Electromyography was performed on the vastus medialis, vastus lateralis, and the rectus femoris during strength measurement at a sampling rate of 1 kHz. The EMG was recorded using bipolar surface disposable electrodes (Blue Sensor, Medicotest) placed on the belly of the vastus medialis, vastus lateralis, and rectus femoris. The inter‐electrode distance was 30 mm. The electrodes were connected to an EMG measurement unit (ME3000, Nihon Medix). The EMG data were transferred into PowerLab (ADInstruments) via an A/D conversion unit. Simultaneous recordings of force and EMG signals during maximal voluntary isometric contraction were performed. To obtain the integrated EMG signal, the EMG signals during over a 1‐s period of steady force output were full‐wave‐rectified and integrated.
Subjects learned to perform maximal voluntary contraction (MVC) in a practice session before measurements were conducted. All subjects were asked to perform MVC of knee extension 3 times at the 70° knee‐flexion position before application of vibration. The measurements were performed with the subject in the sitting position, with the upper body and thigh kept tightly secured to the seat of the isometric exercise machine by belts. The I‐EMGs of all muscles were measured simultaneously during strength measurement. After finishing strength and I‐EMG measurements, vibration stimulation was applied for 20 minutes. Immediately after completion of vibration stimulation, the strength of knee extension and I‐EMG were measured again using the same method as used prior to vibration. For several reasons, the quadriceps femoris was chosen as the object in the present study. First, the quadriceps is the largest muscle in the lower limb and important to maintain posture. Second, a previous study demonstrated that the knee extensors are one of the affected muscle groups in people with a history of falls.19 Third, as the quadriceps has already been examined in many previous studies,20–22 the protocol was established as a valid method.
All data are expressed as means ± SD. In this study, comparisons of changes in strength and I‐EMG were performed between the following 4 groups: (1) young control group, (2) elderly control group, (3) injured side group, and (4) uninjured side group to evaluate the effect of prolonged vibration on the alpha motoneuron activities. One‐factor ANOVA was used to determine the differences of the mean percentage changes in strength and I‐EMG among the 4 groups. The mean percentage changes in strength and I‐EMG after vibration stimulation were calculated as: [(previbration value – postvibration value)/previbration value × 100]. p < 0.05 was taken to indicate statistical significance. Scheffe's F test was used as a post hoc test. Also, one‐factor ANOVA was used to determine the significance of differences in baseline strength value (previbration value) among the 4 groups and Scheffe's F test was used as a post hoc test.
The mean values of knee extension strength before and after 20 min of vibration stimulation in all groups are listed in Table 2. One‐factor ANOVA detected significant differences in strength among the 4 groups. In addition, Scheffe's test revealed that the mean percentage changes in strength of the uninjured (3.09 ± 14.7%) and injured sides (7.49 ± 11.9%) were significantly different from that of the young control group (−13.2 ± 9.57%) but not from that of the elderly control group (−4.60 ± 8.03%). Also, one‐factor ANOVA detected significant differences in baseline strength value among the 4 groups. Scheffe's test revealed that the mean strength of elder control group (333.8 ± 91.0 N), the uninjured (258.2 ± 79.4 N), and injured groups (215.4 ± 112.4 N) were significantly different from that of the young control group (569.5 ± 200.7N).
The mean values of I‐EMG (vastus lateralis, vastus medialis, rectus femoris) before and after 20 minutes of vibration stimulation in all groups are listed in Table 3. One‐factor ANOVA revealed significant differences in I‐EMG of and vastus medialis, but not in the rectus femoris, among groups. Scheffe's test revealed that the mean percentage changes in I‐EMG values for vastus lateralis and vastus medialis in the injured group (vastus lateralis, 17.5 ± 34. 6%; vastus medialis, 19.0 ± 24. 5%) and the uninjured group (vastus lateralis, 8.4 ± 11.7%; vastus medialis, 4.9 ± 12.9%) were significantly different from those of the young control group (vastus lateralis, −19.8 ± 16.5%; vastus medialis, −18.2 ± 10.5%) (Table 3). However, I‐EMG of rectus femoris (10.1 ± 35.9%) of the injured group was not significantly different from that of the young control group (−13.6 ± 15.1%). In addition, even though the mean percentage changes in I‐EMG value of the uninjured group (vastus lateralis, 8.4 ± 11.7%; vastus medialis, 4.9 ± 12.9%) were not significantly different from those of the elderly control group (vastus lateralis, −11.7 ± 10.8%; vastus medialis, −8.8 ± 11.4%), the mean percentage changes in I‐EMG value for vastus lateralis and vastus medialis of the injured group (vastus lateralis, 17.5 ± 34. 6%; vastus medialis, 19.02 ± 24. 5%) were not significantly different from those of the elderly control group (vastus lateralis, −11.7 ± 10.8%; vastus medialis, −8.8 ± 11.4%).
The strength and I‐EMG were reduced after prolonged vibration stimulation in the young healthy subjects. The decreases in strength and I‐EMG in subjects of the young control group represent normal responses to prolonged vibration stimulation.12,20–22 However, the mean percentage changes of strength and I‐EMG of vastus lateralis and vastus medialis of the injured group following prolonged vibration stimulation were significantly different from those of healthy young subjects. These abnormal changes in alpha motor neurons in response to prolonged vibration stimulation indicated gamma loop dysfunction in the injured limbs of these elderly subjects. However, these results could not eliminate the possibility that the injury sustained by these subjects may have affected these abnormal responses. Therefore, the present study was designed to compare the changes in alpha motor neurons in response to prolonged vibration stimulation between the uninjured side of elderly subjects with a history of falls resulting in hospitalization and healthy young subjects. The mean percentage changes of strength and I‐EMG of vastus lateralis and vastus medialis of the uninjured group following prolonged vibration stimulation were significantly different from those of healthy young subjects. Taken together, these observations indicate that the gamma loop in the quadriceps femoris was dysfunctional not only in the injured limbs of these elderly subjects but also in their uninjured limbs. As the gamma loop dysfunction existed even in the uninjured limb of the patients and there were no significant differences in alpha motor neuron activity in response to prolonged vibration stimulation between the injured and uninjured sides, the gamma loop dysfunction found in these subjects was not induced by their old injury. Thus, we inferred that the gamma loop dysfunction was a pre‐existing condition before the injury.
However, the effect of aging on gamma loop function cannot be ignored as previous studies indicated that proprioceptors could degenerate with aging,5 and these receptors play an important role in modulating the gamma loop function.23–27 Therefore, the gamma loop function of elderly subjects who have had no history of falls resulting in hospitalization was also examined in the present study to determine the influence of aging. However, the results of the present study could indicate that gamma loop dysfunction did not exist in the quadriceps of healthy elderly subjects because the mean percentage changes in strength and I‐EMG in the young control group were not significantly different from those of healthy elderly subjects, and the strength and I‐EMG in response to prolonged vibration stimulation in elderly control group tended to be reduced, which could be considered normal response. Therefore, we insisted that aging may not be a factor involved in the induction of gamma loop dysfunction. However, the statistical analysis in present study could not detect differences in the mean percentage of strength and I‐EMG between elderly subjects with no history of falls resulting in hospitalization and the uninjured side of subjects who had such a history of falls. If the gamma loop of subjects in the elderly control group functions as normal as in young normal subjects, significant differences in the mean percentage changes of strength and I‐EMG would have been detected in elderly control group and the uninjured side in elderly subjects with a history of falls resulting in hospitalization. Indeed, the mean percentage changes of strength and I‐EMG in 3 of 10 subjects with no history of falls resulting in hospitalization tended to increase or showed almost no change, which is not considered a normal response; subjects with gamma loop dysfunction as a risk factor of falls may have been included in the elderly control group. Therefore, we speculated that this discrepancy within the elderly control group may explain why no differences in mean percentage changes of strength and I‐EMG were detected between elderly subjects with no history of falls and the uninjured side of subjects with a history of falls. Indeed, in contrast to the results of the present study, a recent study concluded that aging results in gamma loop dysfunction even though only a small number of subjects (4 subjects) were included in the study.28 Further studies are needed to identify the effects of aging on gamma loop function.
In conclusion, as the gamma loop did not function normally in both the injured and uninjured sides of subjects with a history of hospitalized injury, the existence of gamma loop dysfunction could be a risk factor associated with falls. However, gamma loop dysfunction was not detected in healthy elderly subjects. There is a discrepancy within the group of healthy elderly subjects regarding gamma loop dysfunction. These elderly subjects who may have gamma loop dysfunction might have a higher risk of falls. Furthermore, the results of the present study demonstrated that all elderly subjects do not have a dysfunctional gamma loop. Therefore, aging is not the only factor involved in increasing the risk of falls, but other factors must also play a role. The detection of gamma loop dysfunction may be useful for screening to identify elderly subjects at risk of fall injury. Additionally, further research that investigates how neurological disease and aging could interact to reduce gamma loop function in elderly population will need to be performed.
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Key Words: vibration stimulation; falls; aging; muscle strength; electromyography© 2007 Academy of Geriatric Physical Therapy, APTA