Relation between abnormal synergy and gait in patients after stroke

Abstract

Background: The abnormal synergy seen in patients after stroke is considered to limit the ability of these patients. However, in the lower extremity, antigravity torque generation rather than precise movement is needed for functions such as sit-to-stand movement and gait. Therefore, the ability to generate torque may be important either as a primary movement or as an abnormal synergy. We attempted to quantify the torque generation in the lower limb, selectively and as an abnormal synergy, and its relation with gait.

Methods: Selectively generated plantar flexion torque in the ankle and plantar flexion torque secondarily generated accompanying maximal hip extension (i.e., torque generated with abnormal synergy) were measured in subjects after stroke and control subjects. In subjects after stroke, secondary torque generation while controlling hip extension torque as 25%, 50%, and 75% of the maximal hip extension was also measured. The relation of torque generation with the gait speed and timed-up-and go test (TUG) was also analyzed.

Results: In subjects after stroke, there was no difference between the amount of plantar flexion torque generated secondarily and the selectively generated torque, whereas the selective torque was significantly greater in control subjects. Pearson product-moment correlation coefficient analysis revealed that TUG speed is related to secondarily generated torque accompanying maximal hip extension but not with selectively generated torque.

Conclusion: Secondarily generated torque was found to be a factor that affects TUG speed, and the ability to generate torque even through abnormal synergy may help for gait ability in subjects after stroke.

Figure 1

Schematic of the test setup. The subject was placed in the supine position. The subject’s foot was attached to the orthosis-type device with a load cell, and the thigh was attached to the handheld dynamometer. Upon the generation of the plantar flexion torque by the subject, the hydraulic cylinder pushed the load cell as a counterforce. The counterforce, which reflects the plantar flexion torque, was numerically indicated in a personal computer linked to the device.

Figure 2

Correspondence of secondary plantar flexion torque with percent hip extension torque in subjects after stroke. An asterisk (*) denotes statistical significance between conditions (p < 0.05). The figure indicates the plantar flexion torque (standardized by body weight) generated when exerting the hip extension torque at 25%, 50%, 75%, and 100% of the maximal hip extension. There was a significant main effect of exertion condition. The secondarily generated plantar flexion torque (STo) corresponding to 50% of the maximum voluntary hip extension torque (50%STo) was higher than the 25%STo, and the 100% (STo) was higher than the 75%STo, 50%STo, and 25%STo. The 75%STo was higher than the 25%STo.

Figure 3

Electromyography activity during primary torque and secondary torque generation. An asterisk denotes statistical significance between conditions (**; p < 0.01, *; p < 0.05). A, In the control group, the gastrocnemius and soleus activities during PTo were higher than those during STo. B, In subjects after stroke, the tibialis anterior activity during STo was higher than that during PTo.