Physical Therapy Korea

Falls are a common and serious problem in older adults that threaten independence [1] and health [2]. It is estimated that one-third of people over the age 65 and 50% of people over the age of 80 experience at least one fall each year, and 40% of those fall more than once [3]. One in five falls results in a serious injury, such as a fracture or head injury, and more than 95% of hip fractures are due to falls [4]. Previous studies have reported that women experience more falls than men [5,6]. As a result, falls limit physical activity and mobility in the elderly and increase direct medical costs owing to the need for assistance or hospitalization [7].

Various factors are associated with falling, but the most important risk factors in the elderly are muscle weakness and impaired balance [8]. In healthy elderly, muscle strength has been shown to be associated with the standing balance ability [9]. The main muscles required for postural stability are the tibialis anterior (TA) (ankle dorsiflexor [DF]), gastrocnemius (ankle plantar flexor [PF]), hamstring (knee flexor [KF]), and quadriceps (knee extensor [KE]). The ankle DF and PF prevent the center of mass from moving posteriorly and anteriorly to each other beyond the base of support (BOS). In older adults, the hip adductors (HAD) and abductors (HAB) are key muscles that maintain postural control for lateral stability [9]. Before the age of 50 years, women begin to lose strength in their quadriceps and gluteal muscles [10]. Thus, decreased muscle strength negatively affects balance in elderly individuals. Balance ability generally declines with age, leading to an increased fall risk [2]. In the clinic, the examination of balance ability is essential for interventions to prevent falls in the elderly [11].

Computerized dynamic posturography is the gold standard for assessing balance. However, this equipment is expensive and not widely used [12]. In many clinical settings, the Berg Balance Scale (BBS) or Short Physical Performance Battery (SPPB) has been used for balance assessment in older adults. However, these tests focus on frail elderly individuals and have significant ceiling effects [2,13]. Therefore, a more discriminative tool is needed to evaluate balance function in healthy elderly people.

The Y-balance test (YBT), which is a modified of the Star Excursion Balance Test (SEBT), is used to assess dynamic postural control. The YBT assesses reaching distances in three directions: anterior (ANT), posteromedial (PM), and posterolateral (PL), during one-leg standing [14]. Potentially, YBT requires adequate hip joint strength to maintain stability of the pelvis and trunk during testing [15]. The reach distance value is used as an indicator of dynamic postural control, and a longer reach distance indicates better dynamic postural control. It also used to predict the risk of lower-extremity injury, and the YBT score was an important predictor of joint instability [16]. The YBT showed high intra- (0.85–0.91) and interrater (0.99–1.00) reliabilities [14]. In older adults, this test showed excellent test-retest reliability and significant differences compared with young people [17]. Although relationships between various lower extremity components and core stability have been reported [18,19], studies on the relationship between lower-limb strength and YBT scores in elderly women are rare. Muscle weakness is one of the risk factors for dynamic balance, so it is important to know which muscle strength contribute to better performance in the YBT. It is also important for the development of fall prevention training programs in elderly.

Accordingly, this study aimed to examine the relationship between YBT scores and hip flexor (HF), hip extensor (HE), HAB, HAD, KF, KE, DF, and PF strength in elderly women. This study hypothesized that YBT scores and lower-limb strength would be positively correlated in elderly women.

1. Participants

Thirty elderly women participated in the study (Table 1). The inclusion criterion was healthy elderly women aged over 65 years. The exclusion criteria were as follows: inability to walk alone; presence of neurological, cerebrovascular, or muscular disorders; balance problems due to vestibular and visual disorders; history of lower-extremity injury or surgery; and cognitive function problems that prevented understanding of the study description. Before the study, the details of the experimental procedures were explained to all participants, and written informed consent was obtained upon enrollment. The study was approved by the Institutional Review Board at Yonsei University Mirae campus (IRB no. 1041849-202211-BM-195-02).

Table 1 . General characteristics (N = 30).

Variable	Mean ± SD	Range
Age (y)	68.72 ± 2.48	65.0–74.0
Height (cm)	153.6 ± 4.33	144.9–160.0
Weight (kg)	58.13 ± 7.07	45.5–71.8
Body mass index (kg/m2)	24.65 ± 2.83	18.0–29.1
Limb length (cm)	76.9 ± 3.0	70.5–81.5

Mean ± SD, mean ± standard deviation..

2. Instruments

The isometric strength of the lower-extremity muscles, including the HF, HE, HAB, HAD, KF, KE, DF, and PF, was measured using a Smart KEMA strength sensor (KOREATECH Inc.) with a built-in tension sensor that measures the tension generated by pulling on both ends (Figure 1). A nonelastic strap was connected to both sides of the sensor; one side was fixed to a non-moving object and the other side was fixed to the part to be measured. To standardize the tests, the belt length was tightly adjusted to be 2 kgf tension at the starting position for isometric strength measurement. All participants performed maximal isometric contractions for 5 seconds and the data transmitted to a Smart KEMA application via Bluetooth. The mean value of the middle 3 seconds was used for data analysis. The value was divided by the weight of the participant for normalization [20]. In previous studies, the Smart KEMA strength sensor showed high interrater reliability (ICC = 0.85–0.90) [21].

Figure 1. Measurements position of the lower limb strength. (A) Hip flexor, (B) hip extensor, (C) hip abductor, (D) hip adductor, (E) knee flexor, (F) knee extensor, (G) ankle dorsiflexor, and (H) ankle plantar flexor muscles.

3. Protocol

The YBT was performed as explained by Plisky et al. [14]. The YBT was performed on a flat floor with three lines extending in the ANT, PM, and PL directions at an angle of 135° to each other (Figure 2). In this test, the participants chose the leg on which they felt comfortable standing [22]. The participants stood on their preferred leg with the first toe at the center of the line. While standing on the stance leg, the opposite leg was stretched as far as possible along the three lines drawn on the floor. Prior to the test, the participants were allowed to practice two trials on each leg in all directions. If the participant’s standing foot moved or the reaching leg failed to return to its starting position during the trial, the trial was discarded. The test was performed three times in three directions for each leg, and the maximum distance and composite scores were used for data analysis. The maximal reach distances in each direction were normalized to the subject’s anatomical leg length, and the sum of the maximal reach distances in each direction was divided by three times the leg length to obtain the YBT composite scores.

Figure 2. Y-balance test. (A) Anterior reach, (B) posteromedial reach, and (C) posterolateral reach.

Before measuring lower-limb strength, all participants were instructed in the direction of movement and allowed two trials with assistance. An experienced measurer closely stands to the participant to prevent compensatory movements during the test. All muscle strengths were measured at maximum strength within the pain-free range.

The strength of the HF muscle was measured in the sitting position. The participant was seated at the edge of a table with legs hanging down. The thigh strap was tied to the distal femur of the tested leg and the length was adjusted to 2 kgf. The participant then flexed the tested leg toward the ceiling for 5 seconds.

The participant was placed in a prone position on the edge of a table. The thigh strap was tied to the distal femur of the tested leg and the knee was flexed to 90° [15]. The strap tension was adjusted at starting position. The examiner guided the participant’s leg in the direction in which it should be raised. The participant then extended hip in the direction of the ceiling.

HAB strength was measured with the participant in a supine position on a table. The strap was attached to the ankle to be tested. The participant abducted the leg to be tested as hard as possible, without moving the trunk.

The participant was laid down on a table in the supine position to measure the HAD strength. The strap was tied to the ankle to be tested. The participant adducted the leg to be tested as hard as possible without bending the trunk.

KF strength was measured in the prone position with the knee slightly flexed [20]. The strap was applied to the ankle on the tested leg. The participants flexed their legs as much as possible. The examiner pushed the participant’s sacrum during the test to prevent an anterior pelvic tilt.

The participant was seated at the edge of a table with legs hanging down and the knee flexed at 45° to test KE strength [20]. The strap was tied to the ankle of each leg to be tested. The participant was instructed to straighten back and immobilize trunk, then extended the tested knee for 5 seconds.

The participant was seated on a table with one leg bent so that the metatarsal bone was at the edge of the table. The strap was wrapped around the metatarsal bones of the leg being tested. The examiner instructed the participants to flex the foot while bending the toes to prevent contraction of the toe extensor muscles.

To measure PF strength, the participant was placed in a prone position on a table. The knee to be tested was flexed at 90° and the ankle strap was wrapped around the metatarsal bones. The participant then pulled the strap toward the ceiling as strongly as possible. During the examination, the examiner prevented the knees from bending.

4. Statistical Analysis

All data were analyzed using the IBM SPSS software (ver. 22.0, IBM Co.). The Shapiro–Wilk test was used to verify the normality of participant characteristics and dependent variable data. The relationship between lower-extremity strength and YBT distance was assessed using a Pearson’s correlation coefficient. The r value were defined as follows: strong correlations: 0.00–0.25; fair correlations: 0.25–0.50; moderate to good correlations: 0.50–0.75; good to excellent correlations: > 0.75 [23]. The significance level was set at α = 0.05.

A total of 30 elderly women (68.72 ± 2.48 years) were recruited in this study. The participants’ weight was 58.13 ± 7.07 kg, and height was 153.6 ± 4.33 cm. Table 1 presents the general characteristics of the participants. The results for lower-limb strength and YBT distance are shown in Table 2. The HAB strength (r = 0.388, p < 0.05) and HAD strength (r = 0.362, p < 0.05) showed a significant positive correlation with YBT-ANT direction distance. PF strength was positively correlated with the YBT-ANT direction distance (r = 0.391, p < 0.05), YBT-PM direction distance (r = 0.396, p < 0.05), and composite score (r = 0.376, p < 0.05) in Table 3. Figure 3 presents the scatter plot between the lower-limb strength and YBT.

Table 2 . Lower limb strength and YBT results.

	Variable	Mean ± SD
Strength normalized by body mass (% kgf/BM)	HF	22.0 ± 6.1
	HE	15.4 ± 4.8
	HAB	11.2 ± 2.7
	HAD	12.6 ± 3.3
	KE	29.7 ± 10.7
	KF	13.3 ± 4.4
	DF	20.6 ± 7.3
	PF	19.6 ± 7.6
YBT normalized reach distance (%)	ANT	73.5 ± 7.4
	PL	87.6 ± 19.3
	PM	100.0 ± 12.2
	Composite score	87.0 ± 11.2

Mean ± SD, mean ± standard deviation; BM, body mass; YBT, Y-balance test; HF, hip flexor; HE, hip extensor; HAB, hip abductor; HAD, hip adductor; KE, knee extensor; KF, knee flexor; DF, dorsiflexor; PF, plantar flexor; ANT, anterior; PL, posterolateral; PM, posteromedial..

Table 3 . Correlation coefficient between lower limb strength and YBT.

		YBT-ANT	YBT-PL	YBT-PM	YBT-Composite
HF	r	0.162	< 0.001	–0.010	0.030
	p-value	0.421	> 0.99	0.960	0.881
HE	r	0.232	0.006	0.036	0.065
	p-value	0.244	0.977	0.858	0.746
HAB	r	0.388*	0.055	0.180	0.183
	p-value	0.034	0.772	0.341	0.334
HAD	r	0.362*	–0.071	0.173	0.102
	p-value	0.049	0.709	0.360	0.591
KE	r	0.311	0.369	0.013	0.228
	p-value	0.095	0.150	0.945	0.226
KF	r	0.327	0.264	0.255	0.316
	p-value	0.077	0.158	0.174	0.089
DF	r	0.334	0.282	0.146	0.288
	p-value	0.071	0.132	0.440	0.122
PF	r	0.391*	0.256	0.396*	0.376*
	p-value	0.033	0.172	0.030	0.040

YBT, Y-balance test; ANT, anterior; PL, posterolateral; PM, posteromedial; HF, hip flexor; HE, hip extensor; HAB, hip abductor; HAD, hip adductor; KE, knee extensor; KF, knee flexor; DF, dorsiflexor; PF, plantar flexor. *p < 0.05..

Figure 3. The scatter plot between lower-limb strength and YBT. (A) The HAB and HAD strength and the Y-balance ANT direction. (B) The PF strength and Y-balacne ANT, PM, and composite score. YBT, Y-balance test; ANT, anterior; PM, posteromedial; HAB, hip abductors; HAD, hip adductors; PF, plantar flexor.

Falls increase with age, resulting in fractures and injuries [4]. It is important to increase strength and improve balance to prevent falls. The decline in age-related dynamic balance has already been studied in healthy people [24] and the YBT is a useful assessment tool for measuring dynamic standing balance. This study examined the relationship between the YBT scores and lower-limb strength in elderly women. The results showed that HAB, HAD, and PF strengths were positively correlated with YBT scores.

This study founded a fair correlation between HAB and HAD strength and YBT-ANT direction distance. A previous study [25] reported that HE and KF strength were positively correlated with YBT-ANT direction distance in the 40–80s age group. Earl and Hertel [26] reported that the vastus medialis muscle activity was higher in the YBT-ANT direction than in the other directions in participants in their 20s. The two previous studies were similar to our study in that they studied healthy adults; however, there was a difference in the age of the participants. Bouillon and Baker [24] reported that adult women (aged 23–39 years) reached approximately 7 cm further than middle-aged women (40–54 years) in SEBT. The difference in muscle strength with age is due to age-related changes in strength [10], especially in the quadriceps muscle. Therefore, in unstable situations, older adults use more hip strategies than young adults [27]. In older adults, the HAB and HAD are important muscles that maintain postural control for lateral stability [9]. Furthermore, a previous study [28] demonstrated that HAB weakness predicts non-contact anterior cruciate ligament injury. Therefore, HAB and HAD strength are important to control dynamic postural control and performance in the YBT in elderly.

Our study observed that PF strength had a fair correlation with the YBT-ANT, PM, and composite scores. A previous study reported that as TA muscle activity increased, the anterior-posterior displacement of the body's center of pressure decreased and the reach distance increased during SEBT in healthy adults (25.6 ± 4.5 years) [29]. The TA may pull the proximal tibia as the body descends while reaching in all directions, resulting in an increased foot pressure on the supported foot. The relatively reduced PF activity is thought to be due to eccentric contraction of the gastrocnemius to control the tibia [29]. The PF prevents the center of mass from moving beyond the BOS, and is an important muscle that controls ankle torque and postural stability with low forces during standing [30]. According to Tracy [31]’s research, comparison of the ankle muscle strength of elderly and young adults showed no difference in DF maximal voluntary contraction force, but PF maximal voluntary contraction strength decreased by 38% in older adults compared with young adults. Therefore, as PF strength increases, the ability to maintain postural stability improves; accordingly, it can be seen that it is an important muscle used for YBT performance in older adults.

Our study showed relatively low r values, but statistically significant correlations between HAB, HAD, and PF strengths and the YBT scores. Lee et al. [25] found moderate to good correlations of 0.682, 0.719, and 0.653 between HAB and YBY-ANT, PM, and PL reach distances respectively in the 40–80s age group. A previous study [32] in a study of 30 chronic ankle instability patients showed correlations of 0.51 and 0.49 between HAB and YBT-PM and PL distance respectively in 20s. Although our results showed low r values than the two previous studies, the participants were of very different ages. This study consisted of only elderly women aged over 65 years. These results are thought to be due to the fact that there are various variables that can affect the balance ability of the elderly, such as physical activities or underlying diseases [33]. Our study extends these previous findings in that YBT can be used to assess balance ability in the elderly women and analyzed the correlation between lower extremity muscle strength and YBT in the elderly.

This study has several limitations. First, we experimented only with elderly women. Therefore, the results of this study cannot be generalized to elderly men. Further research is needed in elderly men. Second, flexibility or the range of motion of the hip, knee, and ankle joints was not measured. Previous investigators [34] reported that the largest knee flexion range of motion occurred in the anteromedial direction during SEBT. Thus, further research that considers range of motion and flexibility of the lower-limb is required. Third, in our study, the correlation between lower-limb muscle strength and YBT showed relatively low r values.

The results of this study showed that HAB, HAD, and PF strengths were positively correlated with YBT. These results suggest that improvement in HAB, HAD, and PF strength should be considered for dynamic postural stability and potentially reducing fall risk in elderly women.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: SJ, OK. Data curation: EK, HY, OK. Formal analysis: EK. Investigation: EK, YB. Methodology: EK, SJ, OK. Project administration: OK. Resources: EK. Supervision: OK. Validation: OK. Writing - original draft: EK, OK. Writing - review & editing: EK, SJ, HY, YB, OK.