Y. S. Chan, 
Sophia C. W. Ha, Chao-Ying Chen and Roy T. H. Cheung 
 
In this paper, the authors addressed a critically important issue of motor control, i.e. plasticity of 
muscle synergies in human. The design and the results of this study are very original. The data are 
also original and analyzed deeply in a convincing way. I believe this study is valuable for 
neuroscientists, sports scientists and people in the wider field. I enjoyed reviewing this paper. I 
have only one comment to strengthen the authors’ findings. 
 
Specific comments: 
Major 
1) Page 7, lines 2-4: the authors stated that “Thus, merging of muscle-synergies during running 
training may be understood as a process that assigns multiple original W-encoding neuronal 
networks to be driven by one of the original oscillators (Fig. 7C).” 
 
If that is the case, we would like to know how the innervation from the original C1 to W2 in Fig. 7C 
is newly established or reactivated for merging synergies during training. Furthermore, we would 
like to know the role of the reorganization mechanism for the reverse direction in development 
(i.e. fractionation of synergies). You may want to discuss these issues in Discussion. 

 
Specific comments: 
Minor 
1) p 17, line 16: “when an R combination was present (D) or absent (E)” -> “when an R 
combination was absent (D) or present (E)”