Association Between Bottle-Opening Ability and Muscle Weakness in Patients With Coronary Artery Disease

Kodai Ishihara; Kazuhiro P. Izawa; Masahiro Kitamura; Masato Ogawa; Yuji Kanejima; Yoshitaka Naito; Tomoyuki Morisawa; Ikki Shimizu

doi:10.1253/circrep.CR-25-0090

Abstract

Background: Difficulties opening plastic bottles may be associated with muscle weakness. We investigated association between the ability to open plastic bottles and muscle weakness, and determined cut-off values for and sex-related differences in handgrip strength and pinch strength associated with opening plastic bottles in patients with coronary artery disease (CAD).

Methods and Results: This cross-sectional study enrolled 224 consecutive patients with CAD who underwent in-hospital cardiac rehabilitation. Multiple linear regression analyses was used to determine the association between the ability to open plastic bottles and muscle weakness, with cut-off values for handgrip strength, lateral pinch strength, and 3-fingered pinch strength required to open plastic bottles determined by receiver operating characteristic curve analyses. Multiple linear regression showed that the ability to open plastic bottles was independently associated with handgrip, lateral pinch, and 3-fingered pinch strength after adjusting for confounders. Areas under the curves for all patients were 0.96 (95% confidence interval [CI] 0.93–0.99), 0.91 (95% CI 0.83–0.99), and 0.93 (95% CI 0.88–0.99) for handgrip, lateral pinch, and 3-fingered pinch strength, respectively, with corresponding cut-off values of 20.0, 7.1, and 6.1 kgf.

Conclusions: The ability to open plastic bottles was independently associated with handgrip strength and pinch strength in patients with CAD. Evaluating the ability to open plastic bottles may be a simple and practical tool for assessing muscle weakness in patients with CAD.

Central Figure

Muscle weakness has been associated with both cardiovascular events and all-cause mortality in community-dwelling people and patients with coronary artery disease (CAD),¹^–⁵ with handgrip strength often used as an indicator of muscle weakness. The association between pinch strength and cardiovascular events and all-cause mortality has recently attracted attention in patients undergoing maintenance hemodialysis.⁶ Handgrip strength and pinch strength are straightforward and objective measures of muscle strength.⁷ However, their measurement requires specialized and expensive instruments, and assessing handgrip strength and pinch strength is difficult in environments without access to such equipment.

The ability to open plastic bottles was found to be related to muscle weakness and has been used to estimate sarcopenia or handgrip strength in community-dwelling older people and older inpatients.⁸^–¹⁰ In a real-world clinical setting, we have observed that patients with CAD who could not open plastic bottles had low handgrip strength and pinch strength. Patients with CAD often have weaker handgrip strength and pinch strength, which are linked to cardiovascular events and all-cause death, as well as mild cognitive impairment and upper limb disability.⁵^,¹¹^,¹² Hence, we hypothesized that evaluating the ability to open a plastic bottle could be a screening tool to assess handgrip strength and pinch strength. However, studies evaluating cut-off values for and sex-related differences in handgrip strength and pinch strength required to open plastic bottles are lacking. Handgrip strength is known to differ according to sex, with different cut-off values for men and women for the diagnosis of sarcopenia.¹³ Despite these findings, previous studies have not addressed sex-related differences in handgrip strength and pinch strength required to open a plastic bottle or determined sex-specific cut-off values for these measures.⁸^–¹⁰

Therefore, the aims of this study were to determine associations between the ability to open plastic bottles and muscle weakness (handgrip strength and pinch strength) and cut-off values for and the sex-related differences in handgrip strength and pinch strength associated with opening plastic bottles in patients with CAD. Determining a patient’s ability to open a plastic bottle may be a simple, practical tool for assessing muscle weakness in environments in which handgrip/pinch strength measurements are difficult, such as at home and other locations where measuring equipment is lacking.

Methods

Study Design and Participants

In total, 226 consecutive patients with CAD underwent in-hospital cardiac rehabilitation at Sakakibara Heart Institute (Okayama, Japan) between February 2020 and January 2023, including patients with acute coronary syndrome and stable angina. Patients were excluded from the study if they had a diagnosis of dementia, complications of stroke, orthopedic disease that affected manual function, had undergone coronary artery bypass surgery, or had missing data.

The study was approved by the Ethics Committee of the Sakakibara Heart Institute of Okayama (Approval no. B202001-01), and informed consent was obtained from each patient. The study complied with the principles of the 1975 Declaration of Helsinki regarding investigations in human subjects.

Clinical Characteristics of Patients

Data on patient characteristics were obtained from medical records. Baseline characteristics included age, sex, body mass index (BMI), diagnosis, number of significant coronary artery stenoses (those ≥75% and especially left main trunk ≥50%), treatments, left ventricular ejection fraction (LVEF) calculated using the modified Simpson method for cardiac echocardiography, maximum creatine kinase myocardial band (CK-MB) levels (which indicate the degree of myocardial necrosis due to blockage of coronary blood flow), serum hemoglobin concentrations, estimated glomerular filtration rate (eGFR), serum albumin concentrations, comorbidities, and medications. Laboratory and echocardiographic data obtained just prior to patient discharge were also obtained from patients’ medical records.¹¹^,¹²

Assessment of the Ability to Open Plastic Bottles

The ability to open plastic bottles has been used as a screening tool to assess muscle weakness and sarcopenia, and cut-off values have been published for community-dwelling older adults and older inpatients in Japan.⁸^–¹⁰ In this study, participants were asked to open an unopened plastic bottle (Oi Ocha 525 mL; ITO EN, Tokyo, Japan) in the usual way while seated and to perform the action twice, as per previous studies.⁸^,⁹ Opening a plastic bottle was defined in the present study as the successful opening of the bottle at least once across the 2 attempts.⁸^,⁹ The ability of patients to open a plastic bottle was assessed by a physical therapist at the time of discharge.

Measurement of Handgrip Strength and Pinch Strength

Handgrip strength was measured using a grip strength dynamometer (TKK 5401; Takei Scientific Instruments Co., Ltd., Niigata, Japan), whereas pinch strength was measured using a pinch strength dynamometer (Baseline1 Hydraulic Pinch Gauge; Fabrication Enterprises Co., Ltd., White Plains, NY, USA) by a physical therapist at the time of discharge, as described previously.¹¹^,¹² All subjects were evaluated in the standardized testing position recommended by the American Society of Hand Therapists¹⁴ and as described by Mathiowetz et al.¹⁵ to avoid the Valsalva effect. To measure handgrip strength and pinch strength, all patients were seated in a chair with their shoulders neutral and elbows at 90° flexion, forearms neutral in supination/pronation, and the wrist between 0° and 15° ulnar deviation. Lateral pinch strength and 3-fingered pinch strength were measured.¹¹^,¹² To measure lateral pinch strength, the pinch strength dynamometer was positioned between the pad of the thumb and the radial side of the middle phalanx of the index finger. To measure 3-fingered pinch strength, the dynamometer was positioned between the pads of the thumb, index, and middle fingers. The higher of the 2 measured values, each of which was assessed twice, was recorded, and the right- and left-hand values were averaged to obtain handgrip strength (kgf) and pinch strength (kgf).¹¹^,¹²

Statistical Analysis

Continuous variables are presented as the mean±SD or median with interquartile range (IQR), whereas categorical variables are presented as numbers and percentages. To evaluate the significance of differences in patient characteristics and measured outcomes according to the ability to open plastic bottles and sex, we used unpaired t-tests, Mann-Whitney U tests, Fisher’s exact tests, and χ² tests, as appropriate. To evaluate associations between the ability to open a plastic bottle and both handgrip strength and pinch strength, we conducted multiple linear regression analyses with the dependent variables being handgrip strength or pinch strength, and the independent variables being patient characteristics and the ability to open plastic bottles. To calculate the areas under the curves (AUCs) and cut-off points for handgrip strength and pinch strength to predict the ability to an open plastic bottle, receiver operating characteristic (ROC) curve analyses were performed for all subjects, as well as for male and female subjects separately. AUC >0.9 indicates high accuracy, AUC 0.7–0.9 indicates moderate accuracy, and AUC <0.7 indicates low accuracy.¹⁶ We used the Youden index¹⁷ to determine cut-off points for handgrip strength and pinch strength for predicting the ability to open plastic bottles. The overall level of statistical significance was set at P<0.05 (two-tailed). Statistical analyses were performed using R version 4.1.2 (R Foundation for Statistical Computing, Vienna, Austria) and SPSS version 20 (IBM, Armonk, NY, USA).

Results

Patient Characteristics

We excluded 2 (0.9%) patients who had missing data; thus, 224 patients were included in the final analyses. The baseline characteristics of the study population according to the ability to open plastic bottles are summarized in Table 1. The median age of all patients was 69 years (IQR 60–74 years), and 189 (84.4%) patients were male. For all patients, the mean handgrip strength was 30.3±8.5 kgf, the median lateral pinch strength was 8.8 kgf (IQR 7.0–10.1 kgf), and the median 3-fingered pinch strength was 8.0 kgf (6.8–8.8 kgf).

Table 1.

Baseline Characteristics for All Patients and According to the Ability to Open Plastic Bottles

	All patients	Able to open plastic bottles	Unable to open plastic bottles	Effect size	P value
No. patients (%)	224	210 (93.8)	14 (6.3)
Age (years)	69 [60–74]	68 [58–74]	75 [72–80]	0.22	<0.001
Male sex	189 (84.4)	184 (87.6)	5 (35.7)	–	<0.001
BMI (kg/m²)	23.8 [21.5–26.3]	23.9 [21.9–26.7]	21.2 [18.5–24.3]	0.17	0.01
Diagnosis				–	0.01
ACS	198 (88.4)	189 (90.0)	9 (64.3)
Stable angina	26 (11.6)	21 (10.0)	5 (35.7)
Significant coronary artery stenosis				–	0.60
1-vessel disease	142 (63.4)	134 (63.8)	8 (57.1)
2-vessel disease	51 (22.8)	48 (22.9)	3 (21.4)
3-vessel disease	31 (13.8)	28 (13.3)	3 (21.4)
Treatment				–	0.14
PCI	213 (95.1)	201 (95.7)	12 (85.7)
Medication	11 (4.9)	9 (4.3)	2 (14.3)
LVEF (%)	52.5 [44.0–59.0]	53.0 [44.0–59.0]	50.0 [44.3–56.0]	0.02	0.73
Laboratory data
Maximum CK-MB (IU/L)	168.5 [48.8–351.5]	174.5 [50.5–358.5]	57.5 [23.5–160.8]	0.14	0.04
Hemoglobin (g/dL)	13.1±1.7	13.2±1.6	11.6±2.1	0.23	<0.001
eGFR (mL/min/1.73 m²)	57.8 [47.0–69.2]	58.9 [47.4–69.3]	55.4 [39.5–62.3]	0.05	0.44
Albumin (g/dL)	3.6 [3.3–3.9]	3.6±0.4	3.4±0.6	0.10	0.14
Comorbidities
Hypertension	111 (49.6)	102 (48.6)	9 (64.3)	–	0.28
Dyslipidemia	129 (57.6)	122 (58.1)	7 (50.0)	0.04	0.55
Diabetes	79 (35.3)	74 (35.2)	5 (35.7)	–	1.00
Chronic respiratory disease	8 (3.6)	7 (3.3)	1 (7.1)	–	0.41
Orthopedics disease	17 (7.6)	16 (7.6)	1 (7.1)	–	1.00
Medications
ACE inhibitor	108 (48.2)	104 (49.5)	4 (28.6)	–	0.17
ARB	59 (26.3)	56 (26.7)	3 (21.4)	–	1.00
β-blocker	177 (79.0)	167 (79.5)	10 (71.4)	–	0.50
Nitrates	10 (4.5)	10 (4.8)	0 (0)	–	1.00
Calcium antagonist	37 (16.5)	34 (16.2)	3 (21.4)	–	0.71
Analgesics	10 (4.5)	8 (3.8)	2 (14.3)	–	0.12
Living alone	45 (20.1)	44 (21.0)	1 (7.1)	–	0.31
FIM (points)	125 [123–126]	125 [123–126]	122 [116–124]	0.22	<0.001
Handgrip strength (kgf)	30.3±8.5	31.3±7.8	15.6±4.2	0.94	<0.001
Pinch strength (kgf)
Lateral pinch strength	8.8 [7.0–10.1]	8.8±2.0	5.3±1.5	0.39	<0.001
3-fingered pinch strength	8.0 [6.8–8.8]	8.0±1.6	4.8±1.3	0.44	<0.001

Unless indicated otherwise, data are expressed as the mean±SD, median [interquartile range], or n (%). ACE, angiotensin converting enzyme; ACS, acute coronary syndrome; ARB, angiotensin receptor blocker; BMI, body mass index; CK-MB, creatine kinase-myocardial band; eGFR, estimated glomerular filtration rate; FIM, Functional Independence Measure; LVEF, left ventricular ejection fraction; PCI, percutaneous coronary intervention.

Compared with patients who were able to a open plastic bottle, those who were unable to open a plastic bottle were significantly older, less likely to be male, less likely to have acute coronary syndrome, and had a lower BMI, lower maximum CK-MB levels, lower serum hemoglobin concentrations, lower Functional Independence Measure (FIM) scores, and lower handgrip strength, lateral pinch strength, and 3-fingered pinch strength.

The baseline characteristics according to sex are presented in Table 2. Compared with male patients, female patients were significantly older and had lower BMI, lower maximum CK-MB levels, lower serum hemoglobin concentrations, lower FIM scores, lower handgrip strength, lateral pinch strength, and 3-fingered pinch strength, and were less likely to be able to open a plastic bottle.

Table 2.

Baseline Patient Characteristics According to Sex

	Men	Women	Effect size	P value
No. patients	189 (84.4)	35 (15.6)
Age (years)	68 [58–73]	73 [68–78]	0.24	<0.001
BMI (kg/m²)	24.2 [22.4–26.8]	21.2 [19.7–23.5]	0.31	<0.001
Diagnosis			0.07	0.27
ACS	169 (89.4)	29 (82.9)
Stable angina	20 (10.6)	6 (17.1)
Significant coronary artery stenosis			–	0.35
1-vessel disease	116 (61.4)	26 (74.3)
2-vessel disease	46 (24.3)	5 (14.3)
3-vessel disease	27 (14.3)	4 (11.4)
Treatment			–	0.38
PCI	181 (95.8)	32 (91.4)
Medication	8 (4.2)	3 (8.6)
LVEF (%)	52.0 [44.0–58.0]	54.0 [49.5–59.0]	0.11	0.11
Laboratory data
Maximum CK-MB (IU/L)	182.0 [53.0–389.0]	84.0 [27.5–186.5]	0.19	0.004
Hemoglobin (g/dL)	13.4±1.6	11.6±1.2	0.38	<0.001
eGFR (mL/min/1.73 m²)	58.7 [47.8–68.6]	57.1 [37.0–70.6]	0.02	0.81
Albumin (g/dL)	3.6±0.4	3.5±0.5	0.08	0.26
Comorbidities
Hypertension	98 (51.9)	13 (37.1)	0.11	0.11
Dyslipidemia	109 (57.7)	20 (57.1)	0.004	0.95
Diabetes	68 (36.0)	11 (31.4)	0.04	0.60
Chronic respiratory disease	8 (4.2)	0 (0)	–	0.36
Orthopedics disease	15 (7.9)	2 (5.7)	0.03	0.65
Medications
ACE inhibitor	92 (48.7)	16 (45.7)	0.02	0.75
ARB	51 (27.0)	8 (22.9)	0.03	0.61
β-blocker	152 (80.4)	25 (71.4)	0.08	0.23
Nitrates	9 (4.8)	1 (2.9)	–	1.00
Calcium antagonist	31 (16.4)	6 (17.1)	0.007	0.91
Analgesics	6 (3.2)	4 (11.4)	–	0.05
Living alone	38 (20.1)	7 (20.0)	0.001	0.99
FIM (points)	125 [123–126]	124 [121–125]	0.14	0.04
Handgrip strength (kgf)	32.7±6.8	17.6±4.5	0.90	<0.001
Pinch strength (kgf)
Lateral pinch strength	9.1±1.8	5.6±1.1	0.88	<0.001
3-fingered pinch strength	8.3±1.5	5.3±1.3	0.59	<0.001
Able to open a plastic bottle	184 (97.4)	26 (74.3)	–	<0.001

Unless indicated otherwise, data are expressed as the mean±SD, median [interquartile range], or n (%). Abbreviations as in Table 1.

Association of Opening Plastic Bottles With Handgrip Strength and Pinch Strength

Among the 224 patients in this study, 210 (93.8%) could open plastic bottles. The results of multiple linear regression analyses of factors associated with handgrip strength and pinch strength are presented in Table 3. After adjustment for covariates, age (β=−0.18), male sex (β=0.47), BMI (β=0.11), dyslipidemia (β=0.09), chronic respiratory disease (β=−0.09), and the ability to open plastic bottles (β=0.17) remained statistically significant predictors of handgrip strength (P<0.001, adjusted R²=0.62). Further, after adjustment for covariates, age (β=−0.14), male sex (β=0.43), BMI (β=0.15), LVEF (β=0.12), hemoglobin (β=0.16), albumin (β=0.12), dyslipidemia (β=0.13), diabetes (β=−0.14), and the ability to open plastic bottles (β=0.13) remained statistically significant predictors for lateral pinch strength (P<0.001, adjusted R²=0.56). Finally, after adjustment for covariates, male sex (β=0.42), hemoglobin (β=0.18), albumin (β=0.11), dyslipidemia (β=0.10), diabetes (β=−0.12), and the ability to open plastic bottles (β=0.20) remained statistically significant predictors for 3-fingered pinch strength (P<0.001, adjusted R²=0.52).

Table 3.

Associations of the Ability to Open Plastic Bottles With Handgrip and Pinch Strengths in Multiple Linear Regression Analyses

	Handgrip strength R²=0.65 Adjusted R²=0.62 P<0.001, F=25.2		Lateral pinch strength R²=0.59 Adjusted R²=0.56 P<0.001, F=20.0		3-fingered pinch strength R²=0.55 Adjusted R²=0.52 P<0.001, F=17.3
	β	P value	β	P value	β	P value
Age	−0.18	0.001	−0.14	0.02	−0.12	0.06
Male sex	0.47	<0.001	0.43	<0.001	0.42	<0.001
BMI	0.11	0.03	0.15	0.01	0.06	0.33
Diagnosis: ACS	−0.03	0.55	0.01	0.78	0.003	0.96
LVEF	0.05	0.28	0.12	0.02	0.11	0.06
Maximum CK-MB	0.03	0.51	0.04	0.43	−0.006	0.92
Hemoglobin	0.10	0.08	0.16	0.009	0.18	0.01
eGFR	0.05	0.32	−0.06	0.28	0.01	0.93
Albumin	0.09	0.05	0.12	0.02	0.11	0.04
Hypertension	−0.004	0.93	0.01	0.92	−0.004	0.94
Dyslipidemia	0.09	0.04	0.13	0.005	0.10	0.04
Diabetes	−0.07	0.10	−0.14	0.003	−0.12	0.02
Chronic respiratory disease	−0.09	0.04	−0.03	0.52	−0.02	0.70
Orthopedic disease	−0.01	0.76	0.000	0.99	0.04	0.42
Ability to open plastic bottles	0.17	<0.001	0.13	0.01	0.20	<0.001

Abbreviations as in Table 1.

AUC and Cut-Off Values for Handgrip Strength and Pinch Strength to Predict the Ability to Open Plastic Bottles

The ROC curves of handgrip strength and pinch strength to predict the ability to open plastic bottles are shown in Figures 1–3. In all patients, the AUCs for handgrip strength, lateral pinch strength, and 3-fingered pinch strength required to open plastic bottles were 0.96 (95% confidence interval [CI] 0.93–0.99), 0.91 (95% CI 0.83–0.99), and 0.93 (95% CI 0.88–0.99), respectively (Figure 1). Based on the Youden index, the cut-off values for handgrip strength, lateral pinch strength, and 3-fingered pinch strength in all subjects were 20.0 kgf (sensitivity, 0.90; specificity, 0.93; positive predictive value [PPV], 0.99; negative predictive value [NPV], 0.37), 7.1 kgf (sensitivity, 0.78; specificity, 0.93; PPV, 0.99; NPV, 0.22), and 6.1 kgf (sensitivity, 0.87; specificity, 0.86; PPV, 0.99; NPV, 0.30), respectively.

Figure 1.

Receiver operating characteristic curves of handgrip, lateral, and 3-fingered pinch strengths for predicting the opening of plastic bottles in all subjects.

Figure 2.

Receiver operating characteristic curves of handgrip, lateral, and 3-fingered pinch strengths for predicting the opening of plastic bottles in male subjects.

Figure 3.

Receiver operating characteristic curves of handgrip, lateral, and 3-fingered pinch strengths for predicting the opening of plastic bottles in female subjects.

In male patients, the AUCs for handgrip strength, lateral pinch strength, and 3-fingered pinch strength required to open plastic bottles were 0.98 (95% CI 0.95–1.00), 0.85 (95% CI 0.67–1.00), and 0.90 (95% CI 0.81–0.99), respectively (Figure 2). Based on the Youden index, the cut-off values for handgrip strength, lateral pinch strength, and 3-fingered pinch strength in male patients were 24.2 kgf (sensitivity, 0.91; specificity, 1.00; PPV, 1.00; NPV, 0.23), 7.1 kgf (sensitivity, 0.86; specificity, 0.80; PPV, 0.99; NPV, 0.14), and 7.4 kgf (sensitivity, 0.77; specificity, 1.00; PPV, 1.00; NPV, 0.10), respectively.

In female patients, the AUCs for handgrip strength, lateral pinch strength, and 3-fingered pinch strength required to open plastic bottles were 0.88 (95% CI 0.77–0.99), 0.87 (95% CI 0.74–0.99), and 0.89 (95% CI 0.78–1.00), respectively (Figure 3). Based on the Youden index, the cut-off values for handgrip strength, lateral pinch strength, and 3-fingered pinch strength in female patients were 16.4 kgf (sensitivity, 0.77; specificity, 0.89; PPV, 0.95; NPV, 0.57), 5.1 kgf (sensitivity, 0.81; specificity, 0.78; PPV, 0.91; NPV, 0.58), and 4.9 kgf (sensitivity, 0.69; specificity, 0.89; PPV, 0.95; NPV, 0.50), respectively.

Discussion

To the best of our knowledge, this study is the first to show associations between the ability to a open plastic bottle and muscle weakness, as assessed by handgrip strength and pinch strength, in patients with CAD. Moreover, we determined cut-off values for and sex-related differences in handgrip strength and pinch strength associated with the ability of patients with CAD to open plastic bottles. Our findings indicated a robust association between the ability to open plastic bottles and both handgrip strength and pinch strength, even after adjusting for the effects of age, sex, BMI, diagnosis, LVEF, maximum CK-MB level, serum hemoglobin concentration, eGFR, serum albumin concentration, and comorbidities (hypertension, dyslipidemia, diabetes, chronic respiratory disease, and orthopedic disease). Assessing the ability to a open plastic bottle, which requires no specialized measuring equipment, may be a simple and practical tool to evaluate muscle weakness in environments where measurement is difficult because no measuring equipment is available.

Compared with patients who could open plastic bottles, those who could not open plastic bottles were significantly older, were less likely to be male or to have acute coronary syndrome, and had lower BMI, maximum CK-MB levels, serum hemoglobin concentrations, FIM scores, handgrip strength, lateral pinch strength, and 3-fingered pinch strength. These findings largely agree with the characteristics of patients with cardiovascular disease and muscle weakness reported in previous studies.¹⁸^,¹⁹ Therefore, it may be possible to partly generalize the characteristics of patients with CAD and muscle weakness.

In the present study, the association between the ability to open plastic bottles and both handgrip strength and pinch strength remained significant even after adjusting for potential confounders. An association between the ability to open plastic bottles and handgrip strength has been reported in community-dwelling older people and older inpatients in Japan.⁸^–¹⁰ The novel finding of the present study is that in patients with CAD, in addition to the known confounding factors of age, sex, BMI, diagnosis, serum hemoglobin concentration, eGFR, serum albumin concentration, and comorbidities, an association was observed between the ability to a open plastic bottle and handgrip strength after adjusting for LVEF and maximum CK-MB level, which reflect the severity of cardiac dysfunction. The present study also revealed a previously unexplored link between the ability to open plastic bottles and pinch strength. The opening of plastic bottles requires the coordinated movement of the thumb, index, middle, ring, and little fingers.²⁰ The thumb and index finger provide the power to open plastic bottles, whereas the middle, ring, and little fingers act as supports.²⁰ There were close associations between the ability to open a plastic bottle and both handgrip strength and pinch strength because handgrip and pinch grip actions require coordinated movements of the fingers.

The AUCs for handgrip strength, lateral pinch strength, and 3-fingered pinch strength for the opening of plastic bottles indicated high accuracy in all subjects. Moderate-to-high accuracy was also demonstrated for male and female subjects separately. These results indicate that evaluation of the ability to open plastic bottles may reflect handgrip strength and pinch strength in patients with CAD, and may be useful in environments in which measurement of strength and pinch strength can be difficult. The cut-off values reported herein may reflect handgrip strength and pinch strength related to sarcopenia and prognosis in patients with CAD. The Asian Working Group for Sarcopenia 2019 consensus defined low muscle strength as a handgrip strength <28 kgf in men and <18 kgf in women.¹³ In the present study, the cut-off values for handgrip strength, based on the Youden index, were 24.2 kgf in men and 16.4 kgf in women. These cut-off values are higher than the handgrip strength for all subjects for the opening of plastic bottles reported in 2 previous studies (15.0 and 17.7 kgf).⁸^,⁹ A possible explanation for this apparent discrepancy is that the previous studies included community-dwelling older people and older inpatients aged ≥65 years,⁸^,⁹ whereas the age range of patients with CAD in the present study was much wider and included middle-aged patients. Therefore, the cut-off values determine in this study may reflect handgrip strength in both middle-aged and older patients with CAD. These findings indicate that the ability to open plastic bottles could be potentially useful in assessing handgrip strength related to sarcopenia in patients with CAD. With regard to pinch strength, in patients undergoing maintenance hemodialysis (mean age 66.7 years), a decrease in all-cause mortality was reported when patients’ lateral pinch strength exceeded 4.99 kgf.⁶ In the present study, the cut-off values for lateral pinch strength, based on the Youden index, were 7.1 kgf in men and 5.1 kgf in women. These findings suggest that the ability to open plastic bottles may reflect pinch strength related to prognosis. However, these previous studies had limitations and targeted different study populations. Therefore, further studies are needed to assess the clinical implication of handgrip strength and pinch strength associated with the ability to open plastic bottles.

In this study, we found sex-related differences in handgrip strength and pinch strength associated with the ability to open plastic bottles. Female patients in our study were significantly older and had lower BMI, maximum CK-MB, serum hemoglobin, FIM scores, handgrip strength, lateral pinch strength, and 3-fingered pinch strength than male patients, with a lower rate of being able to open plastic bottles in the female group. These results indicate that female patients with CAD had characteristics associated with muscle weakness reported in previous studies¹⁸^,¹⁹ and may be at higher risk of having muscle weakness. Sex-related differences in handgrip strength and pinch strength were consistent with those reported for lean mass and neuromuscular activity.²¹^,²² A previous study reported that sex-related differences in handgrip strength depend on lean mass.²¹ Another study reported that there were different motor unit control strategy characteristics between men and women, with women having a higher rate of discharge and a higher motor unit recruitment threshold than men.²² Moreover, women generally have smaller hands than men, so they can grip bottle caps firmly, which may allow them to concentrate the force needed to open plastic bottles. In a previous study, cap-grasping patterns to open plastic bottles were found to be associated with muscle weakness in community-dwelling older adults.²³ Even if handgrip and pinch strengths are low, the cap of a plastic bottle may be opened if the cap and bottle are gripped in a different way. These findings could potentially explain the sex-related differences in handgrip strength and pinch strength associated with the ability to open the same plastic bottles.

Strengths and Limitations

This study showed associations between the ability to open plastic bottles and muscle weakness (handgrip strength and pinch strength) in patients with CAD. In addition, we investigated cut-off values and sex-related differences in handgrip strength and pinch strength associated with the ability to open plastic bottles in patients with CAD. Our study clearly showed sex-related differences in handgrip strength and pinch strength associated with the ability to open plastic bottles in patients with CAD that have not been reported previously.⁸^–¹⁰

The present study has several limitations. First, this was a Japanese single-center cross-sectional study with small sample size. In addition, the number of patients with CAD who could not open plastic bottles and the number of female patients with CAD were low. Japanese nationwide studies of patients with CAD reported that approximately 20% of these patients are female,²⁴^,²⁵ with similar findings in the present study. The highly skewed sample sizes may have influenced our results. Thus, the generalizability of our findings to a broader international population is limited. Second, the assessment of heart failure and comorbidities was not sufficient; handgrip strength and pinch strength may be influenced by sarcopenia, systemic skeletal muscle weakness associated with heart failure, cervical spondylosis, peripheral nerve disease, and diabetes. Third, the cut-off values for handgrip strength and pinch strength may be different for differently shaped plastic bottles made with other materials because the plastic bottles used in the present study were not internationally standardized, nor were the cap-grasping patterns standardized. Fourth, this study did not examine changes in handgrip strength and pinch strength over time, nor did it assess patients for sarcopenia and frailty. Finally, although this study showed that handgrip strength and pinch strength were associated with the ability to open plastic bottles, handgrip strength and pinch strength could not be estimated based solely on the ability to open plastic bottles. Therefore, further large-scale studies are needed to confirm our findings.

Conclusions

The ability to open plastic bottles was independently associated with handgrip strength and pinch strength in patients with CAD. In addition, we determined cut-off values for and sex-related differences in handgrip strength and pinch strength associated with the ability to open plastic bottles in patients with CAD. Evaluating the ability to open plastic bottles may be a simple and practical tool for assessing muscle weakness in patients with CAD, particularly in resource-limited settings.

Acknowledgments

The authors thank the staff members of the Sakakibara Heart Institute of Okayama and Kobe University who collaborated with this study.

Sources of Funding

The authors received no specific funding for this work.

Disclosures

The authors have no conflicts of interest to disclose.

IRB Information

This study was approved by the Ethics Committee of the Sakakibara Heart Institute of Okayama (Approval no. B202001-01).

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