Department of Exercise Sciences


Descending control to the nonparetic limb degrades the cyclic activity of paretic leg muscles

Lynn M. Rogers, James W. Stinear, Gwyn N. Lewis, David A. Brown
Published in Human Movement Science (2011)

Introduction

During anti-phase locomotor tasks such as cycling or walking, hemiparetic phasing of muscle activity is characterized by inappropriate early onset of activity for some paretic muscles and prolonged activity in others. Pedalling with the paretic limb alone reduces inappropriate prolonged activity, suggesting a combined influence of contralesional voluntary commands and movement-related sensory feedback. Therefore, a thorough consideration of the bilateral neural control of paretic limb cycling impairment is warranted.

Aim

The present study used a series of behavioural tasks to systematically alter the combination of task command and movement related afferent feedback from the nonparetic limb to test hypotheses about the relative influence of these two important control mechanisms. Our primary hypothesis was that descending input from the non-lesioned motor cortex, and not the movement related afference from the non-paretic limb, is primarily responsible for disrupted paretic limb motor output during pedalling.

Method

  • Participants: 15 individuals with chronic post-stroke hemiparesis and 15 neurologically intact individuals volunteered to participate.
  • As depicted in Figure 1, participants were positioned on a custom cycle ergometer equipped with an electric motor to control pedalling velocity. The target leg (paretic limb for person post-stroke, self-reported non-dominant limb for control subjects) performed the same pedalling task in all trials, while the condition of the non-target (non-paretic or dominant) leg was varied. 
  • Tasks with five different non-target leg conditions (with and without movement) were used to investigate the influence of contralateral limb extensor force phasing on pedalling coordination in the target limb (Figure 2).
  • EMG activity was recorded from four lower limb muscles bilaterally: vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), and semimembranosus (SM).
     
Rogers 2011 Figure 1 (insert after method)
Rogers 2011 Figure 2 (insert after method)

Results (Figure 7&8)

  • The main finding of this study was that for persons post-stroke, the previously shown functionally inappropriate VM activity in the OFF phase during “standard” bilateral anti-phase pedalling was exacerbated in this study when accompanied by anti-phased isometric contraction of the non-target limb.
  • Inappropriate VM activity was also exacerbated in this study when accompanied by non-target limb pedalling in-phase, and when accompanied by in-phase isometric contraction of the non-target limb. Importantly this inappropriate OFF phase activity was reduced when the target limb pedalled on its own.
  • A similar but weaker effect was evident in control subjects’ data for the UA condition only when target limb pedalling was accompanied by anti-phased isometric contraction of the non-target limb.
  • These findings suggest that bilateral descending commands are primarily responsible for the inappropriate temporal control of the paretic VM during pedalling.
  • Our novel analytical approach allows us to paradoxically regard the BA condition as an improvement on the UA condition in regard to the extent of inappropriate muscle activity. That is, the BA condition (when both limbs are pedalling in an anti-phase pattern) reduces the extent of inappropriate VM activity induced by the UA condition when the non-target limb was stationary. This finding suggests that movement-related afference diminishes the interference induced by bilateral descending inputs. These findings add considerably to our understanding of the phasic modulation of paretic limb activity during pedalling.
  • Similar but smaller changes in paretic RF activity were observed, suggesting that bilateral commands also play a role in the temporal control of this muscle.
Rogers 2011 Figure 7 (insert after results)
Rogers 2011 Figure 8 (insert after results)

Interpretation

Findings from the current study suggest that (1) there is an interfering influence of the descending command to the non-target limb in both the intact and stroke affected system, (2) non-target limb movement related feedback during anti-phase movement serves to mediate this interfering effect, (3) following stroke these two basic mechanisms remain, but the non-paretic movement related feedback is no longer sufficient to completely mediate the interference, and (4) following stroke, non-paretic movement related feedback in-phase with the movement of the paretic limb further interferes with the timing of paretic limb muscle activity.

Although this study was specifically designed to target the VM, the parallel findings between the RF and VM of neurologically healthy individuals along with similar trends in the paretic RF relative to the paretic VM suggest that these proximal muscles may be influenced by common neural mechanisms.

Acknowledgements

The authors thank and gratefully acknowledge C. Johnston for help in subject recruitment and data collection. This work was supported by an American Heart Association Greater Midwest Region Pre-doctoral fellowship 0615609Z awarded to L. M. Rogers.