Age comparison of H-reflex modulation with the Jendrássik maneuver and postural complexity

The purpose of this study was to examine modulation of the soleus Hoffmann (H)-reflex in response to the Jendrássik maneuver (JM) in standing positions in young and elderly subjects.

Seventeen elderly (mean age=72.0 years) and 23 young (mean age=23.2 years) apparently healthy subjects were examined in two separate experiments. The first experiment was conducted to compare the prone and standing position. The second experiment was conducted in the standing position with isotonic glideboard back support. The isotonic glideboard back support was inclined 30 degrees. In the standing position with back support, the knee and ankle joints were set at 0 degrees of flexion. All subjects were tested with two foot-positions: (1) with no soleus contraction on the platform (simple task) and (2) with an active calf muscle group contraction (complex task). To compare the amplitude of the H-reflex in each experiment between the control trials (relaxed) and JM trials (squeezing tennis balls), the stimulus intensity of 1.1 x motor threshold of the M-response was used for each subject in all body positions.

To ensure experimental control, subjects did not show a difference in the amplitude of the trial M-response between the control and JM trials on any of the body positions. Also, no difference was found in the mean amplitude of the M-response between two different positions in either of the two experiments. Trial M-responses were all between 13 and 17% of M-max in the quiet standing position for the young subjects, and 17 and 21% for the elderly subjects. Results demonstrated that the JM facilitated the H-reflex in both young and elderly subjects. However, differential ability to modulate motoneuron excitability evoked by H-reflex pathways was found between the two groups. Young subjects demonstrated a significant difference in the amplitude of the H-reflex between control and JM trials in each standing position (P<0.05). The elderly subjects, in contrast, demonstrated no significant difference in the amplitude of the H-reflex between control and JM trials during normal standing. When examining standing with back support, the young subjects demonstrated a significant difference in the amplitude of the H-reflex between control and JM trials during both the simple and complex tasks (P<0.05). The elderly subjects, in contrast, demonstrated a significant difference only in the simple postural task (P<0.05).

These results provide evidence of differential human spinal reflex modulation between young and elderly subjects. Further, these results may point towards the role of presynaptic inhibition in mediating these differences, and may lead to a more complete understanding of the different postural control strategies between young and elderly subjects.