Participants
Community-dwelling middle-aged and elderly Japanese were recruited to participate in this study through printed advertisements in a public information magazine. They were informed about the methods, procedures, and risks and provided written informed consent before participating. We excluded individuals who did not follow our instructions or those with medical conditions that could limit their ability to participate in the resistance training program, based on the decision of the physician in charge. Sixty-nine participants aged 69.4 ± 6.5 years (49 women and 20 men) volunteered for this study. Although 19, 16, 6, and 4 participants had a medical history of hypertension, dyslipidemia, heart failure, and diabetes mellitus, respectively, and accordingly, some of them were using medications including atorvastatin and amlodipine, they were all approved to participate in the exercise program by the physician in charge. Their heights, weights, muscle thicknesses, and blood parameters were evaluated before (pre) and after (post) the training period. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee for Human Experiments of Juntendo University, Chiba, Japan (Approval Number: 27–52). This study was registered in UMIN-Clinical Trial Registry (CTR) (ID: UMIN000042759, Date of registration: December 14, 2020).
Training program
Participants were instructed to engage in a low-load resistance training program using their own body weight and an elastic band. They were also instructed to avoid changing their dietary patterns throughout the training period. The program was conducted twice a week for 12 weeks, and the total number of classes was 22 or 23. The program was composed of nine exercises: squats, split squats, push-ups, heel raises, crunches, hip lifts, seated rows, shoulder presses, and arm curls. The first 6 exercises involved the participant’s own body weight, and the last three exercises required an elastic band (Thera-Band®; The Hygenic Corporation, Akron, OH, USA). In the first 2 weeks of the training period, the program was composed of only 4 exercises: squats, push-ups, crunches, and hip lifts, and the participants performed 3 sets of 8 repetitions with a 60-s rest between each set. In each repetition, they were instructed to spend 3 s in both the concentric and eccentric phases. After the first 2 weeks, the number of exercises per session, repetitions, sets per exercise, and exercise times were gradually increased, and the rest interval was gradually decreased every 2 weeks throughout the 12-week training period. For each session, it was indicated to the participants to continue until muscle fatigue. This training program was conducted according to the protocol followed in a previous study, and the details are described by Ozaki et al. [16].
Muscle thickness
Each participant’s muscle thickness was measured with a B-mode ultrasound device using a 5 to 18-MHz scanning head (Noblus; Aloka, Tokyo, Japan). We evaluated the anterior aspects of the thigh (AT) at the midpoint between the greater trochanter and lateral condyle of the femur. The participants were required to rest in the sitting position for at least 30 min before the measurement and to be in a supine position during the measurement. All the measurements were performed by the same operator. Before the study, we conducted preliminary examination to check the reproducibility and calculated the test–retest (inter-session) reliability using intraclass correlation coefficient (ICC), standard errors of measurement (SEM), and minimal difference. The ICC, SEM, and minimal difference for the muscle thickness in the AT was determined in 10 older men and women as follows: 0.992, 0.37 mm, 1.03 mm, respectively. This measurement was also conducted according to the method used in a previous study, and the details are described by Ozaki et al. [16].
Blood parameters
Venous blood samples of approximately 13 mL were obtained following at least 2 h of fasting before (pre) and after (post) the 12-week training program, and the levels of the following blood parameters were assessed: white blood cell count (WBC), red blood cell count (RBC), hemoglobin (Hb), hematocrit (Ht), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), platelet count (PLT), total protein (TP), albumin (Alb), aspartate aminotransferase (glutamic oxaloacetic transaminase; AST [GOT]), alanine aminotransferase (glutamic pyruvic transaminase; ALT [GPT]), alkaline phosphatase (ALP), leucine aminopeptidase (LAP), lactate dehydrogenase (LD [LDH]), γ-glutamyl transferase (γ-GTP), triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), hemoglobin A1c (HbA1c), and fasting blood sugar (FBS). These items were monitored using the complete blood count and blood biochemistry tests, which are included in annual medical checkups conducted in Japan.
We checked each blood parameter’s first quartile value before (pre) the training program, to establish the following cutoff criteria for the reduced levels: WBC = 4400/µL, RBC = 414 × 104/µL, Hb = 12.6 g/dL, Ht = 37.9 %, MCV = 87.7 fL, MCH = 29.2 pg, MCHC = 32.8 %, PLT = 18.8 × 104/µL, TP = 7 g/dL, Alb = 4.1 g/dL, AST(GOT) = 18 U/L, ALT(GPT) = 14 U/L, ALP = 178 U/L, LAP = 45 U/L, LD(LDH) = 169 U/L, γ-GTP = 14 U/L, TC = 181 mg/dL, HDL-C = 52 mg/dL, TG = 85 mg/dL, LDL-C = 102 mg/dL, HbA1c = 5.3 %, and FBS = 95 mg/dL. Using criteria for each blood parameter, participants were divided into the first quartile group, which is the lowest quartile group, as participants in this group have relatively low blood parameter values, or into the combined second, third, and fourth quartile groups with participants having normal blood parameter values.
Statistical analyses
To verify the training effect on muscle thickness at the AT, we analyzed the parameters before (pre) and after (post) the training program. Data were analyzed using the paired t-tests, two-way analysis of variance, analysis of covariance, and binomial logistic regression analysis. When there was a significant interaction in the two-way analysis of variance between time and group, the simple main effects analysis was conducted. In the binomial logistic regression analysis, we included age and sex as confounding factors, and the variable selection was conducted using a stepwise method with p = 0.20. P less than 0.05 was considered statistically significant. Results were expressed as means and standard deviations, and the odds ratio was represented by means and 95 % confidence intervals. Statistical analyses were performed using BellCurve for Excel (Social Survey Research Information Co., Ltd., Japan).