Muscle pain induced by hypertonic saline in the knee extensors decreases single-limb isometric time to task failure

Abstract

Purpose Increased nociceptive activity and the experience of exercise-induced pain (EIP) may contribute to fatigue during endurance exercise. To investigate this, a pain model that produces pain similar to EIP and decouples its relationship to exercise intensity is required. This study (1) compared the quality of pain caused by a hypertonic saline injection into the vastus lateralis in resting and exercise conditions, and (2) investigated whether this pain contributes to changes in time to task failure. Methods On separate days, 18 participants completed a time to task failure at 20% maximal voluntary torque (MVT), a resting hypertonic saline intramuscular injection, and in a further three visits a time to task failure at 10% MVT following injection of isotonic saline, hypertonic saline or a control (no injection). Results In a subset of eligible participants (n = 12), the hypertonic saline combined with 10% MVT produced a qualitative experience of pain (assessed by the McGill Pain Questionnaire) that felt similar to EIP. 10% MVT with hypertonic saline significantly elevated pain intensity in the first 20% of the time to task failure and caused a significantly (P < 0.05) shorter time to task failure (448 ± 240 s) compared with the isotonic saline (605 ± 285 s) and control (514 ± 197 s) conditions. Conclusion These findings demonstrate that hypertonic saline increases the intensity of pain during exercise, which results in a faster occurrence of exercise-induced fatigue. These results provide important evidence supporting pain as a limiting factor in endurance performance.

Acute Quadriceps Muscle Pain Reduces Isometric Endurance Performance of the Contralateral Limb

“…This was however observed with a greater pain intensity than achieved in this study (peak pain 8.5/10 versus 5.6 respectively) Whilst it cannot be ruled out that pain may reduce contralateral muscle strength, the findings of the present study do not suggest that a reduction in maximal force generating capacity (and the consequential increased relative intensity of the endurance task) was responsible for the shorter TTF. This is in contrast with localised muscle pain, which does reduce maximal force generating capacity and then the relative demands of the task (Graven-Nielsen et al, 1997;Smith et al, 2020).…”

“…Visit two comprised of a second familiarisation of the isometric exercise task where the intensity (% maximum voluntary contraction) was adjusted from the first visit if the TTF was lower than four minutes or greater than six minutes. This was to ensure that task failure coincided with the typical pain duration from the intramuscular injection of hypertonic saline (Smith et al, 2020). Visits 3 and 4 comprised of the two experimental visits (see figure 1).…”

“…Therefore, the two may be interrelated and consequently EIP may in part be responsible for the development of fatigue (Mauger, 2013) and be determinantal to endurance performance (Mauger et al, 2010;Astokorki & Mauger, 2017;Stevens et al, 2018). Support for this notion comes from previous work has found that muscle pain reduces the maximal force generating capacity of the muscle (Graven-Nielsen et al, 1997, 2002Khan et al, 2011) and impairs endurance performance (Ciubotariu et al, 2004;Smith et al, 2020). This effect appears to be driven by an exacerbation of central fatigue (i.e., effect spinal and supraspinal areas) (Le Pera et al, 2001;Graven-Nielsen et al, 2002;Khan et al, 2011), with little or no contributing effect on the peripheral apparatus of the muscle (peripheral fatigue) (Farina et al, 2005).…”

“…During intense exercise, muscle pain is present and is caused by an accumulation noxious biochemicals such as inorganic phosphate, lactate and hydrogen ions (H + ) (Mense, 2009) along with an increase in intramuscular pressure (O'Connor & Cook, 1999). This sensation is referred to as exercise-induced pain (EIP), which increases as a function of exercise intensity (Cook et al, 1997) and time (Smith et al, 2020).…”

“…The use of intramuscular injections of hypertonic saline have previously been used to cause acute muscle pain (Graven-Nielsen et al, 2002;Khan et al, 2011;Smith et al, 2020Smith et al, , 2021. For the purpose of exploring the relationship between pain and fatigue, this technique may be advantageous to ischaemia as ischaemia traps blood in the occluded limb and lowers O2 availability.…”