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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 57:B321-B329 (2002)
© 2002 The Gerontological Society of America

Soleus H-Reflex Gain in Healthy Elderly and Young Adults When Lying, Standing, and Balancing

Gordon R. Chalmersa and Kathleen M. Knutzena

a Department of Physical Education, Health and Recreation, Western Washington University, Bellingham

Gordon R. Chalmers, Department of Physical Education, Health and Recreation, MS-9067, Western Washington University, 516 High Street, Bellingham, WA 98225-9067 E-mail: chalmers{at}cc.wwu.edu.

Decision Editor: John A. Faulkner, PhD


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Soleus Hoffman-reflex (H-reflex) gain was compared at the same background level of electromyographic activity across lying, natural standing, and tandem stance postures, in 12 young and 16 elderly adults. When compared to a lying posture, young adults significantly depressed soleus H-reflex gain when in a natural standing (19% decrease) and a tandem stance position (30% decrease; p < .0125 for both positions). For elderly adults, there was no significant decrease in H-reflex gain while standing naturally, but there was a significant 28% decrease when performing tandem stance (p < .0125). The data indicate that, although the mild motor control challenge of natural standing does not induce a decrease in soleus H-reflex gain in the elderly adults, as it does in young adults, in the more difficult task of tandem stance, soleus H-reflex gain is significantly decreased in both young and elderly adults.

NUMEROUS studies have demonstrated in young adults that the size of the soleus Hoffmann reflex (H reflex) is modulated across different motor tasks. For example, the soleus H reflex decreases in size, or gain, when a transition is made from a lying or sitting position to a standing one (1)(2)(3)(4)(5), or from a supported to an unsupported standing position (2)(6). Similarly, when standing is compared with the stance phase of walking (7)(8)(9), walking is compared with beam walking (10), or walking is compared with running (11)(12), soleus H-reflex size, or gain, is lower in the latter cases, when a comparison is made at the same soleus electromyographic (EMG) level.

Examination of the soleus H reflex is an effective means to investigate reflex circuitry and changes in transmission in spinal pathways during motor tasks (13)(14). Task-related regulation of reflex transmission is an important component of motor control, which allows afferent feedback to have differing effects based on the task (15). The strength of the soleus H reflex may be modulated across and within motor tasks to facilitate the mechanical demands of the task (16) and/or to regulate Ia feedback activation of soleus alpha motor ({alpha}-motor) neurons (11)(12)(17). For example, soleus H-reflex excitability is greatest during downhill walking when the need for joint stiffness is greatest, compared with level and uphill walking (16). The H reflex is also modulated through the step cycle. H-reflex size is minimal at the time of heel contact, increases to a maximum during stance, and decreases rapidly just prior to toe off, to be minimal during swing (8)(17). This step-cycle modulation allows the soleus Ia reflex pathway to facilitate weight support and ankle extension during middle to late stance, while allowing ankle dorsiflexion during swing and while the body moves over the foot during early to middle stance (8)(15)(17).

When changing posture, elderly adults regulate the H reflex in a different manner than young adults. On average, when compared to the reflex size in a lying position, elderly adults, upon standing, show either no change or an increase in the maximum soleus H reflex in a reflex recruitment curve; young adults, on average, show a decrease (3)(18)(19).

Because autogenic short latency stretch reflexes contribute to soleus muscle stiffness (20)(21) and are involved in responses to postural perturbations (22)(23)(24)(25), it is an interesting observation that elderly adults, a population that often exhibits impairments in standing posture control (26), do not decrease the soleus H reflex when standing, whereas young subjects do. The lack of a decrease in the H reflex when standing indicates a significant change in motor control strategy for the elderly adults, compared with that of the young adults. This change, however, has only been explored for the postural motor task of natural bipedal stance. It is unknown if similar age differences in H-reflex motor control strategies are observed under more challenging standing postures. Lin (27) demonstrated for the elderly population that a motor control response strategy observation under one test condition cannot be extrapolated to a similar, but more challenging, test condition. Lin (27) found that, when older adults, classified as stable balancers, experienced a slow and small perturbation, by moving the platform they were standing on, the response onset latency of the muscles tested was significantly delayed compared with that observed in young controls. However, when the challenge of the perturbation was increased, the response onset latency of the elderly group occurred at the same short time as that observed in the young subjects. The current study will determine if a more challenging postural motor task than standing with feet apart demonstrates discrepancies in the soleus H-reflex motor control strategies in young and elderly adults.

For the elderly population, intersubject variability in intertask modulation of the soleus H reflex has been observed (3), although the basis for this variability is unknown. Chronic patterns of neuromuscular system activity have some influence on the H reflex. Prior chronic physical activity has a weak positive influence on the size of the resting soleus H reflex (28), and, through practice, humans can learn to reduce soleus H-reflex amplitude when performing a balancing task (29). Accordingly, the ability to depress the soleus H reflex upon standing may be related to prior chronic physical activity patterns.

The goals of this study were (a) to test the hypothesis that there is no difference in soleus H-reflex gain in young or elderly adults when lying and standing naturally, or when lying and standing in a tandem stance, and (b) to determine if there is a relationship between the ability to modulate the soleus H reflex upon standing and prior habitual physical activity level in both young and elderly populations.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Subjects
Twelve young (6 male, 6 female; age mean ± standard deviation, 22 ± 1.4 years; age range, 20–24 years) and 16 elderly subjects (9 male, 7 female; age, 71 ± 4.3 years; age range, 60–78 years) with no known neurological, muscular, or gait pathologies participated in this study. The experiments were approved by the Ethics Committee on Human Experiments at Western Washington University, and all subjects gave their written informed consent prior to inclusion in the study. All of the elderly subjects lived independently in the community, stood and walked without the use of aids, and reported some (n = 11), or no (n = 5) regular aerobic and/or weightlifting exercise participation. All subjects complied with the requirement that they refrain from exercise more strenuous than walking for 24 hours prior to testing (30), and they not consume food, stimulants, or tobacco for 2 hours prior to testing (31). Furthermore, all subjects reported any medications they were taking and it was verified that none took any medications that affected the muscular or nervous systems.

Experimental Procedure and Data Acquisition
H-reflex data were collected and analyzed by using standard techniques (32)(33). All measurements were made on the right leg, with stimulating and recording surface electrodes placed and not removed until completion of data collection, in a room isolated from visual and auditory distractions. Skin at the electrode sites was thoroughly cleaned, and EMG recording electrodes (Ag-AgCl, 10 mm in diameter) were placed on the soleus, along the midline of the dorsal aspect of the leg, with the proximal electrode 1 cm distal to medial head of the gastrocnemius and with a 3-cm interelectrode distance. A 26-cm2 metal plate ground electrode was attached to the medial gastrocnemius. Muscle EMG signals were amplified by a bipolar differential amplifier with a common-mode rejection ratio of 90 dB, and a frequency bandpass of 10–1000 Hz (Grass P5, West Warwick, RI). The EMG signal was digitized at greater than 2000 Hz with a 12-bit data-acquisition card (Micro 1401, Cambridge Electronic Designs, Cambridge, UK), and it was analyzed on line (described in the paragraphs that follow; Spike2 software, Cambridge Electronic Designs). The digitized EMG signal was displayed in a raw format to allow for peak-to-peak measurements of evoked muscle responses, M waves (the first EMG response following stimulation, which is due to direct stimulation of {alpha}-motor neuron axons) (32), and H-reflex waves (the second EMG response following stimulation; stimulation described later) (34). The EMG signal was concurrently displayed in a rectified and then smoothed format (all smoothed EMG signals were calculated as each point averaged over the previous 40-millisecond period) to provide feedback to the subject and to allow for measurement of the background EMG level prior to a stimulus.

EMG activity was recorded from the soleus while the subject walked approximately 12 steps across a level floor, and the peak voltage of the rectified and smoothed EMG record of each of the middle three steps was measured and averaged (35). This mean peak walking EMG value was used to normalize background EMG data.

A waist-height table was equipped with a fold-down wing. The wing reached the floor when folded down and ended with a perpendicular foot platform for the subject to stand on with the back of his or her legs against the table wing. When the table wing was moved up or down, the subject was able to make a transition from a standing to a supine lying position, or vice versa, with negligible movement of the knee and ankle, thereby avoiding displacement of the stimulating and recording electrodes and ensuring that the angle of the right ankle was not varied during the experiment (2).

With the subject standing on the foot platform, a 36-cm2 metal plate anode electrode was attached to the thigh just proximal to the patella, and a cathode (Ag-AgCl, 10 mm in diameter) was located in the popliteal fossa to stimulate the posterior tibial nerve. To position the cathode, low levels of stimulation were used and the position selected was that which allowed the largest possible submaximal H-reflex waves to be evoked without M waves, and where higher levels of stimulation produced M and H waves with a similar shape (33). The stimulating cathode was attached with tape and an elastic bandage. Stimuli (1-millisecond pulse duration) were provided by a Grass S48 stimulator with a SIU5 stimulus isolation unit (Grass), and stimulus timing pulses were digitized concurrently. During all measurements, it was verified that the subject was contracting only the muscles needed for the posture, and the ankle plantar flexors when requested, and that other muscle contractions, such as jaw clenching and the Jendrassik maneuver, known to influence soleus H-reflex amplitude were avoided (36)(37).

With the subject standing, data for a H-reflex recruitment curve were collected by using a stimulus frequency of 0.15 Hz (13)(33)(38). Starting at a low voltage that failed to elicit any H-reflex or M waves, stimulus intensity was increased in increments of 3 V (nominal), up to the point where the M wave failed to increase further. A minimum of five stimuli were delivered at each stimulus intensity. The peak-to-peak height of the M and H waves following each stimulus were measured and averaged for repeated stimuli at the same strength; an H-reflex recruitment curve was plotted. The test stimulus strength for the subject in the subsequent procedures was the voltage that produced a M-wave amplitude that was 25% of the maximum M-wave amplitude (16)(39)(40)(41). This level ensured that the M wave was not obscured by background EMG signals at the higher contraction levels tested.

A random sequence of the three body positions tested was then assigned to the subject: lying supine on the table (lying), standing with feet spaced at a natural, preferred distance apart (natural stance), and standing with the left foot placed directly in front of, and with heel touching the toes of, the right foot (tandem stance). In all three body positions, the right ankle angle was maintained constant by ensuring the right foot was in the corner formed by the table wing and the foot platform, and the back of the right lower leg was against the table wing. The left leg position paralleled the right except in the tandem stance position. With the subject in the natural stance position, shoulder straps to prevent upward movement of the body were fitted snugly over both shoulders, and readjusted as body position was changed, so that ankle plantar flexor contractions would be isometric. The subject's arms were held loosely at the sides, except in the tandem stance position, when some subjects held their arms outward slightly to improve balance. While in the natural stance position, the subject practiced the ankle plantar flexor contractions and subjectively determined a peak target contraction level that was a submaximal effort, and that could be easily reproduced several times with 10-second rest intervals without cumulative fatigue. The mean rectified and smoothed soleus EMG level over the middle 0.25 second of a 1-second contraction at the peak target contraction level was determined to allow lower contraction target levels to be set during subsequent reflex testing.

In each of the three body positions, the following procedure was followed for each subject. The subject was provided with feedback on the current right soleus rectified and smoothed EMG level by means of a computer monitor positioned 24 in. (~61 cm) from the subject's face. The difference between the EMG level for the peak target contraction of the subject and 15% of that value was divided into 12 steps, and the sequence of those steps was randomly assigned as target contraction levels. Low, but functional, levels of contraction were selected for the testing because H-reflex size is most sensitive to task-dependent influences at low contraction levels (42). With a target contraction level presented on the computer monitor, the subject performed a bilateral isometric ankle plantar flexion contraction against the foot platform so that the feedback EMG level matched the target level. Flexion of the trunk, hip, and knees was not allowed. The H reflex was modulated to a level appropriate for the task within a reaction time and simultaneously with muscle activation for the task (11). Accordingly, when the EMG level approximately matched the target and had been held steady for at least 1 second, a stimulation was delivered to the posterior tibial nerve. The subject then relaxed for at least 20 seconds to ensure that no postvoluntary activation depression of the H reflex occurred on the subsequent tests in the position (13). The peak-to-peak amplitude of the M wave following each stimulus was measured to ensure that the stimulus strength delivered to the posterior tibial nerve was constant. Small changes in the distance between the electrode and the nerve as body position changed would cause an excessively large or small effective stimulus strength, and resultant M-wave size (32)(43). If the M-wave amplitude following a stimulus was not within ±15% of the target level for the subject, the measurement was rejected, and the target contraction level was repeated with a slight change in stimulus strength until the M wave was within the ±15% tolerance. The 15% tolerance value was selected because it is less than that used in a similar prior study comparing H-reflex size across motor tasks (11), and pilot studies indicated that it results in an M-wave variance similar, or smaller, to that used in previous related studies (8)(11)(12). The process was repeated for each of the target contraction levels; for each successful stimulus, the peak-to-peak amplitude of the M wave and the H-reflex wave, and the rectified and smoothed background EMG level over 0.1 seconds prior to the stimuli, were determined.

Data Analysis
For each subject, and for each body position, the relationship between the H-reflex wave peak-to-peak amplitude and the background EMG level was plotted (Fig. 1). Background EMG level was also normalized by expressing it as a fraction of the mean peak EMG level observed during three walking steps (10)(44) so that the muscle activation levels tested could be related to levels observed during common functional activities. Soleus H-reflex gain was defined to be the H-reflex wave amplitude at a given level of motoneuron excitability, with the latter reflected by the background EMG level (10)(16)(45)(46)(47) (see Discussion). The midpoint of the background EMG range that was common to all of the postures tested in a subject was used as the EMG level for comparison across tasks. For each body position within a subject, a regression line relating H-reflex wave amplitude and absolute background EMG level allowed the soleus H-reflex amplitude at the midpoint background EMG level to be determined by linear interpolation (16). As a way to determine if soleus H-reflex gain within an individual was different in either of the two standing postures compared with the lying posture, the reflex gains in the natural and tandem stance positions were expressed as a percentage difference from the gain observed in the lying position. The percentage differences in reflex gain for each of the two standing positions within a subject were then averaged across subjects within an age group. This data-analysis approach prevented intersubject differences in soleus H-reflex amplitude, or normalized amplitude, in the lying position from obscuring intrasubject changes in amplitude with a change in posture, as may occur if group averages were determined for each body position before body-position data were compared. To determine if there were differences in stimulus strength across the body positions within an age group or across the age groups, the mean M-wave amplitude for each posture within each subject was determined and then averaged over each posture within each age group.



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  		Figure 1. Electromyographic (EMG) data and soleus Hoffman-reflex (H-reflex) gain calculation for each posture tested, from young (A) and elderly (B) subjects. The Y axis provides the peak-to-peak amplitude of the H-reflex wave and the M wave (the EMG wave produced by direct stimulation of {alpha}-motor neuron axons). The X axis provides the background EMG level over 0.1 seconds prior to the delivery of a stimulus. Tests performed in a lying posture (boxes; solid regression line), natural stance (circles; large dashed regression line), and tandem stance (triangles; small dashed regression line) are shown. Large filled symbols and regression lines are for H-reflex wave amplitude data; small empty symbols are for corresponding M-wave amplitude data. Vertical dashed lines mark the range of muscle activation levels that was common to all three of the postures tested in the subject. The vertical solid line, marking the midpoint of this range, denotes the background EMG level used when comparisons were made in H-reflex amplitudes across the three postures. In A, the H-reflex amplitudes for the natural and tandem stance postures were decreased 10% and 32% from the H-reflex amplitude in the lying position. In B, the H-reflex amplitudes for the natural and tandem stance postures were increased 24% and decreased 4%, respectively, from the H-reflex amplitude in the lying position. When comparing A and B, note the differences in scales on the two axes.

 
Habitual Physical Activity Level Quantification
A questionnaire developed by Baecke and colleagues (48) that included three dimensions of physical activity (occupational, sport during leisure time, and nonsport activities during leisure time) was demonstrated to have sufficient construct validity and test–retest reliability (correlation coefficients 0.88, 0.81, and 0.74 for work, sport, and leisure time physical activity indexes, respectively) to allow habitual physical activity to be measured in men and women between the ages of 20 and 32 years. Voorrips and coworkers (49) modified the questionnaire developed by Baecke and colleagues to allow it to be used with an elderly population. The significant modifications made by Voorrips and coworkers were the use of a longer reference time interval for the elderly group because of concerns about weakness in short-term memory, the use of oral interviews because of concerns about eyesight and/or arthritis, and the allowance for a wider range of activity responses (e.g., household tasks as well as occupational and leisure time activities are recorded). Physical activity was measured in terms of time spent in the activity, and then multiplied by previously published values of the energy expenditure per unit of time for the activity (49). Voorrips and coworkers demonstrated a test–retest reliability correlation coefficient of 0.89, and concurrent validity through correlations to repeated 24-hour recalls and pedometer measurements of 0.78 and 0.73, respectively. Given the nature of the changes made to the validated questionnaire for young adults (48) when it was modified and validated for the elderly adults (49), it is expected that the questionnaire developed by Voorrips and coworkers should be valid for the quantification of habitual physical activity levels of both young and elderly subjects. Accordingly, the habitual physical activity level over the previous year for each subject was quantified by using the questionnaire developed by Voorrips and coworkers.

Statistical Analyses
Results from both sexes in each age group were combined because there are no sex differences in the H reflex (50)(51). Two-tailed independent t tests were used to determine differences between the young and elderly group means for age, habitual physical activity score, M-wave maximum amplitude, and the midpoint background EMG level used when body positions were compared within a subject. The stimulus size target M-wave amplitude was compared across body positions and age groups by a 3 x 2, within–between analysis of variance (ANOVA) with the Greenhouse–Geisser adjustment for sphericity applied (52).

As a way to determine if the percent change in reflex gain was different from zero when standing and lying were compared, a two-tailed single-sample t test was done for each of the two standing positions, for each of the age groups. The alpha level for each of these four related single-sample t tests was adjusted with the Bonferroni adjustment (i.e., 0.05/4 = 0.0125) to reduce the probability of a type I error (52).

To determine if significant differences were meaningful, the effect size for each t test (52) and the eta-squared value for ANOVA tests (53) were calculated, and they are reported when they might aid interpretation of results. As a way to determine if there was a linear relationship between soleus H-reflex gain modulation with a change in posture and habitual physical activity level over the previous year, a Pearson product moment correlation coefficient, r, was calculated, and it was determined if the correlation was significant (52).

Post hoc power and effect size values for the t tests were determined by using nQuery Advisor version 2.0 (Statistical Solutions, Cork, Ireland). All other statistical analyses were performed by using SPSS-PC version 10.0 (SPSS Inc., Chicago, IL). Results are reported as mean ± standard deviation, and p values < .05 are considered significant, except where a Bonferroni adjustment was applied and the adjusted p value needed for significance is stated.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Results From Representative Young and Elderly Subjects
Fig. 1 illustrates the calculation of soleus H-reflex gain for the postures tested within a subject, and the comparison of reflex gain between the two standing postures and the lying position. Note the small variation in M-wave amplitude, reflecting consistency in stimulation intensity, across the contraction levels and the tasks within a subject.

Group Mean Results
There was no significant difference between the young and elderly groups in the normalized midpoint background EMG level used for comparison of H-reflex gain across the postures tested (young, 23%; old, 28% of peak walking EMG) (Table 1 ), although because of the low power of this statistical test (26%), a true difference may have been missed. This muscle activation level used for comparison was one that would be commonly employed in everyday tasks. The M-wave amplitude of the test stimuli, a measure of consistency in stimulation intensity, did not vary with body position in either the young or elderly groups, indicating that differences in reflex amplitude across postures tested are not due to differences in stimulation intensity (Table 1 ). The lower test stimuli M-wave amplitude in the elderly group (p < .05, Table 1 ) reflects the lower M-wave maximum observed in the elderly adults compared with the young adult subjects (p < .05, Table 1 ), as has been previously observed (50)(54); the latter likely reflects reduced muscle mass in the elderly subjects.


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  		Table 1. Summary of Subject Descriptive Data and Results

 
Compared to a lying position, the mean soleus H-reflex gain was significantly lower in the natural stance position for the young subjects (-19 ± 14%, p < .0125), but not the elderly subjects (-8 ± 38%), and it was significantly lower for both the young (-30 ± 19%, p < .0125) and elderly subjects (–28 ± 25%, p < .0125) in the tandem stance posture (Table 1 , Fig. 2). Because of the low power of the statistical test (4%) when the change in reflex gain was examined for the elderly subjects in the natural stance position, a true difference may have been missed. Two elderly subjects were unable to perform the tandem stance task as a result of balance difficulties.



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  		Figure 2. Mean percentage change in soleus Hoffman-reflex (H-reflex) gain in natural and tandem stance postures, compared with the reflex gain in a lying posture, for young and elderly adults. For young adults, when they were standing in a natural or tandem stance, the H-reflex gain was significantly lower than that observed in a lying position. In contrast, for elderly adults, only the tandem stance resulted in a reflex gain that was different from that observed when they were lying down. *Mean change significantly different from zero, p < .0125; bars indicate ± 1 standard deviation.

 
Scatter Plot Results
For both young and elderly adults, there was no statistically significant linear relationship between habitual physical activity level and the percent change in soleus H-reflex gain when shifting from lying to a natural or tandem stance (Fig. 3; r = .34, .17, -.26, and -.29 for parts A, B, C, and D; p > .05 for each correlation). For the young subjects, the one obvious outlier with the very high physical activity score was omitted from the correlation calculation because the single outlier greatly changed the correlation, but not the lack of significance (r = .15 and -.17 for Fig. 3 and Fig. 3 with all subjects). Although there was no linear relationship in the data in Fig. 3, there was an interesting pattern subjectively observed in the elderly subject data, if one compares the elderly adults with lower and higher physical activity levels. The division between lower and higher physical activity for these subjects was defined to be the midpoint (physical activity score of 20) of the range of scores (3–42) observed in the elderly group. Although among the young adults, only one subject showed a slight increase in soleus H-reflex gain when natural stance was compared with lying (Fig. 3), an increase was much more common for the lower activity elderly group (Fig. 3). The upper right quadrant of Fig. 3 is mostly lacking in data points, except for 1 point with a slightly positive value for percent change in reflex gain. This distribution indicates that, with the one low positive value exception noted, we do not observe higher physically active elderly people who increase their soleus H-reflex gain when natural standing is compared with a lying posture.



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  		Figure 3. Relationship between habitual physical activity level over the year prior to testing and the percent change in soleus Hoffman-reflex gain when natural (A and C) or tandem stance (B and D) postures are compared with a lying posture, for young (A and B) and elderly (C and D) adults. Each point represents an individual subject, with almost all subjects performing both natural and tandem stance tests.

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
Determinants of H-Reflex Gain
Soleus H-reflex amplitude depends on the excitation level of the soleus {alpha}-motor neurons (7)(8)(32)(42) and presynaptic inhibition of Ia afferent input to the soleus {alpha}-motor neurons (6)(11)(12)(15)(32)(55)(56), when rate-dependent transmitter release from Ia terminals (32)(38)(57)(58), muscle length (2)(46), and stimulus strength are held constant. Within a subject in the current study, motor neuron excitability was controlled by comparing reflex amplitude at a constant background EMG level across tasks, transmitter release was controlled by keeping the interstimulus interval long (13)(38), muscle length was held constant by preventing ankle movement, and stimulus strength did not vary significantly across the positions tested. Observed changes in reflex gain across motor tasks may therefore be attributed to changes in presynaptic inhibition of Ia afferent input to the soleus {alpha}-motor neurons (9)(11)(12)(17)(18)(55)(59). During natural movement, however, soleus H-reflex amplitude depends on the modulation of the multiple factors that influence it.

Measurement of H-Reflex Gain
Gain is the ratio of a system's output divided by its input (60). In pioneering studies of the stretch reflex, the gain of the stretch reflex was defined to be the muscle activation turned on by a given stimulus, and it was observed that stretch reflex gain increased with preexisting voluntary activity level (61). Consequently, some studies, including the present one, have measured stretch or H-reflex gain as the reflex EMG amplitude at matched motoneuron background EMG levels (9)(10)(16)(45)(46)(47). It has been demonstrated that H-reflex gain may be inferred from the slope of the relationship between the soleus H-reflex amplitude and background EMG level for the task (62), and so this approach has also been used to measure H-reflex gain within and across tasks (2)(11)(12)(19)(63)(64).

For many studies examining H-reflex gain modulation with changes in motor tasks, the same results are obtained regardless of which of the two gain calculation methods is used, because a downward shift in the y-axis position of the regression line of the H-reflex amplitude versus background EMG relationship is often accompanied by a decrease in slope, and vice versa (see, e.g., 2,11,12). This correspondence does not occur in all cases, however. When walking was compared with standing (8), lying prone with standing (19), and lying compared with the tandem stance position in Fig. 1 of the current study, the y-axis position of the regression line relating H-reflex amplitude to background EMG level was shifted down in the latter cases, but slope increased. (For additional examples, see 56 and 65.) The inability for changes in the slope of the H-reflex amplitude versus background EMG relationship to always reflect changes in the reflex gain (as gain is defined herein) may be because the sigmoid shape of the stimulus and response relationship for the reflex causes the slope of the H-reflex amplitude versus background EMG relationship to also depend on the size of the test stimuli (32). Furthermore, the slope of the relationship also depends on whether a correction for variation in maximum M-wave amplitude though the step cycle is applied in locomotion studies (65).

Soleus H-Reflex Gain Modulation in Young and Elderly Adults
For the young adults, the statistically significant lower soleus H-reflex gain in the natural stance position compared with lying was a meaningful and large decrease, based on the suggested criterion for interpreting the effect size calculated with the t test (52). In contrast, Angulo-Kinzler and coworkers in a very similar experiment (19) found that young adults increased soleus H-reflex gain when they shifted from a prone to a standing position. The opposing results are likely due to the differing methods used to calculate H-reflex gain in the two studies. Angulo-Kinzler and coworkers found that the slope of the regression line of the H-reflex amplitude versus background EMG relationship (their definition of gain) increased when young adults changed from lying to standing, although there was a decrease in H-reflex amplitude at equivalent EMG levels in the latter task (gain definition used in the present paper). A decrease in the soleus H reflex was also reported for young adults when standing was compared with lying, when it was measured as an H-wave maximum to M-wave maximum (Hmax/Mmax) ratio (3)(18).

For the elderly subjects, the current results, showing no difference in H-reflex gain when a lying and a natural stance are compared, concur with previous findings for elderly adults when these two body positions are tested (3)(19). The principal new finding of this study was that, when they were provided with a greater motor control challenge of standing with a narrow base of support, the elderly adults were, as a group, able to depress their soleus H-reflex gain to a level that is a meaningful and large difference from the gain observed when lying (meaningfulness based on suggested interpretation of the effect size value for the t test) (52). Notably, and rather unexpectedly, despite a lack of reflex modulation by elderly adults when performing the mild challenge of natural standing, the large decreases in reflex gain of the elderly and young adults in the challenging tandem stance position were comparable (-30% and -28%). The current pattern of results is similar to that reported for elderly adults with stable balance performance when muscle response onset latencies following a balance perturbation were examined by Lin (27). In both cases, although a mild challenge produced a motor control response in elderly subjects that was significantly different from that observed in young subjects, a greater challenge resulted in no differences between the elderly and young subject responses, for the measures examined. The elderly adults apparently did not adequately sense or respond to a mild perturbation, but this did not mean their response to a more significant challenge was altered, compared with that found in young subjects.

H-Reflex Gain Modulation and Habitual Physical Activity
Five of 16 elderly subjects had small to large increases in soleus H-reflex gain in the natural stance position compared with lying, and the ones with the greatest increases were among the elderly group with lower levels of habitual physical activity (Fig. 3). The lack of highly physically active elderly people who greatly increase soleus H-reflex gain when natural standing is compared with lying may not be random. Perhaps among the elderly group, higher physical activity levels facilitate, but are not required for, the maintenance or development of the ability to decrease H-reflex gain when standing. The capacity for experience, in the form of operant conditioning, to allow primates to upregulate or downregulate the H reflex has already been established (66). Humans appear to be similarly able to learn to decrease H-reflex amplitude (67). Furthermore, when balancing on an unstable surface, humans can learn to decrease soleus H-reflex gain (29). Alternately, some other unidentified factor(s) may be influencing the apparently nonrandom relationship between physical activity level and H-reflex depression upon standing for the elderly adults.

Interpretation of a Modulation of Soleus H-Reflex Gain When Standing
Soleus H-reflex data may provide information on Achilles tendon stretch reflex control because the two reflexes are similarly depressed during gait compared with stance (68)(69), and during unsupported stance compared with supported stance (70), although this is not always found (71). In addition, the soleus stretch reflex is modulated through the step cycle in a pattern that is similar to that observed for the H reflex (47)(69)(72)(73). Still, extrapolating from H-reflex data to stretch reflex function must be done cautiously because the two reflexes are not equally affected by test stimuli producing presynaptic inhibition (14)(74).

Autogenic short latency stretch reflexes contribute significantly to the stiffness of the soleus muscle (20)(21) and play a functional role in facilitating responses to postural perturbations (22)(23)(24)(25). A depression of the soleus H reflex has been observed in young adult subjects under conditions in which postural instability and/or task complexity increases (2)(3)(10)(19)(40). Ia feedback is expected to be enhanced during periods of instability (10)(75). When Ia feedback is elevated, a reduction in Ia feedback activation of soleus {alpha}-motor neurons, via a reduced gain in the reflex loop, has been hypothesized to be advantageous to avoid excessive autogenic excitation and to ensure that the motor neurons remain sensitive to central commands (11)(12)(17). This idea is supported by a modeling study demonstrating that, for slow twitch distal leg muscles (a model of the soleus), the stretch reflex loop is a marginally stable control system because responses to rapid disturbances are significantly delayed (76). Under the condition of mild postural instability in a natural standing posture, elderly adults were observed to not decrease soleus H-reflex gain, compared with lying, and this has been associated with greater body sway when standing (3). An inability of the elderly population to decrease soleus H-reflex gain, however, did not apply during tandem stance. Because the H-reflex gain was similarly decreased in both age groups under the difficult balance condition, any motor control benefit resulting from a decreased gain would be expected to be potentially obtained by the elderly subjects as well as the young subjects, although identification of a specific motor outcome benefit was not performed in this initial study.


   Acknowledgments
 
We thank Dr. P. Bawa for comments made during the preparation of this article, and the manuscript reviewers for their comments.

Received September 5, 2001

Accepted May 24, 2002


   References
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 Abstract
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 References
 

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