As far as we are aware, this is the first systematic literature review and meta-analysis that provides an integrated overview of the general effectiveness of HIIT on measures of VO2max (WMD = 2.16, 95% CI: 1.47–2.84), with a significantly greater increase in VO2max following HIIT compared with a MICT protocol (HIIT vs. MICT: WMD = 2.04, 95% CI: 1.01–3.07, p < 0.001). Additionally, training variables modifying the HIIT effects on measures of VO2max, confirmed the outcomes that long term (≥ 12 wks) vs. short term periods (< 12 wks), low (< 2 sessions/wk) vs. high frequencies (≥ 2 sessions/wk), long (≥ 40 min/session) vs. short lengths (< 40 min/session), 6 sets and repetitions, > 60 s time training per repetition, and rest times between sets and repetitions of ≤ 90 s, are more effective for VO2max. Furthermore, the main finding of our analysis showed that HIIT significantly increased 6WMT and muscle power, and reduced TUG, chair test, skeletal muscle CS activities, and EF% indicating that HIIT may be necessary to achieve beneficial cardiorespiratory and skeletal muscle mitochondrial adaptation in older individuals.
The function of HIIT has clinical utility in individuals in need of improved aerobic fitness because HIIT is able to rapidly increase VO2max [1, 2, 4, 11, 43]. The analysis of variables was based on data from 26 articles with a total of 1,000 elderly individuals. The present review showed that HIIT significantly improved VO2max in the elderly, and VO2max increased by 2.16 ml/kg/min and 2.04 ml/kg/min when compared with SED (WMD = 2.16) and MICT (WMD = 2.04), respectively. This was supported by a meta-analysis conducted by Ramos et al., who found that VO2max improved to a greater extent following 12 wks of HIIT (three times/wk) compared with MICT (14–46% vs. 5–16%, respectively) . In agreement with this finding, previous studies performed on treadmills demonstrated that greater improvements in VO2max in response to HIIT compared with MICT remained unchanged in non-active older adults. These positive findings are similar to past meta-analyses looking at HIIT intervention effects on patients with cardiometabolic disorders, cardiorespiratory fitness in children and adolescents, and patients with type 2 diabetes [44–46]. In addition, the previous meta-analysis studies showed that HIIT had a significant medium to large effect on VO2max in normal-weight (SMD = 0.83), overweight/obese (SMD = 0.74), and recreational healthy young adults (SMD = 0.86) . This implies that HIIT may be a more potent stimulus indirectly influencing VO2max in older adults compared to young or overweight adults.
We also determined how training variables such as session length, intensity, frequency, repetitions, training time per repetition, and rest time (rest between sets and repetitions) modified the HIIT effects on measures of VO2max. The present review suggests that training at intensities ≥ 80 VO2max (SMD = 1.83) had larger effects compared with training at intensities < 80 VO2max (SMD = 1.67) in elderly adults. In agreement with this finding, HIIT protocols used for patients with vascular dysfunction with similar intensities may have a greater capacity to improve VO2max in vascular dysfunction patients . In addition, the present review also suggested that training for ≥ 12 wks elicited larger beneficial effects enhancing VO2max compared to HIIT training for < 12 wks. This is consistent with a previous meta-analysis study in overweight/obese populations where HIIT performed for < 12 wks appears to be less effective in improving VO2max (SMD = 0.74) than HIIT for ≥ 12 wks (SMD = 1.20). Additionally, a training time per repetition of > 60 s is more effective for VO2max . The HIIT protocol used in this study consisted of longer terms and the training time per repetition compared with a previous meta-analysis study that included 53 studies, confirmed a training term of ≥ 4–12 wks with a training time per repetition of ≥ 2 min, which was recommended for healthy, overweight/obese, or athletic adults . This implies that HIIT protocols used to achieve beneficial cardiorespiratory fitness adaptations in older individuals should consist of longer terms with shorter training times per repetition.
Additionally, 6 sets and repetitions with rest times between sets and repetitions ≤ 90 s were more effective for VO2max. In the training frequency subgroup, 2 training sessions/wk (WMD = 3.00) produced large VO2max effects related to 3 (WMD = 1.46) and 4 training sessions/wk. In disagreement with this finding, HIIT protocols used for vascular dysfunction patients included 3 training sessions/wk, 4 sets and repetitions, and 3 min of active recovery had a greater capacity to improve VO2max . The HIIT protocol used in this study recommended inclusion of supervised sessions (i.e., 6 × > 60 s HIIT at × 80% VO2max, ≤ 90 s recovery, 2 times/wk for over 12 wks) to effectively improve VO2max. Indeed, in the training session length subgroup, a session length of ≥ 40 min/session was more effective for VO2max. While the American College of Sports Medicine states  that older adults should accumulate 150–300 min/wk (30–60 min/d × 5 times/wk) of moderate intensity aerobic exercise, accumulating only 80 min/wk (40 min/d × 2 times/wk) indicated that HIIT may be more effective in improving CRF in older adults when compared to MICT protocols .
WMT is the most commonly applied measure of endurance walking capacity and is valid for estimating VO2max in the elderly. Improvements in the 6WMT are valuable goals for the elderly in whom functional capacity is severely compromised . Our meta-analysis data also revealed that HIIT has the potential to increase 6WMT by 65.82 m, which is consistent with previous data that demonstrated that HIIT can up-regulate the 6WMT by 68 m in the elderly . Moreover, endurance walking capacity is positively associated with mitochondrial function since CS is a core enzyme of the tricarboxylic acid cycle and directly controls mitochondrial oxidative capacity. A previous study showed that 12 wks of HIIT increased skeletal muscle CS activity (55%) and mitochondrial content in older adults . Our previous animal study indicated that HIIT can up-regulate mitochondrial CS content in the skeletal muscle of aged rats which is consistent with the current meta-analysis results showing a marked increase in CS activity in skeletal muscles of the elderly with HIIT interventions . This coincided with improved endurance walking capacity and increased utilization of glucose and lipids, suggesting that the HIIT-induced increase in CS activity and subsequent increase in mitochondrial ATP biosynthesis may exert protective effects against the loss of age-associated endurance performance in older adults.
Central adaptations may be partly responsible for the greater improvements in CRF in response to HIIT . Cardiac contractile function, as assessed by EF%, was associated with a greater improvement in aerobic fitness. A recent study reported a 4–8% increase in EF% in older adults after 8–12 wks of HIIT training on a treadmill, but not so with MICT . In agreement with previous randomized studies, our study found a marked increase (1.32%) in EF% in elderly individuals derived from a total of 4 meta-analysis experiments after HIIT. Studies in older adults have shown that HIIT can also be effective in inducing left ventricular remodeling, but resulted in no improvements in cardiac diastolic function and greater improvements in systolic function . This is consistent with data from our meta-analysis which demonstrated that HIIT did not show significant results with regard to the SBP and DBP of the elderly, although SBP and DBP are usually consistent in aging and we did exclude individuals with cardiovascular disease . Finally, the current meta-analysis provided evidence that HIIT also improved cardiac function in older adults who are free of cardiovascular and other major clinical diseases.
Recent research suggesting a reduction in TUG time by 0.8–1.4 s demonstrates a clinically significant improvement in physical function . Small differences in TUG time may also improve fall risks . TUG, induced by HIIT, was reduced by 0.58 s and was accompanied by a reduction of 3.86 s in the chair test with similar improvements previously reported in studies using resistance training . This study found that HIIT could increase the muscle power of the elderly by 0.56 standard deviations (SD), and the strength of the upper and lower limbs also tended to increase. Indeed, the growth of even small muscle forces can have a large impact on fitness functional capacity and flexibility. Previous studies also found that after 12 wks of intermittent resistance training, the output power of the elderly increased from 96 to 116%, which is consistent with the results of the present study. This finding has clear practical implications as improvements in leg strength have been shown to make an important contribution to clinically meaningful improvements in chair test times and are related to improvements in functional performance. These results suggest that clinicians should consider whether there is a need for older participants to undertake HIIT if the primary goal is to improve strength and potentially reduce the risk of falls and fractures in older adults.
Age-related changes in body composition are associated with declining physical endurance, power, and slower gaits in older adults, while HIIT leads to decreased adiposity and increased muscle mass, as well as improved clinical outcomes in a number of age-related metabolic disorders, including visceral fat and insulin sensitivity [38, 57]. Our meta-analysis showed that a total of 12 meta-analysis experiments had a larger effect in reducing BF% than the changes of LM and muscle area of the elderly from 11 and 4 studies after HIIT, respectively. These results seem to suggest that HIIT had a greater potential to improve the BF% (WMD = -0.97) in healthy older adults compared with the potential to increase muscle area (WMD = 0.40) and LM (WMD = 0.68). These findings are in line with the results of Bruseghini et al., (2015), who examined the effects of HIIT on BF%, LM, and muscle area in healthy older adults and reported decreases in BF% larger than increases in muscle area size and LM . Indeed, it has been suggested that a greater tendency for HIIT to decrease the accumulation of abdominal fat and induce a lipolysis metabolism is consistent with HIIT interventions found to significantly reduce triglycerides (SMD = -0.34) in older adults. Although previous studies have shown a positive effect of HIIT interventions on total cholesterol, HDL, and LDL in older adults with central obesity, this was not found in the present study possibly due to a lack of lipid metabolic disorders [1, 53, 58].
In the current study, 2 of the studies also measured insulin sensitivity as assessed by blood glucose and 7 measured HOMA-IR following HIIT in healthy elderly adults. No changes were noted except a decreasing trend in HOMA-IR (SMD = -0.44) which may have been due to the lack of power in studies due to small sample sizes. Our findings disagree with results previously reported which indicated that HOMA-IR was reduced significantly following 8-wk all-extremity HIIT which included decreases in insulin sensitivity . Additionally, our findings confirm those of previous investigations that a decrease of glucose (SMD = -0.78) induced by HIIT was accompanied by a gain in LM and skeletal muscle CS activities suggesting that the increase of total muscle mass and mitochondrial oxidative phosphorylation may be help remove glucose even in a group of elderly individuals .
The effects of HIIT on measures of muscle power and health-related outcomes have to be considered as preliminary because based on our selected inclusion criteria our systematic search identified only 2 studies dealing with ULMs and HOMA-IRs and only 3 dealing with LLMs and CSs. Secondly, information regarding individuals’ characteristics were often incomplete (e.g., gender, age) and results were inconclusively reported (e.g., means and standard deviations) so that in several cases we were not able to compute the SMD. In addition, large heterogeneity was found across studies, which implies a large variability in the tested muscle strength variables (i.e., tests for ULM and LLM). Furthermore, except for modifying the training variables used to measure the effects of VO2max, it is a major limitation that such analyses fail to provide insights into how HIIT variables modify the characteristics of physical fitness, muscle size, or health-related outcomes in older adults due to inconclusively reported results.
Despite these limitations, this systematic review and meta-analysis was the first to provide an adequate overview of HIIT effects on measures of body composition, physical fitness, muscle size, health-related outcomes, and HIIT variables on VO2max. The present meta-analysis analyzed older adults who commenced HIIT to mitigate the age-related losses of muscle strength and mass, endurance capacity, EF%, and CS activities. Furthermore, to investigate the effects of training variables on VO2max for slowing age-related CRF loss, a possible combination of HIIT subcategories was created on the basis of the best applicability for practitioners and clinicians.