Cardiovascular disease (CVD) is a health crisis with increasing incidence and prevalence, which may compromise physical performance and contribute to mortality, morbidity, and the economic burden of health care. The prevalence of CVD remains high and is persistently the leading cause of death globally as data shown estimated 17.9 million people died from CVDs in 2019, contributing to 32% of all global deaths [1]. Mitochondria dysfunction is greatly implicated to the pathogenesis and progression of CVD, setting out the emergence of mitochondrial-targeting therapy [2]. As a non-pharmacological tool, exercise training is potentially a safe and effective measure to restore mitochondrial health in CVD [3]. Nevertheless, the mechanisms of exercise on mitochondrial function are not yet elucidated.

Optimal mitochondrial function is fundamentally achieved through the balance of organelle synthesis and degradation in response to energy demand. Under stable conditions, mitochondrial turnover is regulated by the fusion and fission processes. Any damage to the mitochondrial network will recruit fission protein to cleave off the dysfunctional site. The damaged mitochondrial portion will then be cleared by mitophagic degradation, while the functional portion will be fused to other mitochondria. In a high energy-demand state, such as exercise or under hypoxia condition, the rate of mitochondrial biogenesis and turnover is enhanced. Healthy mitochondria consume oxygen and synthesis ATP in the electric transport chain with an efficient degradation of the damaged site. However, disruption in mitochondrial turnover were seen in many CVD, namely atherosclerosis, reperfusion injury, and cardiomyopathy [4].

Mitochondrial dysfunction manifested with structural and functional abnormality in the myocardial ^[5][6][5,6] and skeletal muscles [7]. The metabolic maladaptation of mitochondria in CVD was observed with impaired oxidative phosphorylation (OXPHOS) and decreased ATP levels, increased apoptosis, deregulated autophagy, and severe oxidative stress ^[8][9][8,9]. Endothelial damage induces excessive ROS, which increases the endothelial permeability and monocytes adhesion to the vascular wall, subsequently promoting plague formation ^[10][11][10,11]. Proinflammation induced by mitochondrial dysfunction triggers plaque rupture, resulting in myocardial infarction and cerebrovascular accident (CVA) [12].

Despite the optimal pharmacological treatment, the progression of CVD often results in prognosis and low quality of life. Mitochondria’s role in CVD have been giving novel insights into finding therapeutic ways to fight the disease. However, it is challenging to activate mitochondrial biogenesis without compromising safety, to restore oxidative capacity in cardiomyocyte and vascular cells ^[2][13][2,13]. Therefore, mitochondria-targeted drug therapy remains elusive. Physical exercise is long known as a cardioprotective factor and reduces mortality. A recent cohort study with 30 years of follow-up revealed that long term physical activity was strongly associated with lower mortality for individuals who performed ≈150 to 300 min/week of vigorous physical activity, 300 to 600 min/week of moderate physical activity, or an equivalent combination of both [14]. Regular exercise reduces cardiovascular risks by down-regulating blood pressure, body weight, and LDL cholesterol, increasing HDL and insulin sensitivity, attenuating inflammation, and more recently, modifies mitochondrial dynamics and function [3].

Animal and pre-clinical studies have been suggesting exercise-induced effects on the mitochondrial life cycle, such as increasing PGC-1α, promoting fusion, enhanced oxygen consumption rate, and increased mitophagy. These benefits are yet to be elucidated in human study. TheOur previous studies demonstrated that moderate-intensity continuous and high-intensity interval exercise regimens improved the mitochondrial functions of platelets and lymphocytes in sedentary individuals ^{[15][16][17][18]}[15,16,17,18] and in patients with CVD ^{[19][20][21][22]}[19,20,21,22]. Nevertheless, additional information is needed to understand the exact mechanism of exercise on mitochondrial functionality in patients with CVD ^[23][24][25][23,24,25]. Moreover, the prescription of exercise, including type, intensity, duration, frequency, and progression, to ameliorate mitochondrial function, remained unclear.

2. Physical Exercise Programs

Eleven studies employed aerobic exercise using a static bike, treadmill, or functional movements, of which three used high intensity interval training (HIIT) ^[22][26][27][22,42,44]. Eight studies involved resistance exercise, of which only three worked on multiple muscle groups ^[28][29][30][34,37,38], the others were knee or hand movements. One study explored blood flow restricted knee extension exercise with low intensity ^[31][32]. Two studies had group training in addition to cycling regimen ^[32][33][33,45]. The exercise intensity in aerobic training was tailored to participants’ characteristics, based on peak oxygen uptake (% VO_2peak, 60%) ^[19][22][34][19,22,43], the percentage of peak heart rate (%HR_peak, 70~95%) ^[32][33][33,45], the percentage of maximum heart rate (%HR_max, 70~80%) ^[26][35][29,42], ventilation threshold (VT) [20], and pain threshold ^[36][37][38][39,40,41]. The training duration ranged from four to 24 weeks, with a frequency of twice a week to daily. Time per session ranged from 10 to 60 min. Resistance training was performed with free weights or machines. The exercise intensity was tailored to participants’ characteristics, based on Repetition Maximum (RM, ranged 30~80%) ^[28][29][31][32,34,37], maximum voluntary isometric contraction (MVIC, 15%) ^[39][35], or maximum work rate (WR_max, 50~100%) ^[27][40][30,44]. The training duration ranged from 6 to 18 weeks, with a frequency of thrice a week to daily. Each session took 10 to 100 min or 2–3 sets of 8–10 repetitions. In terms of the disease cohorts, five of the 13 HF studies adopted aerobic exercise and the others were resistance exercise. Three studies compared the training effect between patients and healthy control ^[29][39][40][30,35,37]. Three studies gave advice or no intervention to the control group ^[22][31][22,32], three continued with usual care, while the others had no control group. Concerning PAD, three of the four studies used functional activities such as walking ^[36][37][38][39,40,41], calf raises ^[38][41], or lower limb movement ^[36][39]. Two studies recruited control group with the usual care/general rehab program ^[19][20][19,20]. Aerobic exercise was implemented in studies for CVA, coronary artery disease, and hypertension.

2.1. Mitochondrial Outcomes

The selected studies performed muscle biopsy, spectroscopy, or venous blood test to evaluate the mitochondria function of skeletal muscle cells or platelets. The majority of the studies performed skeletal muscle biopsy on either vastus lateralis ^{[26][27][28][29][30][31][32][33][34][40][41]}[30,31,32,33,34,37,38,42,43,44,45] or gastrocnemius ^[36][38][39,41]. Four studies used spectroscopy to examine skeletal muscle mitochondrial function from the forearm ^[39][42][35,36] and lower leg ^[35][37][29,40]. Chou et al. [22], Lin et al. [20], and Hsu et al. [19] had withdrawn venous blood to analyze platelet mitochondrial bioenergetics in patients with HF, CVA, and PAD, respectively.

2.1.1. Mitochondrial Morphology

Two studies on patients with HF reported controversial findings on the effect of resistance training in mitochondrial phenotypes. Toth et al. ^[29][37] showed no change to the muscle mitochondrial size after 18 weeks of moderate-intensity systemic resistance exercise, whereas Santoro et al. ^[28][34] reported an increase of 23.4% in size with unaltered shape with similar protocol but higher intensity.

2.1.2. Mitochondrial Biogenesis and Dynamics

Two studies ^[40][41][30,31] investigated the effect of resistance training on the mitochondrial volume density in patients with HF and reported significant gain in the mitochondrial volume density. Another two studies ^[32][33][33,45], examined the effect of aerobic exercise, and also reported significant increase in the mitochondrial volume density. In detail, there were increment in TVVM (19%), the surface density of the mitochondrial inner border membrane—SVM_IMB (92%), the surface density of mitochondrial cristae—SVMC (43%), and the surface density of cytochrome c oxidase-positive mitochondria—SVMO_COX+ (between 27 and 41%). In studies that evaluated the protein and enzyme activities contributing to mitochondrial biogenesis, there was no agreement from the selected studies. Two studies ^[29][34][37,43] showed no change in PGC-1α, whereas Wisløff et al. ^[26][42] reported a significant increment after 12-week AIT. One study ^[29][37] reported that Tfam improved significantly after training, while the other ^[34][43] showed no change. Fiorenza et al. ^[27][44] reported no change in ERRα after training. The one study ^[27][44] that investigated the fusion and fission phase reported that MFN1 and OPA1 were down-regulated by HIIT. MFN2 was up-regulated by HIIT but DRP1 had no change.

2.1.3. Mitochondrial Oxidative Capacity

Exercise training significantly increased the mitochondrial oxidative capacity in five trials (SMD = 4.78, CI = 2.99 to 6.57, p < 0.01) (Figure 12). However, the analysis showed high heterogeneity among studies (Q = 16.10, df = 4, p = 0.003, I² = 75%). Eleven papers reported mitochondrial oxidative capacity, of which two reported resting muscle oxygen consumption (mVO₂), five studies examined electron transfer system (ETS) or OXPHOS ^{[19][20][22][31][38]}[19,20,22,32,41], three studies reported the mitochondrial ATP production rate (MAPR) ^[30][35][42][29,36,38], two studies assessed phosphocreatine (PCr) measures ^[32][35][29,33], and six studies measured citrate synthase activity ^{[27][29][30][31][36][38]}[32,37,38,39,41,44]. Chou et al. [22] reported a 12-week of HIIT cycling regimen significantly increased the maximal and reserve platelet OCR capacities, and enhanced the Complex I- and II-mediated OCRs from ETS activity in patients with HF. Groennebaek et al. ^[31][32] studied blood flow restricted knee extension exercise and reported a 6-week training increaseof 23% of the state 3 respiration supported by complex I and II in patients with HF. Lin et al. [20] reported 12-week cycling at ventilation threshold significantly enhanced succinate-involved OXPHOS level, maximal OXPHOS and ETS in platelet in patients with PAD. On the other hand, van Schaardenburgh et al. ^[38][41] reported that the 8-week home-based walking/calf raise exercise improved neither the capacity of OXPHOS nor ETC in patients with PAD, but the calf raise training group had significant improvement in the CS activity in platelet. Hsu et al. [19] studied platelet mitochondrial bioenergenetics in CVA patients and reported that the 4-week cycling plus general rehab significantly enhanced OXPHOS and ETS by activating the FADH2 (Complex II)-dependent pathway. Southern et al. ^[39][35] and Murrow et al. ^[37][40] showed controversial results regarding the effects of exercise training on mVO₂. One showed no change in patients with HF, while the other showed significant improvement in subjects with PAD after training. Three studies, involved patients with HF, reported the effects of exercise training in MAPR. Adamopoulos et al. ^[35][29] did a RCT studying the effect of an 8-week cycling program on the oxidative capacity of calf muscle cells in patients with HF; the study reported that reduced PCr recovery half-time with improved MAPR after training were observed through spectroscopy. Meanwhile, Straton et al. ^[42][36] reported similar findings of increased PCr resynthesis rate with improved MAPR in forearm muscle cells, also observed through spectroscopy, after 4-week daily hand exercise using hand-held dynamometer. A RCT performed by Williams et al. ^[30][38] confirmed the improvement in MAPR, through muscle biopsy, after 3 months of circuit resistance exercise. Theis study also reported an increase in CS activity. There are controversial findings regarding the enzymatic activity of CS. Four studies reported a significant increase in CS activity followed by their training protocol ^{[27][30][31][38]}[32,38,41,44], whereas the other training protocol of three studies reported no change or reduction in CS activity ^[36][38][39,41]. The 18-week resistance training protocol involving patients with HF, performed by Toth et al. ^[29][37], showed no effect in CS activity. Hiatt et al. ^[36][39] reported reduced and no change in CS activity after 12 weeks of lower limb resistance exercise and walking, respectively, in patients with PAD. Similarly, the 8-week home-based walking protocol by van Schaardenburgh et al. ^[38][41] also showed no effect in CS activity.

2.1.4. Mitochondrial Antioxidant Capacity and Quality

Only one study from the selected studies investigated the effects of training on mitochondrial antioxidant capacity. Fiorenza et al. ^[27][44] reported upregulated SOD2 but decreased SOD1, along with augmented CAT and NOX. Divergent responses were seen in the markers of mitochondrial antioxidant protection. Two studies evaluated the mitochondrial quality. Zoll et al. ^[34][43] reported no change in COX after aerobic exercise in patients with coronary artery disease. Fiorenza et al. ^[27][44] showed a lack of change in COX-IV and SIRT3 abundance after HIIT cycling in people with hypertension. No alteration was observed in the mitochondrial quality as explained by the two studies.