Abstract
We analysed the effects of task symmetry during bilateral accurate force production tasks performed by the two feet. In particular, we tested a hypothesis that bilateral deficit would lead to higher indices of synergies defined as co-varied adjustments in the two forces across trials that reduced total force variability. The subjects produced steady-state force followed by a quick force pulse into the target. The two feet could be acting both into plantar flexion and into dorsiflexion (symmetrical tasks), or in opposite directions (asymmetrical task). We used the framework of the uncontrolled manifold hypothesis to quantify two variance components, one of which did not change total force (V UCM), while the other did (V ORT). Synergy indices during the asymmetrical task were higher than in either symmetrical task. The difference was due to higher V UCM (compared to the symmetrical plantar flexion task) or lower V ORT (compared to the symmetrical dorsiflexion task). The synergy index showed a drop (anticipatory synergy adjustment, ASA) starting 100–150 ms prior to the force pulse initiation. The ASA tended to be shorter and of a smaller magnitude for the asymmetrical task. This is the first demonstration of bilateral synergies during accurate force production by the legs. We conclude that bilateral deficit has no or weak effects on two-leg synergies. The results fit the earlier introduced scheme with two groups of neural variables defining average performance of a redundant system and patterns of co-variation among its elemental variables, respectively.
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Abbreviations
- ASA:
-
Anticipatory synergy adjustment
- DF-DF:
-
Bilateral dorsal flexion
- (dF TOT/dt)MAX :
-
Time of the maximal rate of F TOT development
- F L :
-
Force of the left leg
- F MAX :
-
Peak force of the pulse
- F R :
-
Force of the right leg
- F TOT :
-
Total force
- MVC:
-
Maximal voluntary contraction
- PF-DF:
-
Plantar flexion at the non-dominant side and dorsal flexion at the dominant side
- PF-PF:
-
Bilateral plantar flexion
- R :
-
Pearson’s correlation coefficient
- R Z :
-
z-Transformed Pearson’s correlation coefficient
- SS:
-
Steady state
- t 0 :
-
Start of the F TOT increase
- t ASA :
-
Time of ASA initiation
- UCM:
-
Uncontrolled manifold
- V ORT :
-
Variability destabilizing the performance variable, “bad variability”
- VUCM :
-
Variability stabilizing the performance variable, “good variability”
- ΔV :
-
Index of a synergy
- ΔV Z :
-
z-Transformed index of synergy
- ΔΔV :
-
Amplitude of ASA
- ΔΔV Z :
-
z-Transformed amplitude of ASA
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Acknowledgments
Nejc Sarabon would like to acknowledge the support of the Slovenian Research Agency, Grant No. L5—4293.
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Sarabon, N., Markovic, G., Mikulic, P. et al. Bilateral synergies in foot force production tasks. Exp Brain Res 227, 121–130 (2013). https://doi.org/10.1007/s00221-013-3494-4
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DOI: https://doi.org/10.1007/s00221-013-3494-4