Abstract
Purpose
Walking against a constant horizontal traction force which either hinders or aids the motion of the centre of mass of the body (COM) will create a discrepancy between the positive and negative work being done by the muscles and may thus affect the mechanics and energetics of walking. We aimed at investigating how this imbalance affects the exchange between potential and kinetic energy of the COM and how its dynamics is related to specific spatiotemporal organisation of motor pool activity in the spinal cord. To understand if and how the spinal cord activation may be associated with COM dynamics, we also compared the neuromechanical adjustments brought on by a horizontal force with published data about those brought on by a slope.
Methods
Ten subjects walked on a treadmill at different speeds with different traction forces. We recorded kinetics, kinematics, and electromyographic activity of 16 lower-limb muscles and assessed the spinal locomotor output by mapping them onto the rostrocaudal location of the motoneuron pools.
Results
When walking with a hindering force, the major part of the exchange between potential and kinetic energy of the COM occurs during the first part of stance, whereas with an aiding force exchanges increase during the second part of stance. Those changes occur since limb and trunk orientations remain aligned with the average orientation of the ground reaction force vector. Our results also show the sacral motor pools decreased their activity with an aiding force and increased with a hindering one, whereas the lumbar motor pools increased their engagement both with an aiding and a hindering force.
Conclusion
Our findings suggest that applying a constant horizontal force results in similar modifications of COM dynamics and spinal motor output to those observed when walking on slopes, consistent with common principles of motor pool functioning and biomechanics of locomotion.
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Abbreviations
- \( \beta_{\text{F}} \) :
-
Orientation of the average GRF relative to the vertical
- al, av, af :
-
Lateral, vertical and fore–aft acceleration of the COM
- BW:
-
Body weight
- COM:
-
Centre of mass of the whole body, the point where the weighted relative position of a distributed mass sums to zero
- Ekf, Ekv, Ekl :
-
Energy due to the fore–aft, vertical and lateral movement of the COM
- EMG:
-
Records of the electrical activity produced by skeletal muscles
- \( E_{\text{p}}^{*} \) :
-
Potential energy of the COM
- E s :
-
‘Pendular energy savings’, amount of energy saved by the pendular energy exchange
- \( \dot{e}\left( t \right) \) :
-
Rate of pendular energy savings, which represents the magnitude of energy exchanged
- FC:
-
Foot contact
- Ff, Fl, Fv :
-
Fore–aft, lateral and vertical component of the GRF
- F t :
-
Horizontal traction force
- GRF:
-
Ground reaction force
- L :
-
Stride length
- m :
-
Body mass
- r (t) :
-
Recovery within the step, which evaluates the Ep–Ek transduction (in %) at each instant of the walking step
- h :
-
Vertical displacement of the COM
- TO:
-
Toe off
- v belt :
-
Velocity of the belt
- vl, vv, vf :
-
Lateral, vertical and fore–aft velocity of the COM
- \( {\dot{\text{W}}}_{1}^{ + } \;{\text{and}}\;{\dot{\text{W}}}_{2}^{ + } \) :
-
First and second peak of positive power during a step, respectively
- \( \dot{W}^{ - } \) :
-
Major peak of negative power during a step
- \( W_{\text{ext}} \) :
-
Work done by the subject to move the COM relative to the surroundings
- \( \dot{W}_{\text{ext}} \) :
-
Power used by the subject to move the COM relative to the surroundings
- W t :
-
Work done by the horizontal traction force
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Acknowledgements
This work was supported by the Italian Ministry of Health (Ricerca corrente, IRCCS Fondazione Santa Lucia), Italian Space Agency (Grants I/006/06/0 and 2019-11-U.0), Italian University Ministry (PRIN Grants 2015HFWRYY_002 and 2017CBF8NJ_005).
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PAW and AHD: conceptualization. AHD and RMM: data curation. AHD: formal analysis. FL and PAW: funding acquisition. AHD and YPI: investigation. AHD, YPI and RMM: methodology. FL and PAW: project administration. AHD: software. YPI, FL and PAW: supervision. AHD: writing—original draft. AHD, RMM, YPI, FL and PAW: writing—review and editing.
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Communicated by Jean-René Lacour.
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Dewolf, A.H., Ivanenko, Y.P., Mesquita, R.M. et al. Neuromechanical adjustments when walking with an aiding or hindering horizontal force. Eur J Appl Physiol 120, 91–106 (2020). https://doi.org/10.1007/s00421-019-04251-1
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DOI: https://doi.org/10.1007/s00421-019-04251-1