Assessment of motoneuron excitability using recurrent inhibition and paired reflex depression protocols: a test of reliability.

Motor output may be regulated by both pre- and post-synaptic mechanisms. The purpose of this study was to investigate the reliability of two measurement protocols, which purport to examine spinal mechanisms responsible for gating motoneuron excitability. Nine subjects (aged 29 +/- 5 years) were tested using two soleus H-reflex protocols; 1) recurrent inhibition (RI) and 2) paired reflex depression (PRD). The dependent variable for each protocol was the peak-to-peak amplitude of the conditioned Hoffmann reflex (H-reflex). Seven trials were obtained for each subject under each condition as well as control values to assess test-retest reliability. After all trials were collected the subjects rested for at least five minutes after which the process was repeated. Each subject returned to the lab after a period of no less than 24 hours at which time the process was repeated. Protocol #1: Control reflexes (20% of maximal motor response) were obtained during quiet stance. After obtaining control trials two reflex responses were elicited which were separated by 10 ms on each trial to assess recurrent inhibition (Pierrot-Deseilligny et al., 1976; Bussel and Pierrot-Deseilligny, 1977). Protocol #2: Again a double-pulse technique was used to assess reflex activation history on motoneuron pool output (Trimble et al., 2000). This protocol utilized two reflex stimuli of the same intensity separated by 80 ms. The peak-to-peak amplitude of the control, RI conditioned and PRD conditioned H-reflexes exhibited intraclass reliability estimates of .97, .97 and .93 respectively. To achieve a reliability of rI > or = .80, it is recommended that a minimum of 2 trials be used for the RI protocol and that 4 trials be used for the PRD protocol. The results indicate that both techniques provide a means to objectively and reliably measure spinal mechanisms for gating motoneuron pool output.

Velocity training induces power-specific adaptations in highly functioning older adults.

OBJECTIVE: To test the efficacy of high-velocity training in healthy older persons. DESIGN: A 12-week randomized trial, with subjects blocked for gender and residence, comparing high-velocity resistance training with a self-paced walking program. SETTING: Retirement community. PARTICIPANTS: Forty-three volunteers over the age of 70 years. INTERVENTION: Power group: high-velocity leg exercises 3 times weekly with weekly increases in resistance combined with 45 minutes of moderate, nonresistance exercise weekly. Walking group: moderate intensity exercise 30 minutes daily, 6 days weekly. MAIN OUTCOME MEASURES: Variables measured included leg press peak power and leg extensor strength. Functional performance outcomes included: 6-minute walk distance, Short Physical Performance Battery, Physical Performance Test, and Medical Outcomes Study Short-Form Health Survey. RESULTS: Peak power improved 22% (p =.004) in the power group (3.7 +/- 1.0 W/kg to 4.5 +/- 1.4 W/kg) but did not change in the walking group (3.99 +/-.76 W/kg to 3.65 +/-.94 W/kg). Leg extensor power at resistance of 50%, 60%, and 70% of body weight increased 50%, 77%, and 141%, respectively, in the power group (p <.0001, repeated-measures analysis of variance). Strength improved 22% in the power-trained individuals and 12% in the walkers (p <.0001). Training did not improve functional task performance in either group. One subject developed a radiculopathy during training. CONCLUSIONS: Resistance training focusing on speed of movement improved leg power and maximal strength substantially, but did not improve functional performance in healthy high-functioning older volunteers.

Environmental changes in soleus H-reflex excitability in young and elderly subjects.

The purpose of this study was to examine the role of task complexity on soleus H-reflex modulation in young and elderly subjects. Twelve young (mean age = 29.2yrs) and 10 elderly (mean age = 75.1 yrs) apparently healthy individuals were examined under four experimental conditions: (1) eyes open on normal floor surface; (2) eyes open on foam floor surface; (3) eyes closed on normal floor surface; and (4) eyes closed on foam surface. Under each condition, soleus H-reflexes, and background electromyography (EMG) (40 ms window prior to stimulation in both the soleus and the tibialis anterior) were recorded. Postural sway in each condition was also assessed using a Kistler force platform. Results indicated differential reflex modulation between young and elderly subjects. Specifically, young subjects depressed the amplitude of the reflex as task complexity was increased. Elderly subjects, on the other hand, increased the amplitude of the H-reflex when utilizing visual cues, but decreased H-reflex amplitude when vision was occluded. Postural sway in both groups increased as task complexity was increased. There existed a significant difference in the relationship between tibialis anterior activation and soleus H-reflex between the two groups. These results demonstrate differential reflex adjustments between young and elderly subjects as task complexity is increased, and may provide useful information pertaining to postural control in the elderly.

Age differences in postural sway during volitional head movement.

OBJECTIVE: To examine the role of a volitional self-paced head-turn movement on the postural sway characteristics of healthy young and elderly subjects. DESIGN: Cross-sectional design. SETTING: Motor control research laboratory. SUBJECTS: Ten young adults and 10 elderly subjects. MAIN OUTCOME MEASURES: Postural sway characteristics of each subject were examined using a Kistler force platform. Each subject was tested under four experimental conditions: (1) static postural sway with vision; (2) static postural sway without vision; (3) postural sway with vision and self-paced head-turn movement; and (4) postural sway with no vision and a self-paced head-turn movement. Subjects performed six 15-second trials in each experimental condition. Dependent variables analyzed on each trial were mean sway amplitude (in millimeters), sagittal sway standard deviation, lateral sway standard deviation, and frequency of sway (in hertz). RESULTS: During the static conditions (e.g., no voluntary movement), the young subjects produced significantly less postural sway than the elderly in both the vision condition (sway amplitude in the young, 3.80 mm; in the elderly, 4.89 mm) and the no-vision condition (young, 5.44 mm; elderly, 5.95 mm). This increased sway was the result of greater lateral sway in the elderly for the vision condition (3.73 vs. 2.68 mm), and greater sagittal sway for the elderly in the no-vision condition (5.55 vs. 4.70 mm). There were no significant differences between the groups in the frequency of sway. When asked to initiate and complete the head-turn, elderly subjects significantly increased their mean sway amplitude and decreased their frequency of sway, whereas the young subjects did not significantly alter their postural sway profiles. CONCLUSIONS: These results demonstrate different postural sway control strategies for young and elderly subjects when asked to perform volitional movements.

Inhibition of the soleus H-reflex in standing man.

There exists evidence to support the notion that the segmental reflex system is not fixed and inflexible, but rather is highly modifiable under a variety of circumstances. In this study the H-wave and M-wave recruitment curves were obtained from 19 subjects, utilizing the procedures outlined by Hugon. Each subject was tested on one day under two randomly administered conditions: (1) standing; and (2) prone. Once in place, the recording and stimulating electrodes were not removed until the completion of the study, to ensure that exact placement was maintained. A percutaneous electrical stimulus (1 ms pulse) was utilized to elicit the pulse. The current was monitored with a current probe, and was increased in 2 mA increments from zero until a maximal M-wave was obtained. An analysis of variance revealed significant increases in the amplitude of the H-wave (P < 0.05) when the subject was prone with no significant increases in the M-wave. The results indicate significantly higher H/M ratios with the subjects in the prone position. Therefore, it is concluded that H-reflex amplitude is tonically depressed when the subject is maintaining a standing position.