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Medicine & Science in Sports & Exercise:
doi: 10.1249/MSS.0b013e3182207ed8
Applied Sciences

Association between Knee Extensor Strength and EMG Activities during Squat Movement

FUJITA, EIJI; KANEHISA, HIROAKI; YOSHITAKE, YASUHIDE; FUKUNAGA, TETSUO; NISHIZONO, HIDETSUGU

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Author Information

National Institute of Fitness and Sports in Kanoya, Kagoshima, JAPAN

Address for correspondence: Hiroaki Kanehisa, Ph.D., National Institute of Fitness and Sports in Kanoya, 1 Shiramizu, Kanoya, Kagoshima 891-2393, Japan; E-mail: hkane@nifs-k.ac.jp.

Submitted for publication January 2011.

Accepted for publication April 2011.

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Abstract

Purpose: The present study aimed to clarify how the force-generating capability of quadriceps femoris (QF) is associated to its surface EMG activity during a body mass–based squat movement.

Methods: Isometric knee extension torque (KET) during maximal voluntary contraction and EMG activities of the rectus femoris and vastus lateralis muscles during a body mass–based squat movement were determined in 53 men and 48 women age 19–90 yr, including 18 frail elderly persons who used the long-term care insurance system. The rectified EMG signals during the squat movement were averaged and normalized as the relative value (%EMGmax) to that during maximal voluntary contraction. The %EMGmax values for rectus femoris and vastus lateralis were averaged and used as an index representing the level of muscular activities of QF during the squat movement (QF %EMGmax).

Results: QF %EMGmax was nonlinearly related to KET relative to body mass (KET/BM). 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 rapidly increased as KET/BM decreased.

Conclusions: The current results indicate that the activity level of QF during a body mass–based squat movement is influenced by its force generation capability. For individuals with a KET/BM less than 1.9 N·m·kg−1, body mass–based squat movement is considered to be a fairly high-intensity exercise. 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.

It is well documented that the mass and force-generating capacity of skeletal muscles decrease with aging (12,14,21). In particular, the knee extensor muscles show a greater age-related loss compared with other muscles located in limbs (14,21). This is linked to the augmentation of difficulty of performing the activities of daily living (ADL), such as walking, rising, and stepping (9). In addition, the weakness of knee extensor muscles in the elderly contributes to the deterioration in postural stability (13), which may increase the risk and frequency of falls. Thus, clarifying the level of knee extensor strength at which the elderly experience greater difficulty when performing ADL will be valuable information for screening individuals who may encounter the aforementioned risks, as well as for designing optimal strength training programs for them.

Some studies have examined the physiological measurements of muscular effort during ADL by recording surface EMG (9,18) or calculating a percentage of the available maximal torque generation capacity (9) during a given task. According to Landers et al. (18), older women showed higher EMG in the rectus femoris (RF) muscle, normalized to those during isometric maximal voluntary contraction (MVC) task, in the standing task compared with younger women. Because EMG normalized for that at MVC can be an index of relative muscular stress while performing ADL (11), Landers et al. (18) suggested that older women with less knee extensor strength experience a greater difficulty in performing the standing task than younger women. Furthermore, Hortobágyi et al. (9) reported that older adults performed ADL, involving climbing up and down stairs and rising from a chair, near their maximal torque-producing capabilities of the knee extensor muscles.

The previous findings cited above suggest that the EMG activities of knee extensor muscles under ADL tasks, expressed as relative values to those recorded in MVC, can be related to their force generation capability. At the same time, the possible relationship between the two variables will be a useful measure when quantifying the physiological effort for performing the corresponding tasks. In fact, Takai et al. (33) observed a linear relationship between the relative EMG activities during ADL tasks in the quadriceps femoris (QF) muscle and knee extensors torque relative to body mass. In addition, Hortobágyi et al. (9) reported a moderately close association between knee extensor moments and vastus lateralis (VL) EMG activities during ADL, normalized to those during MVC. However, these findings were based on comparisons among the limited subject samples, i.e., younger versus elderly individuals. It is unknown how the force generation capability of knee extensor muscles relates to its EMG activity level during a body mass–based task in a sample with a large variation in age, including both younger and frail elderly persons.

Previous studies examining the relationships between either muscular strength or power and the performance scores of various body weight bearing tasks have shown that the corresponding relationships are not linear beyond a certain threshold level and that an increase in strength or power does not improve the performance scores (3,30). For example, Ferrucci et al. (6), who examined elderly women age 65 yr and older, reported that individuals with good muscle strength showed a departure from linearity in the measures of muscular strength and physical performance of the lower extremities. Furthermore, Buchner et al. (4) also found a nonlinear relationship between leg strength and gait speed in the elderly age 60–90 yr. In the relationships between muscle strength and performance scores, the additional strength beyond the threshold has been considered as a reserve capacity or a safety margin (30). If these findings are taken into account together with the previous findings on EMG during ADL (9,18), it may be assumed that the magnitude of EMG activities during body mass–based tasks is nonlinearly related to knee extensor strength with a certain breakpoint, at which the corresponding relationship between the two variables will differ. If the breakpoint exists, the substantial data on the knee extensor strength will provide useful information for discussing the force generation capability of knee extensor muscles, corresponding to the reserve capacity threshold for performing ADL without greater effort.

The purpose of the present study was to clarify how the magnitude of EMG activities during a body mass–based task can be related to the force generation capacity of QF. To this end, we selected a squat movement as body mass–based task because a sit-to-stand movement requires greater muscle strength than other daily activities, such as walking or stair climbing (29,35). During the body mass–based squat movement, the EMG activities of RF and VL were determined in 53 men and 48 women age 19–90 yr, including 18 frail elderly persons. We examined the relationship between EMG during the squat movement, which was normalized to those under MVC and isometric knee extension torque (KET) under MVC, which was expressed relative to body mass.

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METHODS

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Subjects.

A total of 101 men (n = 53) and women (n = 48) age from 19 to 90 yr, including 18 frail elderly persons who have used a long-term care insurance system, voluntarily participated in this study. The subjects were categorized as follows: frail elderly (n = 18, using the long-term care insurance system); older (n = 28, age ≥65 yr), middle-age (n = 40, age 30–64 yr); and younger (n = 15, age <30 yr) groups. Table 1 shows the means and SD of age, height, and body mass in each subject group. Body mass was measured to the nearest 0.1 kg using a body fat analyzer and weighing scale (TBF-305; Tanita, Tokyo, Japan). The frail elderly group was referred to us by the comprehensive community support center and in-home care management offices in Fukuoka City. None of them had a history of diseases such as CNS disorders and were unable to go out unsupported and refrained from outing alone because of decreased physical capacity. This study was approved by the Ethics Committee of the National Institute of Fitness and Sports in Kanoya and was consistent with their requirement for human experimentation. The subjects and their families were fully informed about the procedures to be used as well as the purpose of the study. Written informed consent was obtained from all the subjects.

Table 1
Table 1
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Experimental design.

The current study used a quarter squat as a body mass–based task in which subjects were asked to stand up from a position with the knee joint flexed to 45° and then to return to the starting position in time with a given cadence. After a period of standardized warm-up and familiarization with the measurement procedures, the subjects were asked to perform the squat movement five times successively at a speed of about once every 2 s. While the subjects performed the task, we positioned our hands in front and behind the subjects without touching them to avoid the risk of the subjects falling or losing body balance. The changes in knee joint angles during the squat task were monitored using an electronic goniometer (SG150; Biometrics, Gwent, United Kingdom). An electronic goniometer with an angle feedback device (PH-521; DKH, Tokyo, Japan) that emitted a sound when knee flexion reached 45° was fastened to the lateral aspect of the left knee. All subjects including the frail elderly successfully completed the squat movement task without losing body balance or falling out of the cadence. The measurements of the torque during MVC knee extension and surface EMG of QF during the MVC and squat tasks were performed for the right leg. In prior studies, which examined the association between knee extensor strength and EMG activities during ADL, two-joint muscle RF (18) or single-joint muscle VL (9) was used as a representative muscle of QF. The squat movement used here involves both hip and knee extensions. In the current study, therefore, the EMG were obtained from RF and VL. A rest period of more than 5 min was set between each of the practice sessions and squat movement and MVC tasks to avoid an accumulation of fatigue that might be caused by the trials. No participant complained of knee discomfort during and after the squat movement and MVC tasks.

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Measurement of knee extension MVC torque.

KET during isometric MVC was measured using a custom-made dynamometer (D-08011B; Takei, Niigata, Japan) with tension/compression load cells (LUR-A-SA1; Kyowa, Tokyo, Japan). In the KET measurements, the right ankle of the subject was fixed to the strap linked to the dynamometer with knee and hip joint angles of 90° (full extension = 0°). Postural changes during torque measurements were avoided by constraining the hips and back in the seat using lap belts. Torque data from each trial were amplified using a strain amplifier (DPM-751A; Kyowa). The torque signals obtained via a 16-bit analog/digital converter (PowerLab/16S; ADInstruments, Sydney, Australia) were recorded on a personal computer at a sampling frequency of 2 kHz. To standardize raw measurements and localize the action to the appropriate muscle group, the subject sat on an adjustable chair with support for the hip and back. Before the maximal testing, the subjects were asked to exert submaximal force isometrically in the test position to familiarize themselves with the test procedure. After a process of warming up and a rest period of 3 min, the subjects were encouraged to exert maximal force for about 5 s on a ramp three times, with at least 3 min between trials to exclude the influence of fatigue. The peak value in the torque curve was evaluated as KET. The highest value among the three trials was taken for the subsequent analysis. The KET was expressed as the relative value to body mass (KET/BM).

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EMG measurements.

Surface EMG was recorded from the right RF and VL by bipolar configuration during the MVC task and squat movements. The Ag–AgCl bar electrodes (thickness = 1 mm, length = 7 mm, interelectrode distance = 12 mm) were placed over the muscle bellies of RF and VL after the skin surface was cleaned with alcohol and rubbed with sandpaper. The electrode for RF was placed at the middle of the distance from the lateral condyle of the femur to the greater trochanter. The electrode for VL was placed at one-third of the distance from the superior border of the patella to the greater trochanter. The electrodes were connected to a preamplifier and a differential amplifier with a band-pass filter set to 5–500 Hz (DL-141; S&ME, Tokyo, Japan). The electrodes were connected to the 16-bit analog/digital converter (PowerLab/16S; ADInstruments) with the attachment unit (DL-720; S&ME), and EMG signals were recorded on a personal computer at a sampling frequency of 2 kHz. The EMG signals were full-wave rectified and integrated using analysis software (Chart version 5.11; ADInstruments). The EMG data for the MVC trial were averaged for 1 s around half a second at the point of maximal torque (Fig. 1). The EMG data for the squat movements were analyzed for those during the second, third, and fourth trials (Fig. 2). The EMG data for the three squat movements were averaged and normalized as the relative values to that during MVC trials (%EMGmax). The %EMGmax values for RF and VL were averaged and used as an index representing the level of muscular activities of QF during the squat movement (QF %EMGmax).

Figure 1
Figure 1
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Figure 2
Figure 2
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Statistical analysis.

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.

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RESULTS

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KET/BM.

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).

Figure 3
Figure 3
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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).

Figure 4
Figure 4
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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.

Figure 5
Figure 5
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DISCUSSION

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.

This work was supported by a Focus Study Projects Grant (12010602) from the National Institute of Fitness and Sports in Kanoya to Eiji Fujita.

The authors acknowledge no conflict of interest, and no companies or manufacturers will benefit from the results of the study.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Journal of Sports Science and Medicine, 12(1): 60-65.

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Keywords:

BODY MASS–BASED TASK; FORCE GENERATION CAPABILITY; BREAKPOINT; AGING; ACTIVITY OF DAILY LIVING

©2011The American College of Sports Medicine

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