Dewald - 1995 - Abnormal muscle coactivation patterns during isometric torque generation

Citation

Dewald JP, Pope PS, Given JD, Buchanan TS, Rymer WZ. Abnormal muscle coactivation patterns during isometric torque generation at the elbow and shoulder in hemiparetic subjects. Brain. 1995 Apr;118 ( Pt 2):495-510. PUBMED

10 Word Summary

Stroke appears to reduce the number of motor programs in hemiparesis.

Abstract

To study abnormal spatial patterns of muscle activation in hemiparetic stroke, we compared EMG activity in paretic and contralateral elbow and shoulder muscles of 10 hemiparetic subjects during 1.5-s voluntary isometric contractions, against five to eight different loads. Isometric forces were generated in eight directions, referenced to a plane orthogonal to the long axis of the forearm, and were recorded by a three degrees of freedom load cell, mounted at the wrist. Surface and intramuscular EMGs of six elbow and six shoulder muscles were recorded from both impaired and contralateral upper extremities of each subject. The spatial characteristics of EMG activation of individual muscles were summarized using two measures. The first, called the 'net resultant EMG vector' is a new measure which calculated the vector sum of EMG magnitudes for each of the eight directions, and the second, index of EMG focus, is a measure of the range of EMG activation recorded for each load level. Use of these measures permitted us to describe spatial EMG characteristics quantitatively, which has not been done previously. We observed consistent and statistically significant shifts in the resultant EMG vector directions in the impaired limb, especially in shoulder and other proximal muscles. Significant increases in the angular range of EMG activity were also identified and were most evident at the elbow. Correlation analysis techniques were used to assess the degree of coactivation of different muscle pairs. There were consistent EMG coactivation patterns observed across all subjects (both normal and hemiparetic). However, in spasticparetic limbs, additional novel coactivational relationships were also recorded, especially between elbow flexors/shoulder abductors and elbow extensors/shoulder adductors. These novel coactivation patterns represent a reduction in the number of possible muscle combinations, or in the number of possible 'synergies' in the paretic limb of the stroke subject. This reduction in number of 'synergies' could result from a loss of descending command options; from an increased reliance on residual, descending brainstem pathways (such as the reticulospinal and vestibulospinal projections); from changes in spinal interneuronal excitability; or from a combination of several of these factors. The relative merits of these hypotheses are addressed.

Notes

• Hemiparesis resulting from stroke is associated with a reduction in the corticospinal and corticobulbar projections from the brain-stem to the spinal cord.
  • These projections are thought to carry voluntary motor commands.
• 3-DoF load-cell measured the "end-point" forces in the coronal plane and the moment in the supanation/pronation direction at the wrist to determine joint torques at the elbow and shoulder.
• EMG tuning curves become increasingly broad with increasing "damage" to motor output of stroke patients.
  • At low levels of activation there is even less tuning
    • Does this have anything to do with fiber recruitment?
• Hypotheses for what is going wrong
  • Descending spinal pathways are constrained
  • Corticospinal pathways that are damaged reduce motor control
  • Interneuronal reflex pathways are damaged