Descriptive data are shown as means ± SD in the text and figures. A one-way ANOVA was used to test the differences among the subject groups in the measured variables. If the F-statistic of the ANOVA was significant, differences among the subject groups were tested using a Games–Howell test. These data were analyzed using SPSS ver.15.0 for Windows statistical software (SPSS, Inc., Tokyo, Japan). A linear piecewise continuous regression model (26) was used to analyze the relationship between KET/BM and QF %EMGmax under squat movement. The level of significance was set at P < 0.05.
The KET/BM ratio was significantly lower (P < 0.001) in the frail elderly group (0.7 ± 0.2 N·m·kg−1) than in the older (1.5 ± 0.9 N·m·kg−1), middle-aged (3.0 ± 0.9 N·m·kg−1), and younger (5.0 ± 0.7 N·m·kg−1) groups (Fig. 3). The differences between the older and either the middle-aged or younger groups and between the middle-aged and younger groups were also significant (P < 0.001).
QF %EMGmax during squat movement.
QF %EMGmax during the task movement was 72.0% ± 19.2% in the frail elderly group, 51.6% ± 22.7% in the older group, 25.3% ± 9.6% in the middle-aged group, and 13.8% ± 4.1% in the younger group (Fig. 4). All differences among the subject groups in QF %EMGmax during the quarter squat were significant (P < 0.05–0.001).
Relationship between KET/BM and QF %EMGmax.
QF %EMGmax was nonlinearly related to KET/BM (Fig. 5). The linear piecewise continuous regression analysis showed that there was a breakpoint of 1.9 N·m·kg−1 in the relationship between the two variables. In individuals with KET/BM less than 1.9 N·m·kg−1, QF %EMGmax sharply increased from 29% to 107% as KET/BM became lower.
The current result indicated that QF %EMGmax during body mass–based squat movement was influenced by the force generation capability of QF. This is concordant with the previous report on the EMG activities during ADL (25). A novel finding obtained here is that QF %EMGmax was nonlinearly related to KET/BM with the breakpoint of 1.9 N·m·kg−1. In this relationship, a slight change of KET/BM sharply affected QF %EMGmax in the individuals with less than 1.9 N·m·kg−1, whereas a significant change of KET/BM yielded a relatively small change in QF %EMGmax in individuals with more than 1.9 N·m·kg−1 of KET/BM. This supports the assumption set at the start of the study and indicates that, for the individuals with less than 1.9 N·m·kg−1, the magnitude of effort to perform the squat task rapidly increases as KET/BM decreases.
To our knowledge, only three studies have tried to investigate the level of the force generation capability of knee extensor muscles required to successfully perform ADL (29,30,35). Among the limited reports, Ploutz-Snyder et al. (29) tried to identify the threshold of knee extensor strength in men and women age 52–92 yr, among whom the performance in ambulatory tasks is compromised. From their findings, individuals with a KET/BM of less than 3.0 N·m·kg−1 are at a substantial risk for impaired function in chair rise, gait speed, and climbing up and down stairs. This value is higher than the KET/BM at the breakpoint observed in this study (1.9 N·m·kg−1). The discrepancy may be due to the differences between the prior and present studies in the task and/or the methods used for determining the threshold. Ploutz-Snyder et al. (29) categorized the subjects as “no difficulty” and “difficulty” on the basis of observed performance of the chair rise task and identified the threshold as the value that optimally discriminates between the two classes.
Furthermore, Rantanen et al. (30) reported that the minimum threshold of knee extensor strength required for walking 1.22 m·s−1 was 1.1 N·m·kg−1 in disabled older women. In addition, Yoshioka et al. (35) investigated the computation of kinematics and the minimum peak joint moment of sit-to-stand movement. They concluded that the relation between the peak joint moments at the hip and knee joints was complementary and that the sum of those torques needed to be greater than 1.53 N·m·kg−1 to perform a successful sit-to-stand movement. In their results, when the peak hip joint moment was minimized (0.24 N·m·kg−1), the peak knee joint moment increased to 1.28 N·m·kg−1, and conversely, when the peak knee joint moment was minimized (0.51 N·m·kg−1), the hip joint moment increased to 0.98 N·m·kg−1. Compared with these values, the KET/BM (1.9 N·m·kg−1) at the breakpoint obtained in this study is somewhat higher. In the current study, all subjects successfully completed the task, even if the difficulty in performing the task might have differed among the subjects. As described earlier, the physiological effort required to do tasks can be evaluated by recording EMG (11). Furthermore, the percentage of knee extensor moment produced during ADL to the maximal knee extensor moment can be associated with relative VL EMG activity (9). Therefore, although the observed breakpoint of 1.9 N·m·kg−1 does not correspond to the minimum threshold of knee extensor strength for successfully performing the squat movement, it should be considered as a reserve capacity threshold of knee extensor strength required to perform the squat movement without great difficulty.
Even if the explanation mentioned above is acceptable, physiological reasons why QF %EMGmax was nonlinearly related to KET/BM with a breakpoint of 1.9 N·m·kg−1 remain unclear. In the relationship between the two variables, most of the frail elderly and older groups were plotted in the region of less than 1.9 N·m·kg−1 of KET/BM. It has been shown that age-related loss in leg extensor power is more pronounced than that in the isometric knee extensor strength (32). Also, Lindle et al. (20) found a significant age-related loss in the concentric torque relative to thigh nonosseous fat-free mass in men and women age 20–93 yr. Considering these findings, it seems that age-related loss in the available maximal force generation capacity under a multijoint dynamic task such as squat movement would be greater compared with that in a single-joint movement such as knee extension. If so, it might be a reason for the steeper increase in QF %EMGmax for the frail elderly and older groups.
Another possibility is that the magnitude of the coactivation of the hamstrings and quadriceps during the squat movement might have differed between either the frail elderly or older group and the younger group. In the present study, EMG recordings for the hamstrings have not been performed. However, it is well known that older individuals compared with younger ones show higher antagonist coactivation during multijoint movements such as level walking (10,28,31), stair walking (19), downward stepping (8), and balance control tasks (23). Although the cocontraction of hamstrings as antagonists plays an important role for stabilizing the knee joint (15), it results in a negative moment in relation to the moment developed by agonists, reducing the net resultant moment output (22). Therefore, it seems that, for the frail elderly or older individuals, an additional effort would be required to resist negative effort to the force production of knee extensors, produced by the coactivation of the hamstrings, for performing the squat movement. This might be a reason why QF %EMGmax in the frail elderly or older individuals was beyond the value expected from the association between KET/BM and QF %EMGmax for the younger individuals.
Apart from the physiological background for the breakpoint of 1.9 N·m·kg−1, the observed relationship between KET/BM and QF %EMGmax will be a useful piece of information to discuss the effects of body mass–based exercises as resistance training. Aniansson and Gustafsson (1) have claimed that the effects of resistance training using body mass–based tasks vary widely. It has been considered that the load applied to the leg muscle with a body mass–based task such as squat, sit-to-stand from a chair, and walking is light. In fact, the intensity as resistance training is low in previous studies (1,17,25). However, substantial data on the muscular activities of the knee extensors during body mass–based resistance training have not been shown clearly so far. In the current result, the mean value of QF %EMGmax for the frail elderly group was 72%. McDonagh and Davies (24) have suggested that the lower limit of the intensity of weight resistance training, which is required to improve the maximum strength, is 66% of one repetition maximum load. For most of the frail elderly, therefore, the body mass–based squat will be a useful measure for improving knee extensor strength if it is taken as an exercise of resistance training. For the middle-aged and younger groups, however, the intensity of the squat task will be too low, and for the older group, the existence of the training effect may depend on their initial levels of KET/BM before intervention.
Before summarizing the current results, we should comment on the factors that might influence the current results, with relation to the limitation of the study design used here. First, the present study did not determine the EMG activities of the vastus medialis muscle. Trappe et al. (34) have shown that VL is the largest among the four muscles of QF and there is no significant difference between the young and the elderly in the percentage of the volume for each of four muscles of QF to the total volume of QF, which supports the use of VL to represent the QF muscle in aging research. In the report of Takai et al. (33), there was no significant interaction between age and muscle (RF, VL, and vastus medialis) in EMG activities during sit-to-stand movement, expressed as relative values to those during MVC. On the other hand, there is the idea that the oblique portion of vastus medialis (VMO) plays a role in controlling normal patellofemoral joint motion (27). In a loaded isometric knee extension task, the ratio of VMO to VL in EMG activity tended to increase with increasing load, suggesting an increase in the activity of VMO relative to VL (7). Therefore, we cannot rule out that the relationship between QF %EMGmax and KET/BM might differ from that observed here, if the EMG activity of VMO is determined and added to calculate QF %EMGmax. Second, it should be noted that, as shown in Figure 5, QF %EMGmax for a frail elderly was greater than 100%. This suggests that the subject might have not been able to fully activate knee extensors in the MVC measurement. Some studies have shown that older adults are able to achieve complete activation during maximal voluntary contractions (2,5,16). In the present study, however, whether the subjects could fully activate QF muscle during MVC measurement was not determined. In addition to the point mentioned above, this is also a matter that should be examined in future studies.
In conclusion, the present study showed that the activity level of QF during a body mass–based squat movement was influenced by its force generation capability. For individuals with a KET/BM less than 1.9 N·m·kg−1, a squat movement is considered to be a fairly high-intensity task. The breakpoint of 1.9 N·m·kg−1 may be assumed to be a threshold level of knee extensor strength, which should be maintained for performing the activities of daily living without great difficulty.
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