RELATIONSHIP OF LEG STRENGTH, H-REFLEXES AND BALANCE IN YOUNG AND OLDER ADULTS.

The purpose of this study was to examine the relationship between leg strength (Hamstring and Quadriceps), H-reflex modulation and static balance ability in young and older subjects. Ten young (mean age - 22.4 yrs) and 18 older (age > 60 yrs) adults were tested. The older subjects were further divided in three groups of 6 subjects (3 males13 females): 60-69 yrs (mean age = 65.1 yrs), 70-80 yrs (mean age = 76.6 yrs) and over 80 yrs (mean age = 82.6 yrs). All older subjects were physically active, with no history of neuromuscular and/or orthopedic disorders. Each subject reported to the lab for one day of testing, during which three tests were administered: 1) H-reflex; 2) leg strength; and 3) static balance. For H-reflex testing, the procedures outlined by Hugon (1973) were followed. Surface electrodes (Therapeutic Unlimited) were applied to the soleus muscle of the right leg, and the soleus H-reflex was administered via a 1 ms electrical squarewave pulse (Grass Instruments S88 stimulator). The H-reflex recruitment curve was determined with the subject standing (maintaining balance), and prone. The Hh4 ratio (ratio of maximum H-reflex to maximum muscle response) was calculated under each condition. Reflex modulation was defined as the difference between the Hh4 ratio in the two conditions. For leg strength, both the quadriceps and hamstrings muscle groups were tested on a Cybex I1 isokinetic dynomameter. Five trials were administered at a speed of 60 degrees per second: Balance was measured with a Kistler force platform. The subject was instructed to stand motionless for six 15-second trials. The amount of sway (sagittal and lateral in mm) and the area of sway (cm2) was calculated with a customdesigned computer program (sample rate - 50 Hz). Of the six trials, three were performed with eyes open and three with eyes closed. Results indicated that the elderly adults had significantly lower H-reflexes than the young subjects in both the standing ( 31. 6% - 59. 9 % ) and the prone (31. 9 % - 7 3. 5 % ) conditions. Also, the young subjects demonstrated modulation of the H-reflex in the two postures, whereas the older adults did not. As expected, the older subjects experienced more sway that the younger subjects in both the eyes open (1668.34 - 1024.86 cm ) and the eyes closed (2268.55 - 1165.5 1 cm2) conditions. Also, without vision, the older subjects increased their sway by 35.9%, whereas the young subjects increased 13.7%. For the older adults, quadriceps strength (r = -0.40) and hamstring strength (r = -0.44) were moderately correlated with postural sway. These results indicate that both H-reflex modulation and leg strength may contribute to postural stability in the older adult. Further, as training induced changes in the reflex system have been established (Wolpaw, 1990), it is concluded that intervention programs targeting Hreflex modulation and leg strength may prove beneficial for fostering postural stability in the older adult