Phase angle is associated with muscle endurance and estimated maximum oxygen uptake in healthy young males

Abstract

[Purpose] This study aimed to clarify the utility of Phase Angle by examining its relationship with muscle endurance and whole-body endurance in healthy young Japanese males. [Participants and Methods] This cross-sectional exploratory study included 24 healthy young male participants, aged 19.1 ± 0.3 years. Body composition by Bioelectrical impedance analysis, including whole body Phase Angle, was measured. To assess the muscle endurance of the quadriceps femoris and hamstrings, participants performed a test using an isokinetic dynamometer. The angular velocity was set at 180°/sec, a standard velocity used in endurance assessments. Oxygen uptake and heart rate were measured during a multistage continuous treadmill test, and maximum oxygen uptake was estimated by calculating a linear regression equation based on the relationship between heart rate and oxygen uptake. Pearson’s product-moment correlation analysis was performed to examine associations between Phase Angle, maximum oxygen uptake, and isokinetic measurement variables. [Results] Only Phase Angle showed significant correlations with extension 180°/sec maximum peak torque/body mass (%), muscle fatigue, and maximum oxygen uptake. [Conclusion] These results suggest that Phase Angle can be identified as a potential marker associated with muscle and whole-body endurance in young, healthy Japanese men. Phase Angle measurements may offer valuable insights for assessing physical condition in this population.

INTRODUCTION

Bioelectrical impedance analysis (BIA) is a non-invasive tool for nutritional and health assessment that has been used in many clinical and sports performance settings to characterize cellular health¹⁾. Previous studies have shown that Phase Angle (PhA) decreases with age^{2, 3)} and is correlated with muscle strength—such as grip strength and knee extensor strength—in middle-aged and older adults^{4, 5)}. Furthermore, Akamatsu et al.⁶⁾ and Matsumoto et al.⁷⁾ observed a significant positive correlation between the phase angle and grip strength in an adult⁶⁾ and amateur soccer players⁷⁾, respectively.

The relationship between maximum voluntary knee extension strength and PhA has been investigated in young Japanese males⁸⁾. However, the relationship between PhA and indicators of muscle function—specifically endurance and fatigue measured using an isokinetic dynamometer—remains unclear.

Maximum oxygen uptake is a standard criterion for assessing endurance capacity. To account for body size, maximum oxygen uptake is a typically normalized by dividing by body mass. Recent advancements in BIA have simplified the measurement of body composition, making the information more accessible. Consequently, there is a need to examine the relationship between maximum oxygen uptake and endurance not only relative to body mass but also using values normalized to fat-free mass and fat mass. Although Yamada et al.⁹⁾ reported a correlation between PhA and aerobic capacity based on walking tests, they did not directly measure oxygen uptake. If PhA is associated with whole-body and muscular endurance, it could serve as practical alternative to the more time- and equipment-intensive measurements of muscle strength and oxygen uptake. Establishing the utility of simple PhA measurements is therefore important for clinical practice and exercise prescription.

This study aimed to clarify the utility of PhA by examining its relationship with muscle endurance and whole-body endurance in healthy young Japanese males. Muscle endurance was assessed using an isokinetic dynamometer to measure isokinetic strength and whole-body endurance was determined by maximum oxygen uptake.

PARTICIPANTS AND METHODS

This cross-sectional exploratory study included 24 healthy young male participants, aged 19.1 ± 0.3 years. Exclusion criteria were: the presence of implantable medical devices or orthopedic prosthesis; skin wounds at the electrode placement sites, any clinical condition or medication usage known to impact water compartments or body cell mass; and participation in competitive sports. The age range was restricted to 19–20 years to control for age-related variation in the outcome measures. Female individuals were excluded to avoid potential confounding effects on the menstrual cycle on hydration status and bioelectrical measurements.

All assessments were conducted at the Exercise Laboratories of the Faculty of Physical Therapy, International University of Health and Welfare, Odawara campus, between May and June 2025. The study protocol was approved by the Ethics Committee of the International University of Health and Welfare (Approval No. 24-TA-127), and conducted in accordance with the Declaration of Helsinki.

BIA was performed using an InBody 380 body composition analyzer (InBody Co., Ltd., Seoul, Korea). Measurements were performed at least a 2-h fasting period. Participants were measured in a standing position after following a period of rest. Body composition, including whole-body PhA, was measured. Whole-body PhA was derived from resistance and reactance values obtained at a frequency of 50 kHz.

To assess the muscle endurance of the quadriceps femoris and hamstrings, participants performed a test using an isokinetic dynamometer (Biodex System 4, Biodex Medical Systems Inc., Shirley, NY, USA). The angular velocity was set at 180°/s, a standard velocity used in endurance assessments. Participants performed 10 maximal repetitions with verbal encouragement. Although previous studies^{10, 11)} used 30¹⁰⁾ and 50¹¹⁾ repetitions, we used 10 repetitions to maintain participant motivation and focus.

We assessed relative isokinetic strength (%) for extension and flexion movements of the dominant lower limb were used the maximum peak torque from the test in our analysis. Muscle fatigue was expressed as a percentage using the formula (work during last 3 repetitions/work during first three repetitions ×100)¹⁰⁾. Although we have implemented System 4, we use System 3 (User Guide Rev3.2.7) for measurements to ensure data continuity. Therefore, gravity correction is performed to match the participant’s joint angle at a position 26° from the anatomical position. After positioning participants and adjusting the equipment, the operator set the range of motion at 110°, and had participants perform familiarization trials consisting of concentric extension and flexion movement at 180°/sec; with 60-s pauses between trials.

Oxygen uptake and heart rate were measured during a multistage continuous treadmill test. The protocol consisted of eight stages. Each lasting three minutes. Participants walked at a constant speed of 3.5 km/h with slopes of 0, 5, 10, and 15%. These conditions were repeated with additional weight: 1 kg distributed in four pockets on torso clothing, plus 1 kg on each wrist and 2 kg on each ankle. In order to estimate maximum oxygen intake from heart rate, it was necessary to minimize artifacts in the electrocardiogram monitor waveform. The speed was fixed at 3.5 km/h, and the exercise style was maintained as walking, with a weight belt used to increase the load intensity. Oxygen uptake was measured breath by breath using an exhaled gas analyzer (AEROMINITOR AE-310S, Minato Medical Sciences Co., Ltd., Tokyo, Japan), and heart rate was monitored using via electrocardiogram (WEP-1200, Nihon Kohden Co., Ltd., Tokyo, Japan). Data from the last minute of each stage was used for analysis. A linear regression equation was established to describe the relationship between heart rate and oxygen uptake. Maximum oxygen uptake was estimated by extrapolating the regression line to predict maximum heart rate, calculated as 220 – age. The regression model demonstrated strong validity for all participants, with correlation coefficients r≥0.90 and coefficient of determination (R²) ≥0.81.

Measurements of body composition, peak isokinetic torque, and oxygen uptake were conducted for each participant between 9:30 and 11:30 on separate days, with the order of assessments randomized. All examiners were licensed physical therapists experienced with the respective equipment. Different therapists performed measurements based on their equipment specialization. No results were shared among examiners until all data collection was complete.

BIA and oxygen uptake results are presented as means and standard deviations. Pearson’s product-moment correlation analysis was performed to examine associations between PhA, maximum oxygen uptake, and isokinetic measurement variables. All statistical analyses were performed using IBM SPSS for Windows version 25 (IBM Corp., Armonk, NY, USA), with the significance level set at 5%.

RESULTS

The body composition characteristics and estimated oxygen uptake of the participants are shown in Table 1. The results of the correlation analysis are shown in Table 2. PhA showed a significant correlation with relative isokinetic strength (180°/sec) normalized to body mass (r=0.466, p<0.05). Conversely, only PhA demonstrated a significant negative correlation with muscle fatigue during extension movements (r=−0.499, p<0.05). No significant correlations were observed between oxygen uptake and any isokinetic dynamometer measurements. Furthermore, significant positive correlations were found between PhA and oxygen uptake, both when adjusted for body mass and fat-free mass (r=0.545, p<0.01; r=0.588, p<0.01, respectively).

Table 1. Body composition characteristics and estimated VO₂max expressed as mean ± standard deviation.

	Males (n=24)
Age (years)	19.1 ± 0.3
Body mass (kg)	63.9 ± 6.7
BMI (kg/m2)	21.5 ± 2.7
Fat mass (kg)	10.0 ± 3.4
Fat-free mass (kg)	54.0 ± 4.5
Phase angle (°)	6.4 ± 0.3
Estemated VO2max (mL/min)	2,772 ± 531
Estemated VO2max/Body mass (mL/min∙kg)	43.5 ± 7.5
Estemated VO2max/Fat mass (mL/min∙kg)	310.6 ± 132.0
Estemated VO2max/Fat-free mass (mL/min∙kg)	51.2 ± 7.5
Extension 180°/s maximum/Body mass (%)	141.1 ± 20.5
Flexion 180°/s max/Body mass (%)	89.0 ± 19.0
Extensor muscle fatigue (%)	28.3 ± 8.4
Flexor muscle fatigue (%)	31.3 ± 10.4

BMI: body mass index.

Table 2. Pearson product-moment correlation analysis results.

	Extension 180°/s max/Body mass (%)	Flexion 180°/s max/Body mass (%)	Extensor muscle fatigue (%)	Flexor muscle fatigue (%)	Phase angle (°)
Phase angle (°)	0.466§	0.314	−0.449§	−0.265	-
Estimated VO2max/Body mass (mL/min∙kg)	0.396	0.333	−0.080	0.020	0.545§§
Estimated VO2max/Fat mass (mL/min∙kg)	0.269	0.191	0.080	−0.050	0.203
Estimated VO2max/Fat-free mass (mL/min∙kg)	0.343	0.196	−0.090	0.080	0.588§§

^§p<0.05, ^§§p<0.01.

DISCUSSION

This is the first study to demonstrate that PhA is associated with muscle endurance capacity in healthy young Japanese males, as assessed by isokinetic strength measurements and muscle fatigue indicators. Furthermore, we found a significant positive correlation between PhA and maximal oxygen uptake. These results suggest that PhA could serve as a practical predictor of both muscle endurance quality and overall endurance capacity, potentially reducing the need for direct measurement of oxygen consumption using masks or muscle strength assessment using isokinetic dynamometers. This finding highlights the clinical relevance of PhA measurements for assessing physical status in healthy male population.

PhA showed significant association with relative peak knee extension torque and muscle fatigue, but not with relative peak knee flexion torque and muscle fatigue. Thorstensson et al.¹²⁾ reported that the proportion of fast-twitch muscle fibers influences torque magnitude, particularly under high-speed exercise conditions. In our study, maximum knee extension torque exceeded flexion torque, which may reflect a high proportion of fast-twitch fibers in extensor muscles that could also affect fatigue characteristics.

Rosa et al.¹³⁾ found that PhA in the dominant leg correlated significantly with both isometric and isokinetic (180°/sec) muscle strength in extension and flexion, suggesting a relationship between PhA and type II (fast-twitch) fiber proportion during rapid movements, as proposed by Ryushi and Fukunaga¹⁴⁾. However, since Rosa et al.¹³⁾ studied a mixed-sex middle-aged cohort with a wide range, these correlations might be influenced by age and sex differences. Further research is needed to clarify the mechanisms underlying these direction-specific movement differences.

Mitochondria in muscle cells have long been recognized for their crucial role in aerobic exercise performance and endurance¹⁵⁾. PhA, as an indicator of cellular health, reflects cellular nutritional status, cell membrane integrity, and cell function¹⁶⁾.

Our results demonstrate that PhA is associated with muscle endurance and whole-body endurance capacity. Given that PhA has been identified as a significant covariate of mitochondrial oxygen consumption¹⁷⁾, it may consequently serve as a partial indicator of mitochondrial function.

This study has several limitations. First, as a cross-sectional exploratory investigation, it cannot establish causal relationships, and results should be generalized with caution. The number of participants was small, consisting of only 24 healthy males. Studies with fewer than 30 participants often lack sufficient precision in estimation, raising concerns about generalizability and increasing the likelihood of Type II error, which may obscure clinically meaningful differences. Second, because we recruited participants who did not engage in regular training, the number of repetitions was set at 10. However, additional research should be conducted with a higher number of repetitions. In addition, it is also necessary to consider measurements with different angular velocity settings. Third, regarding oxygen intake, an indicator of whole-body endurance, it is highly appropriate to consider it in relation to whole-body PhA. However, because right knee extension torque and fatigue level were also used as main outcomes, more detailed consideration is needed by examining it by body part, particularly PhA of the right lower limb.

To our knowledge, this is the first study to identify PhA as a potential marker associated with both muscular and general endurance capacity in young, healthy Japanese males. PhA measurements may offer valuable insights for assessing physical condition in this population.

Conflict of interest

The authors declare no conflict of interest in this work.