Ageing is characterised by a gradual reduction in skeletal muscle mass of approximately 30%–50% between the ages of 40 and 80 years old [1
]. The loss of skeletal muscle mass results in muscular weakness, which makes carrying out everyday tasks more difficult and increases the risk of physical disability with age [2
]. This loss of skeletal muscle mass is associated with a reduction of mitochondrial content, leading to a loss of aerobic capacity with age [2
]. The loss of aerobic capacity is also associated with a decline in physical function in later life [3
]. As well as changes in aerobic metabolism, there is a decline in blood glucose control with ageing [2
]. Insulin resistance with ageing has been associated with increased adiposity, impaired beta cell response and impaired muscular response to the rise in circulating glucose [4
]. Therefore, maintaining muscular function, metabolic health and aerobic fitness is extremely important for ensuring healthy outcomes in later life.
Aerobic and resistance training have been shown to have positive effects on the ageing processes, attenuating declines in muscle mass, physical function and metabolic health [5
]. Following 16 weeks of aerobic exercise (three progressing to four sessions per week at 70% VO2
max), there was a 9% increase in VO2
max which was the same regardless of age [6
]. Likewise, twenty weeks of aerobic exercise (four sessions per week at 70% VO2
max) have also been shown to improve VO2
peak by 8% and sit to stand performance by 9% [7
]. Similar improvements in sit to stand (10%) have also been seen with twenty weeks of resistance training (three sessions per week at 70% 1 repetition maximum, (RM) [7
]) whilst 8 weeks of resistance training (three sessions per week at 75%–80% 1 RM) resulted in mitochondrial adaptations with a 20–40% increase in expression of key enzymes related to aerobic metabolism in skeletal muscle [8
]. This increase in aerobic metabolism of the skeletal muscle was associated with a lower cardiorespiratory response to submaximal exercise [8
]. In contrast, the response of blood glucose control has been shown to be blunted in older adults (5% improvement in insulin sensitivity) compared to younger adults (72% improvement in insulin sensitivity) following 16 weeks of aerobic exercise. After 6 months of aerobic exercise (three times per week at 70% VO2
max), the area under the blood glucose curve was reduced by 3.1% and 2 h post blood glucose concentration was reduced by 3.6% [9
]. When weight loss was included with aerobic exercise then the response was much greater (17.3% reduction in area under the curve, 22.5% reduction in 2 h blood glucose; [9
]). Following 8 weeks of resistance training, blood glucose control is improved, with a 20% reduction in the area under the blood glucose curve following ingestion of 75g carbohydrate and lower 2 h blood glucose concentration [8
]. The greater adaptation of blood glucose control seems to be related to the ratio of muscle mass to intramuscular fat mass with both aerobic and resistance exercise. Despite this, participation in traditional training modalities is low in older adults [10
], with low adherence after supervised training [11
]. Therefore, there is a need to identify different approaches to exercise with older adults.
Sprint interval training (SIT) can be defined as repeated supramaximal intervals of a duration of less than 30 s, with a longer period of recovery [12
]. SIT has been shown to improve blood glucose control in younger adults when performed 3 times per week (4–6 30 s sprints with 4 min recovery, [13
]). Likewise, twice-weekly SIT (10 × 6 s sprints with 60 s recovery) has been shown to improve blood glucose control (6% reduction in glucose area under the curve, 11% reduction in 2 h glucose concentration) and physical function (20% decrease in timed get up and go) in middle aged people [14
]. In older adults, twice-weekly SIT (10 × 6 s sprints with 60 s recovery) has been shown to improve aerobic capacity (8% increase in predicted VO2
max) and physical function (11% decrease in timed get up and go) after 6 weeks [15
]. Following 10 weeks of the same SIT protocol, physical function was shown to be improved along with arterial stiffness and circulating lipids [16
]. The lowest frequency reported for SIT is one session every 5 days (6 × 30 s sprints at 50% peak power with 3 min recovery) which resulted in a clinically relevant increase in peak power and greater lean body mass in older adults after 6 weeks [17
]. This suggests that less than twice per week may be a sufficient training stimulus for improvements following SIT.
Despite the effectiveness of SIT in promoting health benefits, it is not clear what the minimum frequency of training that is required to deliver these adaptations in inactive, but healthy older adults. Studies to date have used two or three training sessions per week but nobody has specifically looked at frequency. Therefore, the aim of this study was to explore the impact of SIT frequency on blood glucose control, aerobic capacity and physical function in older adults. It was hypothesised that twice a week training would produce greater adaptations than training once per week, but both would be effective compared to no exercise.