

Citation: Kim, J.; Park, S.-K.
Differences in Physical
Characteristics of the Lower
Extremity and Running
Biomechanics Between Different Age
Groups. Int. J. Environ. Res. Public
Health 2022, 19, 4320. https://
doi.org/10.3390/ijerph19074320
Academic Editor: Sechang Oh
Received: 6 February 2022
Accepted: 30 March 2022
Published: 4 April 2022
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International  Journal  of
Environmental Research
and Public Health
Article
Differences in Physical Characteristics of the Lower Extremity
and Running Biomechanics Between Different Age Groups
Jongbin Kim 1 and Sang-Kyoon Park 2,*
1
Division of Kinesiology, Silla University, Busan 46958, Korea; kjb36@silla.ac.kr
2
Motion Innovation Center, Korea National Sport University, Seoul 05541, Korea
*
Correspondence: spark@knsu.ac.kr; Tel.: +82-10-5378-9617
Abstract: (1) Background: The objective of this study was to determine physical and biomechanical
changes in age groups upon running. (2) Method: 75 male adults (20–80s) participated in the study.
Bone mineral density and lower extremity joint strength were measured according to age-increase
targeting. Based on age, correlations among running characteristics, impulse, impact force, maximum
vertical ground reaction force, loading rate, lower extremity joint 3D range of motion, joint moment,
and power upon running motion were calculated. (3) Result: Older runners tended to show lower
bone mineral density, extremity maximum strength, stride time, and stride distance, with smaller
RoM and joint power of ankle and knee joints in the sagittal plane, compared with younger subjects.
However, there were no signiﬁcant correlations between age and impact variables (i.e., impulse,
impact force, peak GRF, and loading rate) during running. (4) Conclusion: Older runners tend to
show weaker physical strength characteristics, such as bone mineral density and muscle strength
and lower joint functionality of ankle and knee joints during running, compared with younger
runners. Therefore, strengthening the lower extremity muscle and improving dynamic joint function,
especially for ankle joints, can be helpful for injury prevention during running.
Keywords: bone mineral density; maximal strength of lower joint; aging; running; kinematics
1. Introduction
Physical aging causes high blood pressure, diabetes, obesity, and bone mineral density
(BMD) decrease; thus, physical changes such as aerobic capacity decline, coordination
decrease, muscle function weakening, decreased gait ability, and risk of chronic disease are
induced [1–3]. Among the changes in physical characteristics due to aging, BMD gradually
decreases from 35 years of age, and osteoporosis is caused from 50 years of age [4]. Muscle
volume starts to decrease by 10% compared with people in their 20s and this decrease
is accelerated from 65 years of age. At 70 years of age, the mean strength is 60% of that
of people in their 20s [5]. Consequently, independent life becomes impossible due to
fracture damage and mental health, and all this may work as a potential factor for life
quality decline [6–8]. To delay physical aging, regular exercise is essential. Regular exercise
furthers functional physical health, including physical strength retention and enhancement,
cardiovascular function improvement, muscle strength increase, ﬂexibility increase, and
mental health. Likewise, regular runs are effective for health enhancement and aging
prevention [9–13].
As interest in running has recently increased for health enhancement, participants
in running races of various distances and courses have also steadily increased. Because
running’s temporal, spatial, and cost limitations are slight, its universality is proven as a
health enhancement exercise, compared to other physical activities [14]. Regular physical
exercise and compound exercise are suitable methods to maintain BMD through proper
physical activities in everyday life, and they are frequently used for BMD improvement
and osteoporosis treatment [15]; however, bone health may be negatively affected [16].
Int. J. Environ. Res. Public Health 2022, 19, 4320. https://doi.org/10.3390/ijerph19074320
https://www.mdpi.com/journal/ijerph
Int. J. Environ. Res. Public Health 2022, 19, 4320
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BMD increase between the lumbar and the thigh showed no signiﬁcant difference in
endurance running but exhibited a remarkable variance in weight in a study targeting
long-distance runners aged 18 to 44 [17]. Various types of studies are required to analyze
the effects of exercises on BMD. According to physical aging, the decreased rate of lower
extremity strength is higher than the upper extremity strength. The muscle thickness of the
gastrocnemius and thigh skeletal muscles decreases as one gets older, and consequently,
joint power gradually reduces [18–20]. The lower extremity strength is maintained and
increases with resistance exercise and weight-bearing (load) exercise [21,22]. Previous
studies reported an improvement of 27% total muscle strength, 27% knee joint ﬂexion, and
17% extension strength among elderly people who performed lower extremity resistance
exercises for 14 weeks [23]. The elderly’s ankle and knee joints’ ﬂexion and extension force
was 70–80% of that of male adults in general. Regular physical activities to reduce risk
factors, revealed due to physiological aging, were found adequate for the retention of and
delayed decrease in bone mineral and strength [24].
A study on running motions reported that sagittal plane movements impact absorption,
foot stability, balancing, and acceleration in the stance section when running. Additionally,
most injuries, such as ankle sprain, stress fracture, backache, and muscle rupture, occurred
in the stance section [25].
According to the analysis of the result, depending on age, the range of motion of the
hip joint was greater, and the range of motion of the knee and ankle joints was smaller
in the elderly people group than the young adult group. The impact force occurring in
the initial stage stance section was larger in the young adult group compared with the
elderly group [26–30]. In a dynamic analysis targeting adults between 18 and 60 years
of age, stride decreased along with vertical ground reaction force, ankle moment, and
power [31]. If the impact was not absorbed by adequately increasing the range of motion
of the knee and ankle joints while running, it was reported that a lower extremity joint
injury might occur [32]. However, very low joint stiffness caused by an increased range
of motion may induce running injuries of the soft tissues and musculoskeletal system of
runners’ lower extremities. Regular physical activities positively affect the lower extremity
BMD and joint strength. Therefore, regular physical activities reduce musculoskeletal
system injuries through a reduction in physical change due to aging [33]. However, it is
insufﬁcient to explain aging through the fragmental comparison of males in their 20s and 80s
biomechanically with running motions among regular physical activities through existing
previous studies. This study examined the effect of aging on physical characteristics, joint
movement, and joint power, targeting males from 20 to 80 who regularly ran. The objective
of this study was to determine physical and biomechanical changes in age groups upon
running. It was hypothesized that older runners might compromise their lower extremities’
dynamic joint function during running due to declined physical characteristics compared
with younger runners.
2. Materials and Methods
2.1. Participants
The participants in this study were 75 healthy males who did not receive treatment
and had surgery history due to lower extremity musculoskeletal system problems within
the past six months, who ran 10 km or more three times a week, who had participated at
least once in a marathon race, and whose right leg was their dominant leg (Table 1). All the
participants voluntarily participated in this study. The experiment was carried out after
gaining approval from the IRB (20180611-046).
Int. J. Environ. Res. Public Health 2022, 19, 4320
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Table 1. Subject characteristics for each age group.
Group
20s
30s
40s
50s
60s
70s~80s
Total
Number of participants
12
12
11
12
12
16
75
Age (year)
24.67
±2.46
33.83
±2.79
45.18
±3.38
55.42
±2.36
64.33
±2.99
75.13
±3.93
51.12
±18.04
Height (m)
1.75
±0.05
1.76
±0.04
1.72
±0.05
1.71
±0.04
1.67
±0.05
1.67
±0.06
1.71
±0.06
Weight (kg)
73.58
±5.92
74.58
±6.11
67.64
±6.95
66.83
±4.02
63.42
±7.37
62.67
±6.09
68.07
±7.46
2.2. Procedure
After explaining the procedure and purpose of this study to the participants, they
consented to the test. To collect body composition information, their height and weight
were measured. The participants’ thighs and ankles were secured with a ﬁxing band in
a state where they did not move once lying down on the examination table and looking
at the ceiling, with their lower extremity joint BMD being measured using the Dual En-
ergy X-ray Absorptiometry (DEXA, QDR-1000; Hologic, Waltham, MA, USA) equipment.
Their chest, abdomen, and thigh were held with a ﬁxing band to measure their maximum
lower extremity joint strength. There was no movement in the other joints except in the
observed joints upon matching the dynamometer axis and joint using isokinetic exercise
(Humac Norm, Stoughton, MA, USA), as shown in Figure 1 [34]. The hip joint extension
(gluteus maximus) and ﬂexion (iliopsoas), knee joint extension (quadriceps) and ﬂexion
(hamstrings), and ankle joint dorsiﬂexion (tibialis anterior) and plantar ﬂexion (gastroc-
nemius) were measured once, setting the angle and speed at 60◦/s each time. In doing
so, practices were performed three times, and peak torque was measured ﬁve times. To
prevent fatigue, break time was given between measurements, while a loud voice offered
motivation to exert maximum strength upon measurement.
 
Table 1. Subject characteristics for each age group. 
Group 
20s 
30s 
40s 
50s 
60s 
70s~80s 
Total 
Number of 
participants 
12 
12 
11 
12 
12 
16 
75 
Age (year) 
24.67 
±2.46 
33.83 
±2.79 
45.18 
±3.38 
55.42 
±2.36 
64.33 
±2.99 
75.13 
±3.93 
51.12 
±18.04 
Height (m) 
1.75 
±0.05 
1.76 
±0.04 
1.72 
±0.05 
1.71 
±0.04 
1.67 
±0.05 
1.67 
±0.06 
1.71 
±0.06 
Weight (kg) 
73.58 
±5.92 
74.58 
±6.11 
67.64 
±6.95 
66.83 
±4.02 
63.42 
±7.37 
62.67 
±6.09 
68.07 
±7.46 
2.2. Procedure 
After explaining the procedure and purpose of this study to the participants, they 
consented to the test. To collect body composition information, their height and weight 
were measured. The participants’ thighs and ankles were secured with a fixing band in a 
state where they did not move once lying down on the examination table and looking at 
the ceiling, with their lower extremity joint BMD being measured using the Dual Energy 
X-ray Absorptiometry (DEXA, QDR-1000; Hologic, Waltham, MA, USA) equipment. 
Their chest, abdomen, and thigh were held with a fixing band to measure their maximum 
lower extremity joint strength. There was no movement in the other joints except in the 
observed joints upon matching the dynamometer axis and joint using isokinetic exercise 
(Humac Norm, Stoughton, MA, USA), as shown in Figure 1 [34]. The hip joint extension 
(gluteus maximus) and flexion (iliopsoas), knee joint extension (quadriceps) and flexion 
(hamstrings), and ankle joint dorsiflexion (tibialis anterior) and plantar flexion (gas-
trocnemius) were measured once, setting the angle and speed at 60°/s each time. In doing 
so, practices were performed three times, and peak torque was measured five times. To 
prevent fatigue, break time was given between measurements, while a loud voice offered 
motivation to exert maximum strength upon measurement. 
 
Figure 1. Measurements of BMD and maximum joint strength: (a) BMD, (b) hip, (c) knee, and (d) 
ankle joint. 
After performing a warm-up, the participants wore upper and lower tights for the 
motion capture test, and 64 reflective markers were attached to the body (Figure 2). They 
wore personal standard running shoes, and the standing calibration was carried out to 
ensure the participant’s body anatomical position before the running’s measurement. 
Eight infrared cameras (Oqus 300, Qualisys, Sweden) captured the running motions. The 
sampling frequency was set at 100 Hz. They ran on a treadmill embedded with two force 
plates (Instrumented treadmill, Bertec, corporation, Columbus, OH, USA), and the sam-
pling rate was set at 1000 Hz. After sufficient rest to minimize muscle fatigue, measure-
ment was carried out by gradually increasing speed for five minutes to induce natural 
running before the measurement. The running speed was set at 3.1 m/s [26]. During their 
run on the treadmill with a selected speed of five minutes, the motion was collected from 
30 gait cycles of the right leg (Figure 3). 
Figure 1. Measurements of BMD and maximum joint strength: (a) BMD, (b) hip, (c) knee, and
(d) ankle joint.
After performing a warm-up, the participants wore upper and lower tights for the
motion capture test, and 64 reﬂective markers were attached to the body (Figure 2). They
wore personal standard running shoes, and the standing calibration was carried out to
ensure the participant’s body anatomical position before the running’s measurement.
Eight infrared cameras (Oqus 300, Qualisys, Sweden) captured the running motions. The
sampling frequency was set at 100 Hz. They ran on a treadmill embedded with two force
plates (Instrumented treadmill, Bertec, corporation, Columbus, OH, USA), and the sampling
rate was set at 1000 Hz. After sufﬁcient rest to minimize muscle fatigue, measurement
was carried out by gradually increasing speed for ﬁve minutes to induce natural running
before the measurement. The running speed was set at 3.1 m/s [26]. During their run on
the treadmill with a selected speed of ﬁve minutes, the motion was collected from 30 gait
cycles of the right leg (Figure 3).
Int. J. Environ. Res. Public Health 2022, 19, 4320
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Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
4 
 
 
Figure 2. Reflective markers on the body: (a) frontal view, (b) lateral view, (c) backward view.
 
Figure 3. Experimental setup for running on a treadmill. 
2.3. Data Processing 
The stance phase was defined as the moment a participant’s right heel struck the
off on the treadmill to analyze biomechanical variables. Each joint’s position and gro
reaction force data were obtained using Qualisys’s Qualisys Track Manager Program
data processing. To remove the noise of the data, Butterworth second-order low-pas
tering at 12 Hz was performed for the 3D position coordinate data [35]. The ground r
tion force data were set at a power spectrum density (PSD) of 99% of the value of the
off frequency [36]. Using Visual 3D (C-Motion, Germantown, MD, USA) and M
R2014a (The Mathworks, Natick,USA), the (+) joint angle from the range of motion o
ankle, knee, and hip in the sagittal plane means a flexion and dorsiflexion angle, whil
(−) joint angle means an extension and plantar flexion angle. Joint moment (Nm/kg)
joint power (W/kg) (+) mean a concentric contraction, while the (–) counterpart m
eccentric contraction. For the ground reaction force direction the X-axis was set as le
and right (+), the Y-axis was set as the front (+) and back (−), while the Z-axis was s
vertical (+). 
2.4. Statistical Processing 
For statistical processing, SPSS Ver. 25.0 software (IBM, Armonk, NY,USA) wa
plied. Regarding the physical characteristics, kinematics, and kinematic data obta
through the analysis program, Pearson’s product-moment correlation coefficients w
calculated to find the relationship between age and physical characteristics, as well as
mechanical variables. Based on the sample size calculation, a minimum of 42 subjects
required for the expected correlation coefficient (R) of 0.35 with a power of 70% (β) 
A significant level of statistics was set at an alpha level of 0.05. 
3. Results 
3.1. Correlations between Age and Physical Characteristics (BMD) and Peak Torque 
Figure 2. Reﬂective markers on the body: (a) frontal view, (b) lateral view, (c) backward view.
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
4
 
 
Figure 2. Reflective markers on the body: (a) frontal view, (b) lateral view, (c) backward view
 
Figure 3. Experimental setup for running on a treadmill. 
2.3. Data Processing 
The stance phase was defined as the moment a participant’s right heel struck the
off on the treadmill to analyze biomechanical variables. Each joint’s position and gr
reaction force data were obtained using Qualisys’s Qualisys Track Manager Program
data processing. To remove the noise of the data, Butterworth second-order low-pa
tering at 12 Hz was performed for the 3D position coordinate data [35]. The ground 
tion force data were set at a power spectrum density (PSD) of 99% of the value of the
off frequency [36]. Using Visual 3D (C-Motion, Germantown, MD, USA) and M
R2014a (The Mathworks, Natick,USA), the (+) joint angle from the range of motion o
ankle, knee, and hip in the sagittal plane means a flexion and dorsiflexion angle, whil
(−) joint angle means an extension and plantar flexion angle. Joint moment (Nm/kg
joint power (W/kg) (+) mean a concentric contraction, while the (–) counterpart m
eccentric contraction. For the ground reaction force direction the X-axis was set as le
and right (+), the Y-axis was set as the front (+) and back (−), while the Z-axis was s
vertical (+). 
2.4. Statistical Processing 
For statistical processing, SPSS Ver. 25.0 software (IBM, Armonk, NY,USA) wa
plied. Regarding the physical characteristics, kinematics, and kinematic data obta
through the analysis program, Pearson’s product-moment correlation coefficients 
calculated to find the relationship between age and physical characteristics, as well a
mechanical variables. Based on the sample size calculation, a minimum of 42 subjects
required for the expected correlation coefficient (R) of 0.35 with a power of 70% (β)
A significant level of statistics was set at an alpha level of 0.05. 
3. Results 
3.1. Correlations between Age and Physical Characteristics (BMD) and Peak Torque 
Figure 3. Experimental setup for running on a treadmill.
2.3. Data Processing
The stance phase was deﬁned as the moment a participant’s right heel struck the toe-
off on the treadmill to analyze biomechanical variables. Each joint’s position and ground
reaction force data were obtained using Qualisys’s Qualisys Track Manager Program for
data processing. To remove the noise of the data, Butterworth second-order low-pass
ﬁltering at 12 Hz was performed for the 3D position coordinate data [35]. The ground
reaction force data were set at a power spectrum density (PSD) of 99% of the value of the
cut-off frequency [36]. Using Visual 3D (C-Motion, Germantown, MD, USA) and Matlab
R2014a (The Mathworks, Natick, MA, USA), the (+) joint angle from the range of motion of
the ankle, knee, and hip in the sagittal plane means a ﬂexion and dorsiﬂexion angle, while
the (−) joint angle means an extension and plantar ﬂexion angle. Joint moment (Nm/kg)
and joint power (W/kg) (+) mean a concentric contraction, while the (–) counterpart means
eccentric contraction. For the ground reaction force direction the X-axis was set as left (−)
and right (+), the Y-axis was set as the front (+) and back (−), while the Z-axis was set as
vertical (+).
2.4. Statistical Processing
For statistical processing, SPSS Ver. 25.0 software (IBM, Armonk, NY, USA) was
applied. Regarding the physical characteristics, kinematics, and kinematic data obtained
through the analysis program, Pearson’s product-moment correlation coefﬁcients were
calculated to ﬁnd the relationship between age and physical characteristics, as well as
biomechanical variables. Based on the sample size calculation, a minimum of 42 subjects
was required for the expected correlation coefﬁcient (R) of 0.35 with a power of 70% (β) [37].
A signiﬁcant level of statistics was set at an alpha level of 0.05.
Int. J. Environ. Res. Public Health 2022, 19, 4320
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3. Results
3.1. Correlations between Age and Physical Characteristics (BMD) and Peak Torque
According to the correlation analysis between age and BMD, statistical signiﬁcance
was displayed among the total (r = −0.380, p ≤ 0.001), legs BMD (r = −0.506, p ≤ 0.000), and
T-score (r = 0.442, p ≤ 0.000), and it showed a negative correlation. In the lower extremity
strength relationship, statistically signiﬁcant negative correlations were displayed between
hip joint extension (r = −0.399, p ≤ 0.000) and ﬂexion (r = −0.612, p ≤ 0.000), knee joint
extension (r = −0.535, p ≤ 0.000) and ﬂexion (r = −0.525, p ≤ 0.000), ankle joint dorsiﬂexion
(r = −0.407, p ≤ 0.000), plantar ﬂexion (r = −0.494, p ≤ 0.000) (Table 2).
Table 2. Correlations (R, p-value) between age and the variables (i.e., BMD, strength, running
parameters, and running biomechanics).
Age
BMD
Total
Legs
T score
−0.380 *
−0.506 **
−0.422 *
0.001
0.000
0.000
Strength
Gluteus
Maximus
Iliopsoas
Quadriceps
Hamstrings
Tibialis
Anterior
Gastrocnemius
−0.399 *
−0.612 *
−0.535 *
−0.525 *
−0.407 *
−0.494 *
0.000
0.000
0.000
0.000
0.000
0.000
Running Parameter
Stride Time
StrideDistance
−0.336 *
−0.536 *
0.003
0.000
Impact
Impulse
Impact Force
GRF Peak
Loading
Rate
−0.021
0.100
0.018
0.033
0.864
0.417
0.881
0.784
Joint Angle
Ankle Dorsi
Flexion
Ankle
Plantar
Flexion
Ankle RoM
Knee
Flexion
Knee
Extension
Knee RoM
0.001
0.321 *
−0.352 *
−0.115
0.181
−0.361 *
0.991
0.006
0.002
0.335
0.129
0.000
Hip Flexion
Hip
Extension
Hip RoM
0.164
0.289 *
−0.064
0.166
0.013
0.593
Joint Moments
Ankle Dorsi
Flexion
Ankle
Plantar
Flexion
Knee
Flexion
Knee
Extension
Hip Flexion
Hip Extension
0.328 *
−0.177
0.253 *
−0.166
0.212
0.225
0.004
0.140
0.030
0.167
0.072
0.060
Joint Power
Ankle
Absorption
Ankle
Generation
Knee
Absorption
Knee
Generation
Hip
Absorption
Hip
Generation
−0.334 *
0.326 *
−0.185
0.357 *
−0.044
0.082
0.004
0.006
0.115
0.002
0.711
0.494
BMD: bone mineral density, GRF: ground reaction force. * p < 0.05 indicates signiﬁcant correlations between age
and the variables. ** p < 0.01.
Int. J. Environ. Res. Public Health 2022, 19, 4320
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3.2. Correlations between Age and Running Characteristics and Impact
As age increased, a statistically signiﬁcant negative correlation was revealed between
stride time (r = −0.336, p ≤ 0.003) and stride distance (r = −0.536, p ≤ 0.000) in running
characteristics (Table 2). However, there were no signiﬁcant correlations between age
and impact variables (i.e., impulse, impact force, GRF (ground reaction force) peak, and
loading rate).
3.3. Correlations between Age and Joint Kinetics of the Lower Extremity Joint
According to age increase, statistical signiﬁcances of correlation were found in the
ranges of the ankle (r = −0.352, p ≤ 0.002) and knee joint motion (r = −0.361, p ≤ 0.000)
in the sagittal plane (Table 2). In addition, ankle and knee moments (ankle dorsiﬂexion
moment: r = 0.328, p ≤ 0.004, knee ﬂexion moment: r = 0.253, p ≤ 0.030), as well as ankle
joint power (absorption: r = −0.334, p ≤ 0.004, and generation: r = 0.326, p ≤ 0.006) in the
sagittal plane, were signiﬁcantly lower in older runners compared with younger runners,
based on the analysis of correlations (Table 2, Appendix A; Figure A3).
4. Discussion
This study examined the relationship between lower extremity joint physical charac-
teristics and running biomechanical variables and age, targeting males aged 20 to 80 who
regularly ran. The study aimed to discover physical changes and risk factors of running ac-
cording to aging. Based on the ﬁndings of this study, the hypothesis was partially accepted
as elderly runners showed lower physical characteristics and dynamic joint function while
running compared with their younger counterparts.
When looking at the relationship between age increase and BMD and maximum
strength, targeting males who regularly ran, a negative correlation was found in total
BMD, legs, and T-score. A negative correlation was found in the maximum strength of
the ankle, knee, and hip joints with increased age, with the power being 41.2% in the hip
joint ﬂexion power (Appendix A, Figure A1). In comparing the analysis result of the leg
BMD of 2657 general elderly males and the analysis result of the older adults who regularly
performed running in this study, the leg BMD of this study was 31% higher [38]. Regarding
why running exercise affects BMD, this study infers that bone density decreases if stress
(shock) is given to bone repeatedly by running exercise, as shown in Wolff’s law [39].
Furthermore, due to age increase, the weakening strength of the lower extremity
joint displays a considerable physiological change and may become a cause of fracture
injury. As for males in general, knee joint extension strength decreases by 12–15% every
10 years [33,40]. In this study, the muscle ﬂexibility of the males who ran regularly was
reduced as they aged, especially if there was a large difference in ankle joint dorsiﬂexion
(tibialis anterior) and plantar ﬂexion (gastrocnemius) [27]. Therefore, it is plausible that
more power can be exerted because of the compensation action of the hip joint. On the
other hand, when an older adult continues to exercise, the gastrocnemius and thigh skeletal
muscle develop. As a result of comparing the lower extremity joint maximum strength
in this study to a previous study [41], a delayed reduction in muscle strength in elderly
runners was found. Regular participation in physical activities is vital in reducing physical
change due to aging and the risk factors of musculoskeletal injuries [27,42]. Running
exercise is a valuable method to reduce and prevent BMD. If one steadily runs, diseases
and secondary fracture injuries due to bone reduction symptoms such as osteoporosis can
be prevented in advance, and BMD can be retained. Additionally, running exercise can
reduce the risk of fracture injuries and improve quality of life. Although the BMD volume
and strength of the elderly group who continued to run were lower than those of the young
adult male group, their BMD volume in the lower extremity joint was more signiﬁcant than
that of the elderly group who did not exercise. As age increases, the knee joint extensor
muscle (thigh skeletal muscle) decreases, the iliopsoas that ﬂexes the hip joint weakens,
and the hip joint ﬂexor muscle seems to weaken. However, regular running in the elderly
Int. J. Environ. Res. Public Health 2022, 19, 4320
7 of 12
would delay a rapid decrease in BMD and lower extremity muscle strength, which may be
beneﬁcial for reducing injury risk.
Upon running, the primary cause of injuries is one’s foot repeatedly touching the
ground; the impact (shock) causes stress fracture, patellofemoral disorder, cartilage de-
struction, and lower back pain [43]. As the impact force is generated upon landing, it has
been known that older adults’ injury ratio is high due to physical weakness [44,45]. A
positive relationship was observed upon examining the ground reaction force variables
with age, but a signiﬁcant level was not meaningful after weight standardization. No statis-
tically signiﬁcant difference was revealed between the impact variables as age increases
in the impulse, impact force, maximum ground reaction force, and loading rate. On the
other hand, this study’s magnitude of impact variables was similar to that of the previous
study [31] and, with increased age, the vertical ground reaction force becomes smaller in
elderly runners. This condition is conjectured to occur because stride becomes shorter and
stride frequency increases in elderly runners (Appendix A, Figure A2). However, there
may be a higher risk of accumulated high magnitude of impact for elderly runners due to
weaker BMD and muscle strength.
The decreased range of ankle joint and knee joint motion showed an increase with age,
but the hip joint range of motion showed no changes in the sagittal plane (Appendix A,
Figure A3). Ankle joint range of motion is affected by aging most, and the knee range of
motion becomes larger as running speed increases [30,46]. The result of this study is similar
to the result of previous studies, and it was conﬁrmed that the range of motion in the ankle
joint becomes smaller as age increases [26,27,47]. Previous studies also suggested that the
thigh skeletal muscle and hamstring strength weaken in elderly runners [48]. The result
is linked with how the ankle joint ﬂexor muscle negatively correlates with age. Due to
reduced strength ﬂexibility, an ankle joint sprain, and tibialis posterior and tibialis anterior
injuries can be caused as the ankle joint range of motion becomes smaller. If the knee joint
is not smoothly moved, the impact occurring during running is thought to be delivered
to the whole body. A positive correlation between males running regularly, indicating a
slowly decreased ankle joint plantar ﬂexion moment and knee joint extension moment, was
found as age increased (Appendix A, Figure A3).
A negative correlation shows slowly decreased ankle joint dorsiﬂexion and plantar
ﬂexion power with increased age. Additionally, a negative correlation was revealed be-
tween the knee extension power, indicating absorption of joint energy and age. The runners
propel themselves forward with generations of joint power in the lower extremity during
running. However, if the ankle joint’s power generating ability decreases, compensation
occurs in the knee and hip joints [31,49]. On the other hand, many portions of impact
absorbed in the lower extremity joints are also transferred to the whole body, so there is a
relation between the level of impact of the body and the lower extremity joint angle [50].
If the load transfer is not effectively controlled, a substantial impediment is caused to
the lower extremity musculoskeletal system due to repetitive high impact force [51]. The
load is signiﬁcantly buffered on the ankle joint and then transferred to the knee and hip
joints. As age increases, the load ability of the ankle is reduced, and the risk of ankle injury
may increase [52]. If enough ankle joint strength and bone intensity are not maintained
in elderly runners, there is a high possibility that ankle joint injuries may occur due to
impact occurring on the ground during running. As age increases, decreased dynamic joint
function, showing lower joint power due to weak lower extremity joints, especially in the
ankle, may compromise running performance and induce the risk of joint injuries.
Synthesizing all these, this study found that the musculoskeletal system can be main-
tained by running as age increases. However, the function of the lower extremity joints
signiﬁcantly weakens linearly, especially when the ankle joint function signiﬁcantly de-
creases. The load of the lower extremity joint upon running is thought to be eased by muscle
strength in physical strength. If one does not run correctly due to weakening strength ac-
cording to aging, the physical activity can be connected to injuries, not health enhancement.
Int. J. Environ. Res. Public Health 2022, 19, 4320
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There are some study limitations that need to be addressed for the future direction of
this area. First, a slight difference in biomechanical variables may exist between treadmill
and overground running as this study was conducted on a treadmill. Second, the skill
level and experience of the runners were not controlled among different age groups, but
they may reﬂect different characteristics of running patterns. Finally, a longitudinal study
investigating the effect of aging on physical and biomechanical changes in running may
require in future research.
5. Conclusions
This study targeted males aged 20 to 80 who were regularly running, and it aimed to
determine the relationship among physical characteristics, running variables, and age. First,
the T-score of the males aged 40 to 80 who regularly ran was found to be in the normal scope.
The lower extremity maximum strength decreased in hips and knees among people in their
20s and decreased in the ankle joints among those in their 60s. Speciﬁcally, a considerable
decrease was found in males in their 70s in ankle joint plantar ﬂexion. Second, the stride
length in the running characteristics fell as age increased. Stride showed a difference among
the groups, but stride frequency showed an increased trend as age increased. This is related
to stride frequency when older adults run. Third, it was observed that the knee joint range
of motion and ankle joint movement remarkably decreased in running biomechanics as age
increased. When synthesizing the results, the physical characteristics gradually decreased
as age increased, and the BMD and lower extremity strength of the males who regularly
ran was maintained and improved compared to non-runners. Running characteristics
improved if one regularly ran. Speciﬁcally, the ankle joint movements were remarkably
reduced due to aging, and impact absorption was further shown on the ankle joint as
age increased. As a result of examining physical characteristics and kinematic variables,
the burden on the ankle joint was more evident among males in their 60s due to their
weakening lower extremity joint strength. Therefore, proper running intensity and method
may be applicable to prevent running-related injuries in older runners by distributing
the impact and load among the lower extremity joints based on the ﬁndings. A further
biomechanics study, considering the changes in physical strength and motion by aging, may
suggest a more effective training method for elderly runners to maintain musculoskeletal
health and ensure injury prevention.
Author Contributions: Conceptualization, J.K. and S.-K.P.; methodology, J.K. and S.-K.P.; software,
J.K.; validation, J.K. and S.-K.P.; formal analysis, J.K. and S.-K.P.; investigation, J.K. and S.-K.P.;
resources, J.K. and S.-K.P.; data curation, J.K.; writing—original draft preparation, J.K. and S.-K.P.;
writing—review and editing, J.K. and S.-K.P.; visualization, J.K.; supervision, S.-K.P. All authors have
read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: The study was conducted according to the guidelines of the
Declaration of Helsinki and approved by the Institutional Review Board of Korea National Sport
University (approval number 20180611-046 and date of approval 25 June 2018).
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Not applicable.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
Int. J. Environ. Res. Public Health 2022, 19, 4320
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Appendix A
 
y
g
g
Declaration of Helsinki and approved by the Institutional Review Board of Korea National Sport 
University (approval number 20180611-046 and date of approval 25 June 2018). 
Informed Consent Statement: Informed consent was obtained from all subjects involved in the 
study. 
Data Availability Statement: Not applicable. 
Conflicts of Interest: The authors declare no conflict of interest. 
Appendix A 
 
 
 
(a) 
(b) 
(c) 
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
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(d) 
(e) 
(f) 
 
 
 
(g) 
(h) 
(i) 
Figure A1. Correlations between age and physical characteristics (BMD and peak muscle torque). 
(a) Total BMD, (b) legs BMD, (c) T-scope, (d) hip extensor, (e) hip flexor, (f) knee flexor, (g) knee 
extensor, (h) ankle dorsi flexor, (i) ankle plantar flexor. * p < 0.05 indicates significant correlations 
between age and the variables. 
 
 
 
(a) 
(b) 
Figure A2. Correlations between age and impact and running characteristics. (a) Stride time, (b) 
stride distance. * p < 0.05 indicates significant correlations between age and the variables. 
 
Figure A1. Correlations between age and physical characteristics (BMD and peak muscle torque).
(a) Total BMD, (b) legs BMD, (c) T-scope, (d) hip extensor, (e) hip ﬂexor, (f) knee ﬂexor, (g) knee
extensor, (h) ankle dorsi ﬂexor, (i) ankle plantar ﬂexor. * p < 0.05 indicates signiﬁcant correlations
between age and the variables.
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
9 of 12 
 
 
 
 
(d) 
(e) 
(f) 
 
 
 
(g) 
(h) 
(i) 
Figure A1. Correlations between age and physical characteristics (BMD and peak muscle torque). 
(a) Total BMD, (b) legs BMD, (c) T-scope, (d) hip extensor, (e) hip flexor, (f) knee flexor, (g) knee 
extensor, (h) ankle dorsi flexor, (i) ankle plantar flexor. * p < 0.05 indicates significant correlations 
between age and the variables. 
 
 
 
(a) 
(b) 
Figure A2. Correlations between age and impact and running characteristics. (a) Stride time, (b) 
stride distance. * p < 0.05 indicates significant correlations between age and the variables. 
 
Figure A2. Correlations between age and impact and running characteristics. (a) Stride time, (b) stride
distance. * p < 0.05 indicates signiﬁcant correlations between age and the variables.
Int. J. Environ. Res. Public Health 2022, 19, 4320
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(a) 
(b) 
Figure A2. Correlations between age and impact and running characteristics. (a) Stride time, (b) 
stride distance. * p < 0.05 indicates significant correlations between age and the variables. 
 
 
 
 
(a) 
(b) 
(c) 
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
10 of 12 
 
 
 
 
(d) 
(e) 
(f) 
 
 
 
(g) 
(h) 
(i) 
Figure A3. Correlations between age and lower extremity joint biomechanics upon running. (a) An-
kle ROM, (b) knee ROM, (c) hip ROM, (d) ankle moment, (e) knee moment, (f) hip moment, (g) 
ankle power, (h) knee power, (i) hip power. * p < 0.05 indicates significant correlations between age 
and the variables. 
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Differences in Physical Characteristics of the Lower Extremity and Running Biomechanics Between Different Age Groups.
Kim, Jongbin,Park, Sang-Kyoon