Study Selection
The research databases AMED, Embase, HMIC, BNI, MEDLINE, PsycInfo, CINAHL, HEALTH BUSINESS, PubMed, Web of Science (WoS), and EBSCO were used to conduct a systematic literature search. The search retrieved 21,559 articles. A total of 32 articles were also identified from the reference lists of other studies. 1448 duplicates were identified using Rayyan QCRI and EndNote. Title and abstract screening by two independent reviewers resulted in the exclusion of a further 20,063 studies. Eighty articles were deemed eligible for full-text screening, while 48 were excluded for particular reasons. Finally, 32 articles were eligible for inclusion in the systematic review, and 26 in the meta-analysis (see the PRISMA flow diagram in Fig. 1).
Study Characteristics
The average age calculated from 26 studies was 57.36 years. Two studies recruited participants in the age ranges 30–80 years [26] and 40–65 years [27], respectively; another recruited elderly patients above 50 years [28]. The average exercise session was 45.15 min, while the average follow-up duration was 21.94 weeks. The mean exercise frequency according to our findings was 3.25 days/week.
For all studies, the study population comprised type 2 DM patients. Depending on the study, the participants were also obese [29], hypertensive [30], adults with coronary artery disease (CAD) [31], or newly diagnosed type 2 DM patients [32], had received their diagnosis 5–8 months earlier [26] or < 5 years earlier [33], or were female participants only [34], sedentary patients with 5 years of diagnosis [35], elderly patients with peripheral neuropathy [28], or overweight, dysregulated, and sedentary patients [36]. Most of the studies recruited participants with HbA1c levels of 6.4–7.0 [26, 30, 32, 37,38,39,40,41,42,43,44], followed by levels of ≥ 6.5 [27, 45, 46], 7.0–10.0 [34], 7.2–9.0 [28, 31, 33, 47], 10.0–11.0 [48, 49], or 7.0–11.0 [36].
A variety of exercise interventions were utilized in the studies, but aerobic training (12 studies) [27, 28, 30, 32, 39, 41, 44, 45, 49,50,51,52] and resistance training (RT) (14 studies) [27, 31, 36, 39,40,41, 44, 45, 47, 48, 52,53,54,55] were the most common. More specifically, the exercises used in the studies were as follows: aerobic exercises: moderate-intensity continuous (MIC) training [30, 32], supervised structured aerobic exercise training (SSAET) [51]; anaerobic training: 5 studies [32, 37, 55,56,57], high-intensity interval training (HIIT) [30, 32, 37, 56], sprint interval training (SIT) [32]; resistance training (RT): biodensity resistance exercise [40], progressive resistance training (PRT) [53, 54]; endurance training: 1 study [37]; combined exercise training: combined exercise regimen [38, 44, 45, 47]; walking: [29, 31, 39, 43, 45, 46]; general physical activity: [26, 31, 33, 42, 44]; nonlocomotive physical activity [42]; yoga: [34, 43]; and nonspecific exercises [35].
The common outcome measures used by most of the studies were as follows: HbA1c (all studies), FBG (20 studies) [26,27,28,29, 32, 35,36,37, 40, 42,43,44, 47, 48, 50,51,52, 56,57,58], BMI (17 studies) [27, 29, 30, 34,35,36,37,38, 44, 45, 48, 50, 51, 54,55,56,57], bodyweight (13 studies) [26, 27, 31, 33, 35, 36, 42, 44, 45, 48, 54, 56, 57], waist circumference (11 studies) [26, 29, 33, 35, 41, 44, 45, 47, 48, 53, 54, 56], and QoL (5 studies) [33, 36, 43, 48, 54].
The durations of interventions reported by the included studies were as follows: 8–12 weeks [27, 28, 30, 35, 37, 38, 42, 43, 48, 55,56,57], 16–25 weeks [36, 37, 44, 50, 51, 53], and 6 months to 1 year [26, 29, 31, 39,40,41, 46, 52]. Three days per week (3d/wk) [30, 32, 35,36,37,38, 41, 45,46,47,48, 51, 53,54,55, 57] was the most common exercise frequency, followed by 5d/wk, [39, 50, 56], 1d/wk [59], and 2d/wk [31, 34, 43], daily [29], 7 visits/3 month [44], and 1d/5 weeks [44]. Most of the studies were consistent in the duration of an exercise session: 5–10 min [40], 20–45 min [30, 32, 35, 37, 38, 44, 47, 49, 50], 45–60 min [27, 31, 34, 36, 39, 43, 51, 55, 56], 270 min/wk [41], or 150 min/wk [33, 45, 52].
Progression and increasing intensity of exercise were reported by 12 studies as follows: based on an increasing number of steps at each visit [29]; based on monitoring body weight and maximum oxygen uptake (VO2 max) every three months, progression from 70% to 80% HRR was observed [47]; based on increased exercise intensity with every session [32]; based on increasing the mean treadmill grade and METs progressively [41]; based on a progressive increase in resistance [53, 56]; based on an individualized elevation of exercise intensity (14–16 on the Borg scale) at week 12 [48]; based on a 30-min increase in each phase of a five-phase program [51]; based on increasing nonlocomotive physical activity [42]; based on progression in the completion of 12 repetitions during the final set of exercises in two consecutive sessions [45]; and based on the progression of low-volume HIIT to high-volume HIIT over 6–12 weeks [55]. A detailed description of the study characteristics is available in Table 1.
Table 1 Characteristics of the studies included in this review Study Quality and Risk of Bias
The selection, performance, and detection biases were similar for most of the studies (Fig. 2, 3). The quality and level of evidence of each study are summarized in Table 2. The majority (26) of the studies were of high quality (level 1), but 6 studies were of lower quality (level 2) [33, 39, 42, 44, 45, 57].
Table 2 The quality and level of evidence of each study