A Systematic Review of Isokinetic Knee Muscle Strength in Korean Adults

Kim, Chae-Been; Ahn, Dohyun; Heo, Hae-Ri; Ryu, Seungho; Jo, Eun-Ah; Kim, Ji-Been; Kang, Hyun-Joo; Seo, Dong-Il; Song, Wook; Lee, Han-Joon; Park, Jung-Jun; Chae-Been Kim; Dohyun Ahn; Hae-Ri Heo; Seungho Ryu; Eun-Ah Jo; Ji-Been Kim; Hyun-Joo Kang; Dong-Il Seo; Wook Song; Han-Joon Lee; Jung-Jun Park

doi:10.15857/ksep.2025.00507

Exerc Sci > Volume 35(2); 2026 > Article

Kim, Ahn, Heo, Ryu, Jo, Kim, Kang, Seo, Song, Lee, and Park: A Systematic Review of Isokinetic Knee Muscle Strength in Korean Adults

Review Article

Exerc Sci. 2026; 35(2): 146-154.

Published online: May 15, 2026

DOI: https://doi.org/10.15857/ksep.2025.00507

A Systematic Review of Isokinetic Knee Muscle Strength in Korean Adults

Chae-Been Kim PhD^1,^2,^3,^†

, Dohyun Ahn MS^1,^4,^†

, Hae-Ri Heo BS¹

, Seungho Ryu PhD^1,²

, Eun-Ah Jo PhD^1,²

, Ji-Been Kim PhD⁵

, Hyun-Joo Kang PhD⁶

, Dong-Il Seo PhD⁷

, Wook Song PhD⁸

, Han-Joon Lee PhD⁹

, Jung-Jun Park PhD^1,^2,

¹Department of Sport Science, Pusan National University, Busan, Korea

²Research Institute of Human Ecology, Pusan National University, Busan, Korea

³Institute of Health and Sport Sciences, University of Tsukuba, Ibaraki, Japan

⁴Department of Kinesiology and Sport Management, Texas Tech University, Lubbock, TX, USA

⁵Department of Health Wellness, Yuhan University, Bucheon, Korea

⁶Department of Sports Medicine, Soonchunhyang University, Asan, Korea

⁷Department of Sports Science, Dongguk University, Gyeongju, Korea

⁸Institute of Sports Science, Department of Physical Education, Seoul National University, Seoul, Korea

⁹School of Sport Science, University of Ulsan, Ulsan, Korea

Corresponding author: Jung-Jun Park Tel: +82-51-510-2713 Fax: +82-51-510-3746 E-mail jjparkpnu@pusan.ac.kr

†Authors contributed equally as the first author.

Received Sep 4, 2025 Revised Feb 6, 2026 Accepted Feb 9, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

PURPOSE

Pooled reference data for Korean adults are limited, and Western norms may not be applicable because of ethnic differences. This study aimed to describe sex- and age-specific distributions of knee isokinetic peak torque normalized to body weight in healthy Korean adults.

METHODS

Following PRISMA guidelines, PubMed, Web of Science, Google Scholar, RISS, and KISS were searched for articles published between 1990 and 2023 using combinations of the terms “isokinetic,” “muscle strength,” “muscle function,” and “Korean.” Studies were eligible if they assessed knee isokinetic strength in healthy Korean adults using Cybex or HUMAC NORM. Weighted means and 95% confidence intervals (CI) for peak torque at 60˚/s (% body weight [BW]) were calculated based on sample size.

RESULTS

Thirteen studies (n=426; 245 men, 181 women) were included. In men, left knee extensor peak torque decreased from 267.5 %BW (95% CI 260.7–274.3) in those aged 20–29 years to 143.3 %BW (130.6–155.9) in those aged >60 years. In women, left knee extensor peak torque declined from 227.3 %BW (211.6–242.9) in adults aged 20–29 years to 86.7 %BW (78.7–94.6) in those aged >60 years. Flexor strength showed similar age-related declines, and men consistently exhibited higher peak torque than women.

CONCLUSIONS

This study provides the first combined descriptive estimates for knee isokinetic strength in Korean adults. These values may serve as preliminary reference information for clinical assessment and exercise prescription. Results should be interpreted with caution owing to limited sample sizes in some subgroups. Large, standardized, population-based studies are needed to refine these reference standards.

Keywords: Isokinetic strength, Knee strength, Peak torque, Korean adults

INTRODUCTION

Skeletal muscle is a key organ involved in body movement, postural control, and the regulation of energy metabolism. Loss of skeletal muscle mass and strength is closely associated with an increased risk of falls, disability, cardiovascular disease, and premature mortality [1,2]. Muscle function, including strength, speed, and endurance, is a major determinant of functional capacity in daily life and plays an important role in age-related physical dysfunction and chronic disease prevention [3]. Among the various components of muscle function, muscle strength is relatively easy to assess and has been consistently reported to be associated with physical function and overall health outcomes [4-6]. For this reason, muscle strength is one of the most widely used factors to evaluate muscular fitness in clinical and exercise physiology research. Accordingly, the quantitative assessment of muscle strength in the general adult population is essential for developing strategies to promote healthy aging and preserve functional capacity, highlighting the importance of identifying sex- and age-related patterns.

Muscle strength assessment is performed for various purposes, including the evaluation of exercise performance, monitoring of rehabilitation progress, and health status screening [7]. Handgrip strength and one-repetition maximum (1-RM) testing have traditionally been widely used because of their high accessibility [8]. However, handgrip strength primarily reflects static muscle strength of the upper limbs and does not adequately represent lower-extremity function [9], whereas 1-RM testing requires high mechanical loads and thus carries a risk of injury when applied to older adults or inexperienced individuals during exercise [10]. In contrast, isokinetic muscle function testing induces joint movement under a constant angular velocity, allowing objective comparison of muscle strength across individuals, and it is regarded as a standardized measurement for muscle strength assessment due to its high test-retest reliability, with stable results across different testers and time points [11-14]. In particular, isokinetic knee peak torque is closely associated with functional mobility, such as walking and stair climbing, as well as fall risk, and is therefore widely used in clinical settings as a representative indicator of lower-extremity muscle strength [15,16].

For accurate interpretation of an individual's muscle strength, reliance on absolute values alone is insufficient; rather, it should be determined through relative comparison with reference to normative data that take demographic characteristics such as sex and age into consideration. Such reference values are useful for identifying an individual's current level within a comparable population and may provide an evidence-based foundation for individualized exercise prescription and goal setting. In this regard, the presentation of the sex- and age-specific distribution of knee isokinetic muscle strength in the general adult population has substantial value as foundational data for the early identification of impaired muscle function and the establishment of intervention strategies.

Nevertheless, previous studies on isokinetic muscle strength have been conducted predominantly in specific populations, including athletes, patients with musculoskeletal disorders, and individuals undergoing rehabilitation, leading to limited normative data derived from healthy individuals [17]. Although several studies have attempted to establish normative isokinetic strength values according to sex and age [18,19], these investigations are restricted to limited age groups or focused primarily on athletic or clinical populations. Given this research gap, the representativeness of the data for the Korean general population across the adult lifespan is limited. Furthermore, isokinetic muscle function is known to vary according to not only age and sex but also ethnicity [20], and significant differences in body composition that influence muscle strength expression have been reported between Caucasian and Asian populations [21,22]. Taken together, these findings suggest that knee isokinetic strength reference values established from Caucasian populations should not be directly applied to Korean adults, highlighting the need for independent reference data that reflect the physical characteristics of the Korean population as well as differences by sex and age.

Several domestic studies have examined isokinetic muscle strength in different joints, including the knee, shoulder, and trunk. However, substantial heterogeneity remains across studies regarding the assessment protocols and primary outcome variables [23-25]. Previous studies indicate that the knee is the most frequently assessed joint in research and that the protocols are relatively well standardized, supporting its use as one of the most applicable indicators of muscle strength [26]. Therefore, a systematic review of previous findings with a focus on knee joint strength in Korean adults is needed to improve understanding of overall lower-extremity muscle strength. Moreover, analyzing its sex- and age-related trends to provide foundational evidence for future normative investigations is warranted. Therefore, the purpose of the present study was to systematically review previous studies that assessed knee isokinetic muscle strength in Korean adults and to integrate the reported strength values according to sex and age. This study provides foundational summary data for future muscle strength assessment and for knee-related exercise and rehabilitation research in Korea.

METHODS

1. Study selection and characteristics

The present study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. During the literature search, studies on isokinetic muscle strength were broadly considered; however, data extraction and analysis were restricted to the maximal isokinetic strength of the knee joint. The inclusion criteria were as follows: (1) studies published in Korean or English; (2) studies conducted in Korean populations; (3) studies involving adults aged 19 years or older from the general population, excluding athletes, patients with musculoskeletal disorders, and individuals undergoing rehabilitation after surgery or injury; (4) original research articles conducted in humans; and (5) studies that assessed isokinetic muscle strength of the knee joint using Cybex or HUMAC NORM (Computer Sports Medicine, Inc., MA, USA) isokinetic dynamometers.

The exclusion criteria were as follows: (1) studies with limited full text access; (2) studies that did not report the statistics required for analysis, including mean, standard deviation, and sample size; (3) studies involving minors; (4) studies with substantial heterogeneity in measurement protocols, including the joint assessed, angular velocity, or contraction type, such that meaningful comparison and pooling of the results were not feasible; and (5) studies in which all participants were exposed to a specific training program or therapeutic intervention, thereby limiting the ability to reflect the general baseline muscle strength of Korean adults.

Relevant studies were identified through a stepwise screening process based on the inclusion and exclusion criteria. A total of 1,864 records were initially identified through database searches. After removal of 332 duplicate studies, the titles and abstracts of 1,532 studies were screened. Of these, 988 studies that did not meet the eligibility criteria were excluded, and 544 studies were further assessed through full-text review. Following full-text review, an additional 531 studies were excluded for the following reasons: lack of full-text availability (n=168), insufficient statistical information (n=247), inclusion of minors (n=24), application of a specific intervention to all participants (n=9), and substantial heterogeneity in measurement protocols (n=83). Ultimately, 13 studies were included in the present systematic review. The study selection process and reasons for exclusion are presented in Fig. 1.

Fig. 1.

PRISMA flowchart_Cybex.

2. Data collection

For data collection, both international databases (PubMed, Web of Science, and Google Scholar) and domestic databases (Research Information Sharing Service and Korean Studies Information Service System) were used. The search was limited to articles published between 1990 and 2023, and the following search keywords were applied: (isokinetic OR isokinetic function) AND (muscle strength OR muscle function) AND (Korean).

In the present study, muscle strength was defined as peak torque (Nm) measured at angular velocities of 60°/s and 90°/s, normalized to body weight (kg) and multiplied by 100, and expressed as a percentage of body weight (% Body Weight, %BW) [17]. Accordingly, strength data for the knee extensors and flexors were extracted from studies meeting the eligibility criteria and categorized according to age group and sex. The main characteristics and measurement protocols of the included studies [27-39] are presented in Table 1.

Table 1.

Subject characteristics and summary of measurement protocol

Study	Male (n)	Female (n)	Age group	Joint	Velocity	Repetitions
Yang et al. (2001) [27]	0	14	20–29	Knee	60°/s	3
Bang et al. (2014) [28]	23	0	20–29	Knee	60°/s	3
Yoo et al. (2014) [29]	0	40	Over 60	Knee	60°/s	3
Kim et al. (2016) [30]	80	0	20–29	Knee	60°/s	5
Byun (2016) [31]	0	9	Over 60	Knee	60°/s	4
Park et al. (2017) [32]	20	0	30–39	Knee	60°/s	5
Joo et al. (2017) [33]	0	30	40–49	Knee	60°/s	3
Park et al. (2018) [34]	39	30	Over 60	Knee	60°/s	4
Son et al. (2019) [35]	26	0	30–39	Knee	60°/s	5
	0	34	40–49	Knee	60°/s	5
Lee et al. (2020) [36]	21	0	20–29	Knee	60°/s	5
Choi et al. (2020) [37]	16	0	50–59	Knee	60°/s	5
Kim et al. (2021) [38]	20	0	20–29	Knee	60°/s	5
Kim et al. (2021) [39]	0	24	20–29	Knee	60°/s	3

3. Data analysis

Weighted means were calculated from the selected studies by applying the sample size of each individual study as the weighting factor. In addition, a combined standard deviation was calculated to reflect both within-study variability and variability arising from differences in mean values across studies. Based on this combined standard deviation, the standard error and the 95% confidence interval were subsequently estimated. The formulas used in the present analysis were as follows.

Weighted mean MW=∑i=1kniMi∑i=1kniScombined =∑i=1kni−1si2+∑i=1kniMi−MW2N−1SE=Scombined N95% Confidence Interval =MW±t1−α/2,N−1⋅SE

n_i = sample size of study i; M_i = mean value reported in study i; k = number of included studies;

s_i = standard deviation of study i; si2 = variance of study i; N = total sample size; SE = standard error; df = N − 1

Bilateral strength asymmetry was quantified using a deficit formula based on clinical criteria, whereby the difference between sides was expressed as a percentage of the higher strength value [40].

RESULTS

1. Literature selection and characteristics

A total of 13 studies were included in the final analysis, and summary analyses of knee isokinetic muscle strength according to sex and age were performed using data from 426 healthy Korean adults (245 men and 181 women). All strength variables were expressed as relative strength normalized to body weight (%BW).

2. Isokinetic strength of the knee extensors and flexors according to sex and age

The descriptive statistics for the integrated knee isokinetic strength data extracted from the selected studies are presented in Table 2. Among men aged 20-29 years, left knee extensor and flexor strength were 267.5 %BW (95% CI 260.7–274.3) and 158.5 %BW (95% CI 153.7–163.4), respectively. In women in their 20s, the corresponding values were 227.3 %BW (95% CI 211.6–242.9) and 109.3 %BW (95% CI 100.2–118.4), respectively. Overall, knee extensor and flexor strength reached the highest levels in individuals in their 30s and 40s and then showed a gradual decline with aging.

Table 2.

Descriptive statistics of isokinetic knee strength profiles of Korean adults by sex and age

Sex	Age group	n	Knee Extensors			Knee Flexors			H/Q ratio
Sex	Age group	n	Left, %BW	Right, %BW	Deficit, %	Left, %BW	Right, %BW	Deficit, %	Left, %	Right, %
Male	20–29	144	267.5 (260.7–274.3)	269.4 (261.9–276.8)	0.7	158.5 (153.7–163.4)	161.1 (155.6.0–166.7)	1.6	59.3	59.8
	30–39	46	282.9 (258.9–306.9)	294.3 (267.9–320.8)	3.9	158.8 (149.3–168.3)	165.6 (154.6–176.7)	4.1	56.1	56.3
	50–59	16	217.7 (191.6–243.7)	235.0 (210.3–259.7)	7.4	133.9 (123.7–144.2)	135.9 (122.8–148.9)	1.5	61.5	57.8
	Over 60	39	143.3 (130.6–155.9)	142.0 (129.4–154.5)	0.9	72.3 (62.4–82.3)	74.6 (65.3–83.9)	3.1	50.5	52.5
Female	20–29	38	227.3 (211.6–242.9)	231.1 (216.2–246.0)	1.6	109.3 (100.2–118.4)	115.5 (105.7–125.4)	5.4	48.1	50.0
	40–49	64	232.0 (217.3–246.7)	233.7 (218.5–248.9)	0.7	111.7 (103.0–120.3)	111.4 (102.3–120.5)	0.3	48.1	47.7
	Over 60	79	86.7 (78.7–94.6)	89.7 (81.8–97.5)	3.3	41.5 (36.8–46.3)	44.6 (39.4–49.8)	7.0	47.9	49.7

Normalized peak torque values are presented as weighted means (95% confidence intervals).

DISCUSSION

The present study is meaningful in that it provides integrated summary values of sex- and age-specific knee isokinetic relative strength in Korean adults, for which no systematic analysis had previously been available. Impairment in skeletal muscle function is an important indicator closely associated with aging, musculoskeletal disorders, and reduced quality of life [5,6]. Nevertheless, previous domestic studies have been conducted primarily in special populations, including athletes and patients with musculoskeletal disorders, resulting in a lack of reference data for the general population [19,41]. In response to this gap, the present study systematically reviewed previous studies and summarized sex-and age-specific muscle strength characteristics in Korean adults.

Knee joint strength values were generally higher in men than in women within the same age group, confirming that the physiological sex-related disparity in muscle strength is also evident in Koreans. This finding is consistent with established physiological mechanisms indicating that men possess greater skeletal muscle mass and larger muscle fiber cross-sectional area than women and exhibit greater motor unit recruitment during high-intensity contractions [42-45]. Differences in the proportion of type II muscle fibers and in the level of neuromuscular activation have also been proposed as major contributors to sex-related differences in muscle strength [44,46]. The findings of the present study reaffirm that such sex-related differences are also present in the general Korean adult population and suggest the need for sex-specific standards for muscle strength management.

Relative muscle strength according to age was generally maintained through the third and fourth decades of life in both men and women, followed by a clear decline with advancing age. This pattern is similar to that reported in previous international studies and in Korean studies of handgrip strength [47,48]. Such age-related reductions in muscle strength are attributable to complex physiological degeneration associated with aging, including loss of fat-free mass, impairment of neuromuscular junction function, and selective atrophy of type II muscle fibers [49]. The age-specific summary values derived in the present study reflect these age-related declines in function and may serve as a useful reference in clinical settings for determining whether an individual's muscle strength is appropriate for their age.

In Korean adults, bilateral differences in knee joint strength were generally within 10%, corresponding to the range of functional symmetry [50], while the H/Q ratio was generally below 0.60. Previous studies have suggested that when flexor strength falls below 60% of extensor strength, joint stability may be compromised and the risk of injury may increase [51,52]. In this context, the present findings indicate that strengthening the knee flexors may be important for maintaining joint stability during daily activities in Korean adults. Nevertheless, because these criteria were proposed largely in athletic populations, they should be interpreted as preventive reference indicators rather than absolute pathological thresholds when applied to the assessment of muscle function in the general adult population.

Compared with previous international studies, the isokinetic strength pattern observed in Koreans appears to be broadly consistent with the universal physiological patterns of sex- and age-related variation reported worldwide. Although direct comparison of absolute values is limited by methodological heterogeneity in instrumentation, testing protocols, and normalization methods [53-56], the overall trends in age-related decline and sex differences appear to be consistent [26]. Knee strength values reported in some studies of Brazilian and Chinese adults in their 20s were similar to or slightly lower than the summary values presented in this study [16,56]. These findings suggest that the muscle strength characteristics of Koreans fall within the internationally reported range.

The summary values derived in the present study have several implications for clinical and field-based application. First, they are meaningful in that they provide a descriptive statistical summary of lower-extremity muscle strength levels in Korean adults based on the currently available evidence. Second, they may provide a comparable reference range for evaluating the extent of strength improvement during rehabilitation or exercise interventions and for monitoring age-related declines in muscle function.

Several limitations should be considered when interpreting the findings of the present study. First, because of the limited sample sizes in some sex- and age-specific subgroups, the statistical sensitivity of the weighted means and confidence intervals may have been reduced. Second, because the present study was based on secondary data analysis, variation in measurement protocols across the included studies could not be completely standardized. Third, the left-right deficit values and H/Q ratios presented in this study were not calculated by averaging individual-level ratios; rather, they were recalculated at the group level based on weighted mean values for each group. Accordingly, they may not fully reflect asymmetric patterns at the individual level. Finally, the present study included only data measured using Cybex and HUMAC NORM devices, which may limit the generalizability of the findings. However, this was a methodological decision intended to enhance internal validity and improve data homogeneity by excluding potential mechanical measurement errors derived from differences among devices manufactured by different companies [57,58].

Future studies should establish reference data for isokinetic muscle strength not only for the knee but also for major joints such as the trunk and shoulder through large-scale, multicenter investigations with larger sample sizes and a wider range of ages. In addition, more precise and stratified Korean-specific standards for muscle function should be developed through analyses that take physiological and demographic factors into account, including physical activity level, body fat percentage, risk of sarcopenia, and the presence of chronic disease.

CONCLUSION

The present study provides integrated summary values of sex- and age-specific knee isokinetic muscle strength in Korean adults through a systematic review. The results demonstrated that men generally exhibited higher muscle strength than women within the same age group and that, in both sexes, muscle strength followed a typical pattern of peaking in the third and fourth decades of life and declining thereafter with advancing age. The summary values derived in this study have clinical significance as foundational summary data for understanding muscle function levels in Koreans and for use in rehabilitation goal setting. However, because of limited data availability and heterogeneity in certain age and sex categories, these findings should not be interpreted as absolute normative standards. Further large-scale, multicenter studies with greater population representativeness are needed to establish Korean-specific normative muscle strength data for major joints.

Notes

CONFLICT OF INTEREST

Authors declare no conflicts of interest.

FUNDING

This work was supported by a 2-Year Research Grant of Pusan National University.

AUTHOR CONTRIBUTIONS

Conceptualization: JJ Park, HJ Lee, W Song, DI Seo, HJ Kang, JB Kim; Data curation: D Ahn, HR Heo; Formal analysis: D Ahn, S Ryu; Methodology: D Ahn, CB Kim; Project administration: JJ Park; Visualization: D Ahn, CB Kim, HR Heo; Writing - original draft: D Ahn, CB Kim; Writing - review & editing: CB Kim, S Ryu, EA Jo, JJ Park.

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