Velocity dependent passive plantarflexor resistive torque in patients with acquired brain injury.
OBJECTIVE
This study sought to determine whether factors other than stretch reflex excitability contribute to velocity dependent passive plantarflexor resistive torque following brain injury.


BACKGROUND
In patients with acquired brain injury increased resistance to passive muscle lengthening commonly results from abnormal muscle contraction, secondary to disinhibition of descending motor pathways, in addition to rheologic changes within the musculo-tendinous unit. Hyper-excitable tonic stretch reflex responses (spasticity) have traditionally been considered to be the main factor influencing resistance that is velocity dependent.


METHODS
Ten adults with brain injury and eighteen age matched controls were studied. A computer controlled torque measurement system was utilised to evaluate resistance to dorsiflexion stretches at two velocities (5 degrees and 25 degrees s(-1)). Only stretches which did not evoke muscle contraction were included in the data analysis. The mean difference and 95% confidence limits in passive plantarflexor resistive torque at two stretch velocities, measured over a defined portion of the test movement, were compared between subject groups.


RESULTS
A velocity dependent increase in passive plantarflexor resistive torque was evident when the ankle was dorsiflexed past the neutral position in both subjects with brain injury and controls. However, the mean difference was approximately 10 times greater in neurologically impaired limbs compared with control values.


CONCLUSIONS
These data indicate that an important component of velocity dependent resistance to passive muscle lengthening in adults with brain injury can be mechanical, and unrelated to stretch induced reflex muscle contraction.


CONCLUSIONS
Increased resistive torque during rapid muscle lengthening may represent a compensatory adaptation for reduced distal motor control following brain injury. A velocity dependent increase in passive plantarflexor resistive torque has the potential to improve stability during gait and provide mechanical resistance to sudden external perturbations.
Singer
B J
BJ
The Centre for Musculoskeletal Studies, Department of Surgery, University of Western Australia, Level 2, MRF Building, Royal Perth Hospital, Rear 50 Murray Street, 6000, Perth, WA, Australia. kps@cms.uwa.edu.au
Dunne
J W
JW
Singer
K P
KP
Allison
G T
GT
D000328:Adult; D000843:Ankle Joint / Q000503:physiopathology*; D001930:Brain Injuries / Q000150:complications / Q000503:physiopathology*; D004548:Elasticity; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009122:Muscle Hypertonia / Q000209:etiology / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D012026:Reflex, Stretch; D015203:Reproducibility of Results; D012399:Rotation; D012680:Sensitivity and Specificity; D013314:Stress, Mechanical; D019415:Torque