Mitochondria are unique organelles that are derived from prokaryotic cells that long ago fused with a host cell. They are imperative for the provision of energy-rich ATP and the regulation of cellular longevity across multiple organ systems. Colloquially termed ‘the powerhouses of the cell’, this unofficial moniker is an underrepresentation of the variety of functions these mighty organelles play. However, the fact that mitochondria are widely recognizable beyond the science community is indicative of their relative importance as integral organelles in virtually all cells throughout the body. Moreover, the volume of mitochondrial research within the medical sciences community is steadily on the rise. Over the past two decades mitochondrial-related publications have continued to increase beyond the rate of all other cellular organelles, possibly as a result of the recognition of the convergence of essential signalling pathways and biological processes on mitochondria.
Mitochondria are capable of adapting to altered metabolic demands. Through processes of biogenesis and fusion, newly formed mitochondria are adjoined to neighbouring organelles to increase their capacity for ATP synthesis, the sharing of metabolites, and Ca2+ handling. Conversely, where mitochondria are overabundant and/or dysfunctional, processes of fission and mitochondrial clearance (mitophagy) occur to re-establish metabolic homeostasis and maintain mitochondrial health within the cells. At any moment these opposing processes are in a state of flux, depending on the metabolic needs of the tissue, to calibrate and promote an optimal mitochondrial pool. When the cell is subjected to conditions that perturb metabolic homeostasis, such as exercise or advancing age, this balance in mitochondrial regulation can shift toward an increase in synthesis and fusion, or fission and mitophagy, respectively. The classic mechanism of mitochondrial fusion is achieved through the activity of the proteins mitofusin-1/2 (Mfn1/2) and optical atrophy protein 1/2 (Opa1/2) which facilitate fusion of adjacent outer and inner membranes, respectively, thus establishing an expanded organelle network. Conversely, mitochondrial fission is accomplished by the joint action of proteins dynamin related protein 1 (Drp1), mitochondrial fission factor (Mff) and fission protein 1 (Fis1), the actions of which culminate in the constriction and cleavage of organelle fragments from the network to allow for proper clearance.
Recent research has uncovered the complexity of mitochondrial morphology, exhibited by connections through either electron dense intermitochondrial junctions (IMJs), as well as membranous protrusions termed nanotunnels. It is proposed that IMJs allow for electrical coupling of the mitochondrial pool, as well as the ability to rapidly segregate dysfunctional organelles from the reticulum, while simultaneously allowing for either the repair or removal of the malfunctioning organelle. In contrast, nanotunnels promote the sharing of mtDNA, proteins, metabolites and other small molecules to non-adjacent mitochondria. These small (40–200 nm diameter) double-membrane extensions appear to exist primarily to connect mitochondria in tissues with restricted mitochondrial motility. Nanotunnels may develop as a compensatory mechanism of mitochondrial crosstalk under stress conditions, when more direct mitochondrial communication is not possible.
Mitochondrial morphology and function also differ across tissues, in order to match the specialized metabolic demand of their respective cells, or the changing pathological or physiological conditions that they are subjected to. Skeletal muscle provides an excellent tissue to investigate the various morphological configurations of mitochondria within a cell. When viewed using electron microscopy (EM), mitochondria within muscle have distinct geographical subpopulation. Below the sarcolemmal membrane reside the subsarcolemmal (SS) mitochondria which present the more classic rounded appearance. They specialize in providing ATP for nuclear gene transcription and membrane transport. Interspersed throughout the contractile myofibrillar protein network are the intermyofibrillar (IMF) mitochondria, which are more elongated and interconnected. These mitochondria provide ATP for muscle contraction and they are tightly connected to the sarcoplasmic reticulum (SR). Thus, these may additionally play a prominent role in Ca2+ signalling. In response to whole-body exercise, skeletal muscle mitochondria increase their rates of ATP synthesis to match the metabolic demands of the cell. Coincidentally, a myriad of signalling events converge to activate nuclear and cytoplasmic proteins to orchestrate the initiation of biogenesis, as well as mitophagy of a pre-existing, unhealthy mitochondrial pool.
The effect of various exercise training modalities on mitochondria
It is now well recognized that exercise is a potent stimulus to induce the signalling pathways described above, which ultimately produce robust phenotypic changes in the mitochondrial milieu and improve the quantity and quality of the organelle network, leading to greater muscle health. It was through John Holloszy's pioneering work that we first came to understand the important training parameters that are required to achieve a particular level of adaptation, and how exercise training promotes mitochondrial biogenesis. Favourable mitochondrial adaptations in muscle can only be achieved if training is performed at a sufficient frequency, intensity and duration, for an adequate length of time. Since the 1960s, decades’ worth of research has demonstrated that exercise promotes a robust increase in mitochondrial content, as well as improved oxidative phosphorylation and respiratory capacity per mitochondrion. Additionally, chronic training reduces the production of ROS, indicative of an enhanced capacity for electron flow through the ETC. Given that these adaptations are favourable for improved muscle health, scientists have continually strived to determine the precise signalling pathways mediating mitochondrial quality control, which includes the result of biogenesis and fusion, as opposed to fission and mitophagy.
Since Hoppeler et al. described a correlation between mitochondrial volume and 
, the association of mitochondrial content within muscle and exercise capacity has been a focal point for fitness and exercise practitioners to harness modalities that improve aerobic potential and performance. For many years, endurance training was considered the primary means of achieving mitochondrial adaptations. Endurance exercise training increases total mitochondrial proteins including those involved in β-oxidation, the tricarboxylic acid (TCA) cycle, and the electron transport chain, thus improving the capacity for energy provision to the exercising muscle. In fact, a single bout of endurance exercise is sufficient to induce structural changes in the mitochondrial network that promote augmented function of the organelle network. With prolonged endurance training, mitochondrial volume typically increases as much as 40–50% and this increase in content is paralleled by more modest improvements in respiration and oxidative capacity of the mitochondria on a per organelle basis. Additionally, mitochondrial adaptations to exercise vary across the different fibre types, depending on the initial organelle content, as well as the degree of motor unit recruitment during the training session. In humans, slow-twitch (type I) fibres contain the highest percentage of mitochondria, followed by fast-twitch red (type IIa) fibres, and the whiter type IIx fibres, respectively. With training, the mitochondrial content within any of these fibre types can be enhanced, indicating that mitochondrial adaptations are not dependent on the myosin-based fibre type per se, but instead are based on the stimulus and the recruitment of that fibre.
Terjung and colleagues illustrated the important interplay between intensity and duration in determining the extent of mitochondrial adaptation in each fibre type. Type I fibres in particular are most easily recruited at exercise intensities as low as 40% of 
and lower. As workload increases and exceeds 40% of 
, type IIa fibres are recruited, and only after the exercise intensity surpasses about 75% of 
will type IIx fibres be engaged. Based on this principle – that motor units must be recruited in order to adapt – training approaches have evolved from the traditional long-distance endurance exercise to alternative, time-saving modalities, such as sprint interval training (SIT), high intensity interval training (HIIT) and even resistance exercise, which can lead to diverse mitochondrial adaptations in muscle.
Resistance exercise is typically associated with muscle hypertrophy and improved force-generating capacity, as opposed to fatigue resistance and improved aerobic energy metabolism. Historically, the expansion of muscle cell size via hypertrophy has been viewed as ‘diluting’ mitochondrial content in muscle and increasing the diffusion distances. Indeed, a recent review of the effect of resistance exercise has revealed the disparate results obtained on mitochondria. However, it now appears that resistance exercise has the potential of inducing tangible improvements in maximal coupled respiration measured in permeabilized myofibres, albeit without a parallel increase in mitochondrial gene expression or mitochondrial mass. These adaptations may be more pronounced in older individuals or in diseased muscle such as sporadic inclusion body myositis, where mitochondrial gene expression and, subsequently, content is reduced in the basal state.
Interval training modalities such as HIIT and SIT have gained considerable popularity of late, and are capable of eliciting a similar level of adaptation as traditional endurance training while doing so in considerably less time, with a reduced exercise volume. These exercise conditions involve the recruitment of all motor units, and this contributes to the detection of mitochondrial adaptations in all muscle fibres within a mixed fibre biopsy sample. High intensity training bouts promote the activation of the various signalling kinases that converge on PGC-1α to promote organelle synthesis, as with endurance exercise. However, exercise at higher intensities generates more rapid ATP hydrolysis and greater Ca2+ release to generate force. This should lead to the enhanced activation of signalling kinases such as p38 MAPK, AMPK and CaMKII, and could account for the observations that repeated bouts of both HIIT and SIT produce as much as 25–35% increases in mitochondrial content in as few as 6–7 sessions.
