Muscle fatigue is a complex phenomenon defined as a temporary, exercise-induced reduction in the capacity of the muscle to produce force. Briefly summarizing (see review by Allen et al. 2008), within the muscle, fatigue can be broadly attributed to three mechanisms: reduced Ca2+ release from the sarcoplasmic reticulum (SR), reduced myofibrillar Ca2+ sensitivity, or reduced force produced per crossbridge. During extended muscular activity, metabolic by-products accumulate, and several of these metabolites are postulated to be involved in the reduced force output observed during fatigue. Inorganic phosphate (Pi), the by-product of ATP hydrolysis, is thought to contribute to fatigue via all three mechanisms. At the myofibrillar level Pi affects the myosin power-stroke, resulting in early crossbridge dissociation and promoting a transition from a strong to weak actin-myosin binding state. This results in a lower number of crossbridges in force-producing states, lower average force per crossbridge, and a reduced sensitivity of the contractile proteins to Ca2+, altogether impairing force output. Pi can also interfere with the Ca2+ signal itself. Pi can enhance Ca2+ release by activating the ryanodine receptor (RyR) via an active site on the sarcoplasm side of the SR. Pi may also diminish Ca2+ release by reducing the amount of Ca2+ available to be released from the SR. This could be accomplished by impairing Ca2+ reuptake by the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA), and/or by forming a precipitate with Ca2+ (CaPi) within the SR (Fryer et al. 1995). The focus of the paper by Ferreira et al. (2021,) recently published in The Journal of Physiology, aimed to elucidate the mechanism by which Pi enters the SR. Anion channels are considered the entry point for Pi into the SR and, based on prior speculation, Ferreira et al. (2021) tested the hypothesis that Pi enters the SR through chloride channels (ClC). To this end, they measured free Ca2+ concentration within the intact SR ([Ca2+]SR) of chemically permeabilized fibres of Rana catesbeiana and Rana pipiens semitendinosus muscles. Ca2+ spark frequency was used as an indirect indicator of [Ca2+]SR and assessed using Rhod-2 and Fluo-3 fluorescence at varying [Pi]. Direct measurements of [Ca2+]SR were made using the fluorescent dyes Mag-Indo-1 and Fluo-5N. They found that spark frequency was increased above baseline (0 mm Pi) when Pi concentrations ([Pi]) were below 10 mm, but as expected, further increases in [Pi] greatly reduced Ca2+ spark frequency and [Ca2+]SR. Next, experiments were repeated but with the introduction of 9-anthracenecarboxylic acid (9AC), a known ClC blocker. 9AC prevented the increase in spark frequency at sub-10 mm [Pi] and, as hypothesized, 9AC attenuated the decreases in spark frequency and [Ca2+]SR at higher [Pi]. The authors concluded that by using 9AC to block the entry of Pi, they prevented the precipitation of Ca2+ within the SR. This work is an exciting step forward in our understanding of fatigue processes and may have implications in scenarios of disease and ageing, which can exacerbate the effects of muscle fatigue. The putative pathway for fatigue mediated by CaPi precipitation in the SR is summarized in Fig. 1. While blocking ClCs indeed seems to inhibit CaPi precipitation in the SR at high [Pi], the physiological relevance of CaPi precipitation has not been fully established. It is notable that the frogs were housed at approximately the same temperature as that used in the single fibre experiments (∼20°C). As a result, there is clear potential for CaPi precipitation to contribute to fatigue in frog muscles under physiological conditions. The same cannot currently be said for mammalian muscle, as all studies examining SR CaPi precipitation that we are aware of have been performed at temperatures well below the physiological temperatures of mammalian muscle. As the specific CaPi salts formed within the SR have not been identified, warmer temperatures could enhance or diminish CaPi precipitation. For example, CaHPO4 becomes more soluble when temperature is increased, whereas CaHPO4.2H2O becomes less soluble as temperature is increased (Wang & Nancollas, 2008). Adding to the uncertainty, temperature could affect free [Ca2+]SR or the relationship between pH and CaPi solubility. Additionally, ClCs play an integral role in maintaining muscle cell excitability, and ClC blockade causes myotonia-like effects in well-rested muscle (Van Lunteren et al. 2011). It therefore seems prudent to clearly demonstrate that CaPi precipitation compromises muscle contractility under physiological fatiguing conditions for mammalian muscle before exploring pharmacological interventions. We must also consider what proportion of the fatigue response is attributable to the precipitation of CaPi in the SR, and whether it depends on the type and duration of muscle activity. Pi increases rapidly at the onset of muscle activity whereas sarcoplasmic [Ca2+] initially increases with muscle activity, before decreasing later in activity when force has dropped substantially (Allen et al. 2008). A reasonable explanation for this discrepancy is that adenosine nucleotides inhibit the opening of ClCs, with ATP having a greater effect than ADP, and ADP having a greater effect than AMP (Allen et al. 2008). This could limit passage of Pi into the SR until fatigue is well established. However, Ferreira et al.’s use of 5 mm ATP in their solutions suggests that Pi can enter the SR even in the well-rested state. However, any negative inotropic effect CaPi precipitation has early on in muscle activity seems to be overcome by factors which enhance Ca2+ release. The increase in spark frequency at [Pi] below 10 mm (Ferreira et al. 2021) is an illustration of this point. The increase in spark frequency at [Pi] below 10 mm and the decrease in spark frequency at [Pi] 10 mm and above are probably attributable to different mechanisms. Although Ferreira et al. (2021) present limited data for [Ca2+]SR at lower [Pi], the ∼150% increase in spark frequency reported at 5 mm is substantially greater than the 30% increase in [Ca2+]SR in fibre 011504c seen in Table 2. This departure from the proportionality of spark frequency and [Ca2+]SR is reconcilable with the Pi-mediated activation of the RyR complex (Allen et al. 2008). At higher [Pi] the decline in spark frequency appears to be caused by a reduction in the [Ca2+]SR via precipitation of Ca2+ with Pi inside the SR. The ClC blocker 9AC attenuated Pi-mediated changes in spark frequency. As 9AC caused a small increase in spark frequency on its own, and the other ClC blockers tested (4’4’-diisothiocyanatostilbene-2’2’-di-sulfonic acid and 4-acetamido-4’-isothiocyanato-stilbene-2,2’-disulfonic acid) caused large increases in spark frequency, ClC blockers could activate (or interact closely with) the Pi-binding site on the cytosolic side of the RyR complex in a way that prevents Pi from reaching or further activating this regulatory site. The experiments in Ferreira et al. clearly indicate that 9AC prevented Pi-induced declines in free [Ca2+]SR and CaPi precipitation in the SR. However, 9AC has effects distinct from inhibition of the ClC; this fact was carefully considered and acknowledged by Ferreira et al. (2021). Thus, channels or transporters aside from the ClC could be important conduits for Pi into the SR. As Pi-activation of the RyR did not prevent Pi-mediated inhibition of spark frequency or declines in free [Ca2+]SR at high [Pi], the putative activation of RyR by ClC blockers other than 9AC should not preclude the notion that these ClC blockers could mitigate the Pi-mediated inhibition of spark frequency or declines in [Ca2+]SR when [Pi] is high. As different ClC blockers could vary in effectiveness against different types of transporters and anion channels, an exploration of this variability could help determine the specific access point(s) for Pi into the SR. Ferreira et al. (2021) have provided compelling evidence that Ca2+ and Pi precipitate in the SR and that the passage of Pi into the SR is mitigated by the ClC blocker 9AC. However, since 9AC is not specific to the ClC, Pi could conceivably enter the SR via another 9AC-sensitive pathway. Finally, the significance of CaPi precipitation in fatigue remains unresolved for mammalian muscle under physiological conditions. No competing interests declared. All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. None.