The stepping test, and infrared depth sensor, provide reliable measures of balance in community-dwelling older adults.

[Purpose] The purpose was to examine relationships between variables characterizing the 20-second stepping test movement pattern, assessed with an infrared depth sensor (KinectTM), and measurements of dynamic and static balance. [Participants and Methods] A total of 27 independent-living, older adults (7 males and 20 females) participated in this study. For each participant, the stepping test total movement distance, movement displacement, knee movement distance, and step number were calculated from Kinect closed joint-point coordinate data. Dynamic and static balance were assessed using a NeuroCom Balance Master Platform system. [Results] The stepping test total movement distance had a moderate negative correlation with endpoint excursion (r=-0.469) and a moderate positive correlation with total movement distance, corrected for knee movement distance (r=0.557). Step numbers had a moderate negative correlation with stepping test total movement distance (r=-0.667) and total movement distance, corrected for knee movement distance (r=-0.531). Division into high and low-balance sub-groups (based on endpoint excursion or sway velocity scores) revealed that stepping test total movement distances and movement displacements were greater, and step numbers were fewer, in low balance groups. [Conclusion] The stepping test, combined with a KinectTM-assessed movement pattern provides a simple, objective, reliable means for assessing balance in community-dwelling, independent-living older adults.

A 20-sec Stepping Test and KINECTTM Sensor Provides Objective Quantification of Movement/Balance Dysfunction in Older Individuals.

Background: Tests such as the Functional Independence Measure (FIM) are widely used measures of infirmity and burden of care. However, these scales are largely qualitative and especially problematic when assessing movement-based tasks. Effective, reliable analysis of human movement is technically complicated and expensive, but an infrared depth sensor is potentially a low-cost, portable devise which may provide a quantitative aspect to clinical testing. Our purpose was to assess the utility of a 20-sec stepping test (ST) and KinectTM infrared-depth sensor in providing objective evaluation of balance toward identifying disability in older adults.Methods: Men and women between 64 and 90 years of age, consisting of independent (IG; n = 37) and dependent (DG; n = 38) living at community, geriatric day-care center in Japan. Total movement distance (TMD) and greatest displacement (MMD) were calculated from KinectTM recording of the ST.Results: DG had lower FIM scores than IG. TMD and MMD were significantly greater in DG than IG, while step number and rate were lower in DG. Receiver-operator characteristic analysis showed TMD, TMD/step, MMDstep, and MMD corrected for time and height strongly discriminated between assignment to DG or IG with moderate sensitivity and specificity.Conclusions: Greater TMD and MMD observed during a 20-sec ST appear to indicate disability with moderate sensitivity and specificity in older adults. Measures of movement distance (e.g. TMD, MMD) appear indicative of changes in dynamic balance due to a circuitous movement pattern generated by aberrant step replacement with repeated stepping-in-place.

Prediction and Validation of DXA-Derived Appendicular Fat-Free Adipose Tissue by a Single Ultrasound Image of the Forearm in Japanese Older Adults.

OBJECTIVES: To develop regression-based equations for estimating dual-energy x-ray absorptiometry (DXA) derived appendicular fat-free adipose tissue (FFAT) using a single ultrasound image in the forearm, and to investigate the validity of those equations to calculate FFAT-free appendicular lean mass (aLM-minus-FFATappendicular ) in 311 Japanese adults aged 60 to 79 years. METHODS: Subjects were randomly separated into two groups: 215 in the model-development group (91 men and 124 women) and 96 in the cross-validation group (42 men and 54 women). Appendicular fat mass and aLM were measured by the DXA, and subcutaneous adipose tissue (AT-forearm) and muscle (MT-ulna) thicknesses were measured by ultrasound. Appendicular FFAT was calculated based on the results of a previous study (appendicular FFAT = appendicular fat mass/0.85 x 0.15). The aLM was estimated from MT-ulna using a previously published equation (aLM = 4.89 x MT-ulna x body height - 9.15). Stepwise linear regression analysis was used to determine predictive models for DXA-derived appendicular FFAT from AT-forearm, sex, age, and anthropometrical variables. The best ultrasound prediction equation for estimation of appendicular FFAT was developed and then cross-validated in a subsample of older adults. RESULTS: There was no significant difference between the DXA-derived and ultrasound-predicted aLM-minus-FFATappendicular . A strong correlation was observed between the DXA-derived and ultrasound-predicted aLM-minus-FFATappendicular (r = 0.935, P < .001). Bland-Altman analysis did not indicate a bias in the prediction of the aLM-minus-FFATappendicular for the validation group. CONCLUSIONS: Our results indicated that a single ultrasound forearm measurement can be used to accurately estimate DXA-derived aLM-minus-FFATappendicular in Japanese older adults, which may be advantageous for community-based physical examinations.

Prevalence of Poor Sleep Quality and its Association with Lifestyle Habits, Competition-Based Activities, and Psychological Distress in Japanese Student-Athletes During the COVID-19 Pandemic.

Objectives The present study clarified the prevalence of poor sleep quality and its relation to lifestyle habits, competitive-based activities, and psychological distress among Japanese student-athletes in the initial pandemic period (2020) and 1 year later (2021). Methods In the present study, student-athletes were defined as individuals belonging to university athletic clubs. The data of two cross-sectional surveys (2020: n = 961 and 2021: n = 711) were collected from student-athletes in 6 universities in Japan. First, the prevalence of poor sleep quality (Pittsburgh sleep quality index score > 5) was investigated. Relationships between poor sleep quality and lifestyle habits, competition-based activities, and psychological distress were then explored using multivariate logistic regression analysis adjusted for age, sex, and body mass index. Results The prevalence of poor sleep quality was 33.6% in 2020 and 36.6% in 2021. Poor sleep quality in 2020 was related to late bedtime, taking supplements before bed, part-time job (no late night), stressors of expectations and pressure from others, and psychological distress, whereas that in 2021 was related to early wake-up time, skipping breakfast, taking caffeinated drinks before bed, use of smartphone/cellphone after lights out, stressors of motivation loss, and psychological distress. Conclusions In both 2020 and 2021, one-third of student-athletes had poor sleep quality and psychological distress was its common risk factor. Lifestyle habits and competition stressors associated with poor sleep quality were pandemic-specific in 2020, but similar to the prepandemic period in 2021.

Effects of body mass-based squat training in adolescent boys.

The purpose of this study was to determine the effects of body mass-based squat training on body composition, muscular strength and motor fitness in adolescent boys. Ninety-four boys (13.7 ± 0.6 yrs, 1.60 ± 0.09 m, 50.2 ± 9.6 kg) participated in this study and were randomly assigned to training (n = 36) or control (n = 58) groups. The training group completed body mass-based squat exercise training (100 reps/day, 45 sessions) for 8 weeks. Body composition and muscle thickness at the thigh anterior were determined by a bioelectrical impedance analyzer and ultrasound apparatus, respectively. Maximal voluntary knee extension strength and sprint velocity were measured using static myometer and non-motorized treadmill, respectively. Jump height was calculated using flight time during jumping, which was measured by a matswitch system. The 8-wk body mass-based squat training significantly decreased percent body fat (4.2%) and significantly increased the lean body mass (2.7%), muscle thickness (3.2%) and strength of the knee extensors (16.0%), compared to control group. The vertical jump height was also significantly improved by 3.4% through the intervention. The current results indicate that body mass-based squat training for 8 weeks is a feasible and effective method for improving body composition and muscular strength of the knee extensors, and jump performance in adolescent boys. Key pointsAn 8-wk body mass-based squat exercise training decreased percent body fat in adolescent boys.The body mass-based squat exercise training increased muscle size and strength capability of the knee extensors in adolescent boys.The squat exercise training improves vertical jump height in adolescent boys.

Profile of regional fat and fat-free soft tissue accumulation in male athletes.

BACKGROUND: It is unclear whether or not the breakpoint (BP), at which the proportion of each of fat mass (FM) and fat-free soft tissue mass (FFSTM) to body mass (BM) alter, exists in male athletes. We examined the hypothesis that in male athletes, the regional FM and FFSTM-BM relationships have a BP, but the body mass at BP (BMBP) differs among the arms, trunk, and legs. METHODS: By using a dual X-ray absorptiometry, whole-body and regional FMs and FFSTMs in the arms, trunk, and legs were estimated in 198 male athletes (20.8 ± 2.1 years; 1.73 ± 0.07 m; 72.7 ± 14.8 kg). To detect the BP in the relationship between each of FM and FFSTM and BM, a piecewise linear regression analysis was used. If a BP was detected in the corresponding relationship, the significant difference between the regression slopes above and below the BP was examined. RESULTS: The regression analysis indicated that the BMBP existed in the FM- and FFSTM-BM relationships regardless of region and whole body. For the whole body, BMBP was 81.8 kg for FM and 82.2 kg for FFSTM. In regional FM-BM relationships, BMBP was 80.5 kg for arms, 82.6 kg for trunk, and 63.3 kg for legs, and the regression slopes above the BMBP became higher than those below the BP, and vice versa in regional FFSTM-BM relationships (BMBP 104.6 kg for arms, 80.9 kg for trunk, and 79.0 kg for legs). The relative differences in the slopes between below and above BMBP in the regional FM-BM relationships were higher in the arms and trunk than in the legs, and those in the regional FFSTM-BM relationships in the legs than in the trunk. CONCLUSION: Whole-body and regional FM- and FFSTM-BM relationships for male athletes have breakpoints at which the proportion of the tissue masses to BM alters. The BMBP and differences in the distribution of regional FM and FFSTM around the breakpoint are region specific.

Repeated sit-to-stand exercise enhances muscle strength and reduces lower body muscular demands in physically frail elders.

PURPOSE: To examine the effect of a conditioning program consisting of repeated sit-to-stand exercise on knee extensor strength and muscular activities during body mass-based squat movement in physically frail elders. METHODS: Fourteen men and women aged 75 to 88 years who used the long-term care insurance system participated in the 12-week training program (48 reps/session, 3 sessions/week). Isometric knee extension torque (KET) during a maximum voluntary contraction (MVC) and electromyogram (EMG) activities of the rectus femoris and vastus lateralis muscles during the MVC and a body mass-based squat task were determined at baseline, and following 4 and 12 weeks training. KET was expressed relative to body mass (KET/BM) and EMG activities during the squat task were normalized to that during a MVC and averaged (QF %EMGmax). RESULTS: KET/BM increased from 1.07 ±â€¯0.28 Nm/kg at baseline to 1.26 ±â€¯0.26 Nm/kg at week 4 and 1.31 ±â€¯0.28 Nm/kg at week 12 (P < 0.001), and QF %EMGmax decreased from 67.2 ±â€¯17.2% to 49.3% at week 4 (P < 0.001) and 43.5 ±â€¯7.7% at week 12 (P = 0.016). At each of the three measurement time points KET/BM was inversely correlated with QF %EMGmax (r = -0.78 to -0.86, P ≤ 0.001). CONCLUSION: For physically frail elders, a short-term conditioning program consisting of repeated sit-to-stand exercise is effective in increasing knee extensor strength and reducing the muscular effort required for lowering and raising the body.

Evaluating activities of daily living using an infrared depth sensor: KINECTTM.

PURPOSE: With a growing proportion of elderly people in the population, the maintenance of activities of daily living (ADLs) in elderly people is crucial to keep medical costs down. We investigated the ADL measurement accuracy of KINECTTM and Kinect Studio. To eliminate the subjectivity of conventional methods, we numerically assessed motions with computer analysis. METHODS: Eighteen actions that repeated "move" and "stationary" phases, including movement of arms, legs, head and torso were measured using KINECTTM. Errors and standard deviations of joint coordinates at the stationary points outputted from KINECT Studio were evaluated. Simultaneous measurements were performed with KINECTTM using conventional high-performance motion capture, and the output was treated as a true value for comparison. RESULTS: In most motions, errors of the joint coordinates were within 100 mm; however, there were two cases where errors due to the skeleton-model estimation by KINECT Studio increased. Firstly, when a part of the body unexpectedly moved out of the infrared measurable area, and secondly, when parts of the body overlapped each other on the KINECTTM image. CONCLUSIONS: KINECTTM and Kinect Studio are effective for ADL assessment when positions that cause large errors are excluded. Since KINECTTM has sufficient precision, it should also be possible to develop a more appropriate ADL evaluation system with a new algorithm of skeleton-model estimation that does not depend on KINECT Studio. Implications for Rehabilitation The KINECTTM and Kinect Studio are effective for ADL assessment when positions that cause large errors are excluded With an increasing proportion of elderly people in the population, the maintenance of activities of daily living (ADLs) in elderly people is crucial to keep medical costs down Systems such as the KINECTTM can support these goals.

The impact of DXA-derived fat-free adipose tissue on the prevalence of low muscle mass in older adults.

BACKGROUND/OBJECTIVES: To investigate the impact of eliminating fat-free adipose tissue (aFFAT) on the prevalence of low muscle mass in older adults. SUBJECTS/METHODS: Three hundred and forty-three (153 men and 190 women) well-functioning Japanese older adults (aged 65-79) had their appendicular lean mass (aLM) and appendicular fat mass (aFM) measured using dual-energy X-ray absorptiometry (DXA). aFFAT was then estimated from DXA-derived aFM (aFM = (FM/0.15)*0.85). Both traditional cutoffs and those corrected for aFFAT were used for diagnosing low muscle mass. RESULTS: With traditional cutoff values, the prevalence of low muscle mass using the unadjusted aLM index was 20.1%. After adjusting the aLM index for aFFAT, the prevalence increased to 49.0% (p < 0.001). However, when the cutoff values were also adjusted for aFFAT, the prevalence of low muscle mass only increased to 23.0% (p < 0.001). Further, ~5% of the participants (7 men and 8 women) were newly classified as having low muscle mass after correction for aFFAT. However, several women (n = 5) were not classified as having low muscle mass using the corrected cutoff value, although they would have been when using the non-corrected cutoff. CONCLUSIONS: Adjusting for the effect of aFFAT on DXA-derived aLM significantly increases the prevalence of low muscle mass in older adults. For clinical research and practice, the influence of aFFAT on DXA-derived aLM may need to be taken into consideration when diagnosing low muscle mass.

Body fat percentage assessment by ultrasound subcutaneous fat thickness measurements in middle-aged and older adults.

BACKGROUND & AIMS: B-mode ultrasound accurately measures both muscle mass, body density and percent body fat (% BF) in younger adults, but how well it can estimate % BF in middle-aged and older adults using DXA-derived %BF as the criterion is unclear. We sought to develop % BF prediction equations for middle-aged and older adults using ultrasound subcutaneous fat thickness (SFT). METHODS: A cross-sectional study of Japanese adults (n = 414, 50-79 years) where 276 subjects were randomly assigned to a model development group and the other 138 subjects were assigned to a cross-validation group. B-mode ultrasound measured SFT at nine sites. Dual energy X-ray absorptiometry (DXA) measured % BF, arm fat mass (FM) and leg FM. Stepwise multiple linear regression developed prediction equations from anthropometric data (body mass, height, waist and hip circumference) and ultrasound SFT sites. Bland-Altman plots assessed validity of the prediction equations to measure % BF in the cross-validation group. RESULTS: The best prediction equation for % BF was the following: [% BF = 15.709 + (1.753*anterior trunk SFT) + (5.626*Sex) + (3.635*posterior upper arm SFT) - (4.428*anterior lower leg SFT) - (0.170*height) + (0.264*waist) + (anterior thigh SFT*2.241); r2 = 0.809, standard error of the estimate (SEE) = 3.3 kg]. Arm FM and leg FM prediction equations had r2 values ranging from 0.690 to 0.812 and SEEs of 0.29 and 0.75 kg. A small mean bias was noted for estimating % BF (-0.14%), but large limits of agreement were found (-8.0-7.7%) and systematic error was noted in all of the equations (r = 0.275 to 0.515, p < 0.05). CONCLUSIONS: Despite high r2 values and a small mean bias found between predicted and DXA % BF, wide limits of agreement were found with some systematic error present. Therefore, these prediction equations for middle-aged and older adults may not be sufficiently accurate to use in a clinical setting.

Proficiency in pole handling during Nordic walking influences exercise effectiveness in middle-aged and older adults.

Nordic walking (NW) is a total body version of walking increasingly used as a health-promoting activity by middle-aged and older adults. The present study examined the relationship between force exerted through the pole and physiological response during NW. In this non-randomized exercise trial, 17 participants comprising 8 middle-aged and older recreationally trained Nordic walkers (NWrec: 63.7 ± 8.1 years) and 9 experienced NW instructors (NWinstr: 57.5 ± 7.8 years) underwent outdoor ordinary walking (OW) and NW bouts as fast as possible for 12 minutes. Walking distance, speed, heart rate (HR), energy expenditure (METs and J/kg/m) and upper and lower limb muscle activities using surface electromyogram (EMG) were assessed. A pole with a built-in load cell measured force applied to the pole with peak pole force, pole contact time, % of pole contact time with respect to the gait cycle, and pole impulse derived. We conducted two-way analysis of covariance adjusted for age and BMI. There was a significant group and walking type interaction for walking distance and speed (P = 0.04), METs (P < 0.01), and HR (P = 0.04) with higher values in the NWinstr group during NW than OW. As expected, upper limb EMG activities increased (P < 0.01) with NW in both groups. All pole force measures were significantly higher in NWinstr than NWrec (P ≤ 0.01). Change in walking distance and speed were correlated with pole peak force (r = 0.67, P < 0.01) and pole impulse (r = 0.63, P = 0.01). Similarly, change in METs was associated with peak pole force (r = 0.66, P < 0.01) and pole impulse (r = 0.56, P = 0.02). These results indicate that planting the pole on the ground more forcefully and for longer periods to derive a driving force in NW enhances the effectiveness of the exercise and potentially the health-derived benefits.

Body shape indices are predictors for estimating fat-free mass in male athletes.

It is unknown whether body size and body shape parameters can be predictors for estimating whole body fat-free mass (FFM) in male athletes. This study aimed to investigate whether body size and shape variables can be predictors for FFM in male athletes. Using a whole-body dual-energy X-ray absorptiometry scanner, whole body fat mass (FM) and FFM were determined in 132 male athletes and 14 sedentary males. The sample was divided into two groups: validation (N = 98) and cross-validation (N = 48) groups. Body height (BH), body mass (BM), and waist circumference at immediately above the iliac crest (W) were measured. BM-to-W and W-to-BH ratios were calculated as indices of body shapes. Stepwise multiple regression analysis revealed that BM/W and W/BH were selected as explainable variables for predicting FFM. The equation developed in the validation group was FFM (kg) = 0.883 × BM/W (kg/m) + 43.674 × W/BH (cm/cm)- 41.480 [R2 = 0.900, SEE (%SEE) = 2.3 kg (3.8%)], which was validated in the cross-validation group. Thus, the current results demonstrate that an equation using BM/W and W/BH as independent variables is applicable for predicting FFM in male athletes.

Ultrasound-Derived Forearm Muscle Thickness Is a Powerful Predictor for Estimating DXA-Derived Appendicular Lean Mass in Japanese Older Adults.

To test the validity of published equations, anterior forearm muscle thickness (MT-ulna) of 158 Japanese older adults (72 men and 86 women) aged 50-79 y was measured with ultrasound. Appendicular lean soft tissue mass (aLM) was estimated from MT-ulna using two equations (body height without [eqn 1] and with [eqn 2]) previously published in the literature. Appendicular lean mass was measured using dual-energy X-ray absorption (DXA), and this method served as the reference criterion. There was a strong correlation between DXA-derived and ultrasound-estimated aLM in both equations (r = 0.882 and r = 0.944). Total error was 2.60 kg for eqn (1) and 1.38 kg for eqn (2). A Bland-Altman plot revealed that there was no systematic bias between DXA-derived and ultrasound-estimated aLM; however, eqn (1) overestimated aLM compared with DXA-derived aLM. Our results suggest that an ultrasound MT-ulna equation that includes body height is appropriate and useful for estimating aLM in Japanese adults.

Effects of school-based squat training in adolescent girls.

BACKGROUND: For adolescent girls, less information on the effects of school-based exercise training is available from earlier studies. This study aimed to determine the effects of school-based squat training on body composition and muscular strength in adolescent girls. METHODS: Fifty-two girls (13.8±0.6 years) were randomly assigned to the training and control groups. The training group conducted an 8-week body mass-based squat exercise training (100 reps/day, 45 sessions) as a part of after-school activity. Body composition (bioelectrical impedance analyzer), muscle thickness at the thigh anterior (ultrasound), and maximal isometric knee extension strength (myometer) were determined before and after the intervention. The magnitude of maturation was assessed using Tanner stage criteria of pubic hair before the intervention. RESULTS: After the intervention, percent body fat decreased in the training group, but increased in the control group. The relative changes in lean body mass, muscle thickness and muscular strength were similar between both groups. In the training group, the relative change in knee extension strength was correlated to the magnitude of maturation before the intervention. CONCLUSIONS: For adolescent girls, an 8-week body mass-based squat training is feasible for lowering percent body fat. In addition, the strength improvement for the knee extensors partially depends on the magnitude of maturation at start of the intervention.

Associations of sit-up ability with sarcopenia classification measures in Japanese older women.

To test the hypothesis that sit-up performance is associated with sarcopenia classification measures, 93 older women aged 53-78 years were divided into three groups based on achieved repetitions (30 s) for the sit-up performance test: Group 0 (G 0, n = 33) performed 0 repetitions, Group 1-9 (G 1-9, n = 30) performed between 1 and 9 repetitions, and Group 10+ (G 10+, n = 30) performed over 10 repetitions. Dual-energy X-ray absorptiometry-derived appendicular lean soft tissue mass (aLM), handgrip strength (HGS), usual walking speed, and chair stand were measured, and low muscle mass (aLM index) and poor physical function were defined according to previous studies. Age and body mass index were similar among the three groups. HGS was higher in G 10+ compared with G 0. The prevalence rate of low muscle mass was 30% for G 0, 20% for G 1-9, and 3% for G 10+. Low HGS was observed in both G 0 (24%) and G 1-9 (20%), but not in G 10+. Only two persons in G 0 were classified as slow walking speed. Our results suggest that sit-up performance may be a useful indicator to determine the extent of sarcopenia because low muscle mass and poor function were almost non-existent in individuals who could perform over 10 sit-ups.

Effect of maturation on muscle quality of the lower limb muscles in adolescent boys.

BACKGROUND: The purpose of this study was to clarify the effect of maturation on the muscle quality of the lower limb muscles around puberty. METHODS: Subjects were 117 Japanese boys age 12 to 15 years. The maturity status was assessed by using a self-assessment of stage of pubic hair development based on the criteria of Tanner. On the basis of the criteria, subjects were divided into the prepubescent or pubescent group. Muscle thickness of knee extensors and plantar flexors were measured by a B-mode ultrasound. Muscle volume index (MV) was calculated from muscle thickness and limb length. Maximal voluntary isometric joint toques (TQ) of knee extension and ankle plantar flexion were measured using a myometer. Muscle quality was derived from dividing TQ by MV (TQ/MV). RESULTS: In both muscles, TQ-MV relationships were also similar between the prepubescent and pubescent groups, and there was no significant difference in TQ/MV between the two groups when chronological age was statistically adjusted. CONCLUSION: The current results indicate that, for adolescent boys, the muscle quality of the lower limb muscles is not significantly influenced by maturation.

Influence of maturation on anthropometry and body composition in Japanese junior high school students.

BACKGROUND: The purpose of this study was to examine maturity-related differences in anthropometry and body composition in Japanese youth within a single year. METHODS: Two hundred and ten Japanese youth aged from 13 to 13.99 years participated in this study. Their maturity status was assessed using a self-assessment of stage of pubic hair development. Bioelectrical impedance analysis was used to estimate percent body fat and lean body mass (LBM). Muscle thickness of the anterior thigh, posterior lower leg and rectus abdominis muscles were measured by ultrasound. RESULTS: For boys, height, body weight, and LBM in less mature groups were lower than that in more mature groups. The maturity-related differences were still significant after adjusting for chronological age. On the other hand, muscle thickness values in the lower extremity and abdomen differed among the groups at different stages of pubic hair development, whereas there was no maturity-related difference in the relative values corrected by LBM, except for those thickness values measured at the abdomen. For girls, only the muscle thickness at the anterior thigh and muscle thickness relative to LBM1/3 at the posterior lower leg was significantly affected by maturity status, but significant maturity-related difference was not found after adjusting for chronological age. CONCLUSIONS: At least for Japanese boys and girls aged 13 years, maturity status affected body size in boys, but not in girls, and the influence of maturation on the muscularity of the lower extremity and trunk muscles is less in both sexes.

Association between knee extensor strength and EMG activities during squat movement.

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 (%EMG(max)) to that during maximal voluntary contraction. The %EMG(max) 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 %EMG(max)). RESULTS: QF %EMG(max) 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 %EMG(max) 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.