Table 1 Summary of findings from studies investigating muscle oxygen saturation and energy rich phosphates

McCully et al.,1994 [22]

5, 36 ± 13, male

Two submaximal and two rapid (maximal) plantar flexion exercises were performed

Submaximal bouts: > 7.0, Maximal bouts: < 6.8

Two 5 min bouts and two 1 min bouts. First two bouts were performed with PCr depletion to 60% of resting values, then two bouts were performed till PCr values reached 5–20% of resting values.

Gastrocnemius and soleus muscle

Measured with a homebuilt spectrometer system (PCr; Pi); Runman (HbO₂)

Rate of recovery was slower for PCr than for HbO₂ in maximal exercise. The maximal PCrTc value (68.3 ± 10.5 s) was significantly higher than the submaximal PCrTc values (36 ± 6.5 s) and both the submaximal and maximal HbTc values (27.6 ± 6 and 29.4 ± 5.5 s).

Hamaoka et al., 1996 [26]

5,25–31, male

1) AO for the measurement of resting metabolic rates; 2) measurement of oxidative metabolism during isotonic hand-grip contractions; 3) AO was used at the end of exercise to distinguish between O₂ supply and O₂ demand

–

10, 20, 30, and 40% MVC. 5 min each intensity, 1 contraction every 4 s.

Flexor digitorum superficialis

Otsuka Electronics (PCr, Pi, ADP); HEO-100, OMRON Inc. (VO_2mus)

Averaged VO_2mus and ADP values during muscle contractions were correlated (r2 = .98, p > .01), VO_2mus and PCr were correlated (r2 = .99, p > .01).

Yoshida et al., 1996 [27]

6, 19 ± .4, male

Knee flexion exercises with active and passive recovery. Exercise test consisted of ramp test to obtain the relative work rate, submaximal repeated exercise for passive recovery and submaximal interval exercise for active recovery.

Low- and high-pH groups (no values).

Exercise intensity was selected as 60% of maximal exercise intensity attained in the ramp exercise test. 1) Ramp test: 1 min of resting measurement, exercise intensity was increased by 0.41 W every 15 s until volitional exhaustion. 2) Passive recovery test consisted of 2 min of exercise and 2 min passive recovery, 2 min exercise, followed by 5 min recovery. 3) Active recovery test consisted of 2 min unloaded warm-up, 2 min of exercise, followed by 2 min of unloaded exercise. Procedure was repeated, followed by a resting recovery period of 5 min.

Biceps femoris muscle

EX90, JEOL, Japan (PCr); Hamamatsu, NIR-4 s (Hb-Mb: Total hemoglobin/myoglobin)

PCr time constant during active recovery appeared a little slower than during passive recovery (30.2 and 32.1 vs. 25.3 and 25.9). Hb-Mb returned to baseline only during passive recovery (no values).

Binzoni et al., 1997 [28]

8, −, male

Three bouts of 5 min ischemia and 5 min reperfusion in a supine position.

The pH value decreased (.02 pH).

10 min each cycle. Full ischemia without movement was conducted.

SMIS, Guilford, UK (PCr); NIRO 500 Hamamatsu Photonics, Japan (Hb, HbO₂, Hb_tot)

No changes in PCr and ATP concentration were found. Hb, HbO₂ and Hb_tot changed during ischemic preconditioning.

Binzoni et al., 1998 [29]

50, 29.7 ± 4–37 ± 12, 20 females, 30 males

Group 1: 5 min measurement of resting baseline while the subject was seating; Group 2: 5 min of resting baseline, following a sequence of three cycles, each consisting of 5-min ischemia followed by 5-min reperfusion. Subject was lying supine; Group 3: Same protocol as for Group 2 with Na-NMRS. In addition, control NA-NMRS measurements were carried out in separate sessions before the intervention protocol.

Increase (0.02 ± .004) observed at the end of the exercise protocol and the baseline values.

Group 1: 5 min after an initial 5-min readjustment period; Group 2: 5 min resting, three cycles of 5-min ischemia and 5-min reperfusion; Group 3: As group 2. Additionally, Na-NMR spectra were acquired 10 min after the lying position was performed.

Gastrocnemius muscle

SMIS, Guilford, UK (PCr); NIRO 500 Hamamatsu Photonics, Japan (Hb, HbO₂, Hb_tot)

No concentration changes in PCr occur over time when contraction is not present. A constant de-crease in ∆HbO₂ is accompanied by an increase in ∆Hb. Hb_tot varies among subjects.

Boushel et al., 1998 [30]

7, 25–39, male

Rhythmic handgrip was performed at a cadence of 1 Hz.

The pH value ranged from 7.38 to 6.8 at 15% MVC to 6.38 at 30% MVC.

Experiment was conducted at 15 and 30% MVC. Exercise duration was 2 min. 5 s before the end of exercise, the occlusion cuff was inflated and maintained for 10 min. After releasing cuff pressure 20 min of recovery followed and the protocol was repeated at 30% MVC.

Forearm flexor muscle

Vivospec spectrometer, Otsuka Electronics and Magnex Scientific, Abingdon, UK (PCr); INVOS 3100, Somanetic Corporation, Troy, MI and NIRO 500, Hamamatsu, Japan (NIRS-O₂)

NIRS-O₂-derived rate of metabolism in forearm flexor muscles highly correlated with muscle metabolic rate evaluated by MRS-derived indices of ATP turnover. MRS-derived and NIRS-derived indices of muscle metabolism showed an approximate twofold increase at 15% MVC compared to 30% MVC.

Kutsuzawa et al., 2001 [31]

9, 29 ± 5, 1 female, 8 male,

Repetitive hand gripping of a lever at the rate of 20 grips per minute.

The pH value dropped from 7.05 ± .08 at rest to 6.85 ± .16 at the completion of the exercise phase

7% of the maximum grip strength of the non-dominant arm. 3 min exercise.

Forearm flexor muscle

BEM 250/80 and Otsuka Electronics Co., Osaka, Japan (PCr); OM-100A, Shimadzu Co. Kyoto, Japan (deoxy-Hb, oxy-Hb)

At rest: PCr/(PCr + Pi) .855 ± .025; at exercise end: PCr/(PCr + Pi) .582 ± .104; Deoxy-Hb increased in a monoexponential fashion and reached a plateau, oxy-Hb was a direct reflection of this behavior.

Sako et al., 2001 [32]

12, 25 ± 5, male

Two bouts rhythmic handgrip exercise were performed to measure independently P-MRS and NIRS. One session was conducted to measure resting metabolic rate.

The pH value was 7.02 ± .01 at rest and decreased to 6.7 ± .19 at the end of exercise. At 30 s after exercise pH dropped to 6.61 ± .3.

Experiment was conducted at 12% (n = 3),18% (n = 3), or 24% (n = 6) of MVC. Resting metabolic rate was measured during 15 min of arterial occlusion. Before each bout of exercise, subjects underwent arterial occlusion for 1 min followed by 2 min of resting period. Each exercise was conducted for 3 min

Forearm flexor muscles

Otsuka Electronic Co. Ltd., Osaka (PCr); OM-100A, Shimadzu (Oxyhemoglobin)

Muscle oxidative metabolic rate at 30 s postexercise was evaluated from the ratio of the rate of oxyhemoglobin decline during occlusion and the rate at rest. PCr resynthesis rate and NIRS-related muscle oxidative metabolic rate at 30 s after exercise correlated significantly (r = .965, p < .001).

Hamaoka et al., 2003 [33]

5, 24–36, male

Subjects performed submaximal grip exercise.

The pH value was not measured.

5 sec isometrics and 5 sec relaxation. After 1 min of resting arterial occlusion, 2 min of recovery followed, and thereafter exercise was performed for 3 min at 50% MVC.

Flexor digitorum superficialis muscle

Otsuka Electronics Inc. (PCr); HEO-200, OMRON Inc. Japan (Oxygenated, deoxygenated, and total hemoglobin)

Without AO similar kinetics between rate of deoxygenation and PCr concentration at the onset of exercise were observed. Correlation between the rate of deoxygenation and PCr concentration at the onset of exercise (r = .96, p < .05) were found.

Kime et al., 2003 [34]

7, 29 ± 3, male

Subjects performed a maximal isometric handgrip exercise for 10s and the arm was completely relaxed immediately after exercise.

Minimum intracellular pH 6.85.

Exercise duration was at 10 sec. Intensity was maximal. Reoxy-rate measurement was extended to 60s.

Otsuka Electronic Co. Ltd., Osaka (PCr); OM-100A, Shimadzu (Oxy-Hb)

Reoxy-rate and PCr time constant showed significant positive correlation (r2 = .939, p < .001).

Kime et al., 2003 [35]

7, 28 ± 3, male

Static hand grip exercise; initial 4 seconds was used for calculating reoxygenation rate

Minimum intracellular pH 6.84 ± .04.

10 seconds at 30, 60, and 90% MVC.

Otsuka Electronics (PCr); OM100A, Shimadzu, Kyoto, Japan (Oxy-Hb)

Reoxy-rate was correlated to the PCr time constant (r = 0.825, p < .05 at 90% MVC exercise). No significant results at 30 and 60% MVC exercises were found.

Nagasawa et al., 2003 [36]

8, 24–35, male

Two bouts of dynamic handgrip exercise with the same procedure to measure VO_2mus recovery after exercise with NIRS combined with arterial occlusion and to measure in a second session P-MRS in a magnet.

Minimum pH value during recovery in all subjects was above 6.95.

Duration was dependent of incremental test length. Intensity was increased by 5% of MVC every 2 min, starting from 10% MVC up to 20% MVC.

Flexor digitor-um superfi-cialis.

BEM 250/80; Otsuka Electronics Co., Osaka Japan (PCr); HEO200, Omron Inc., Japan (VO_2mus)

VO_2mus was based on decline rate of oxygenated hemoglobin. Time constant for the VO_2mus in recovery periods and the Tc for PCr recovery were 33.1 ± 9 and 35 ± 8.5 s. The Tc for muscle oxygen consumption recovery correlated significantly with the Tc for PCr recovery (r = .92, p < .01).

Forbes et al., 2006 [37]

7, 24 ± 4, male

Rest measurement, single-leg plantar flexion exercise, and recovery in control (PETCO₂ 33 mmHg) and hyperventilation (PETCO₂ conditions.

pH mean drop of .07

20 min rest, 6 min exercise, and 10 min recovery. Exercise was conducted at 75% of pH threshold.

Lateral gastrocnemius

IMRIS, Winnipeg, MB, Canada and Surrey Medical Imaging Systems, Guilford, UK (PCr); Wright Instruments, Enfield, Middlesex, UK (HHb, HbO₂, Hb_tot))

Hyperventilation-induced hypocapnia, compared to control condition, was associated with 1) a slower time course and greater breakdown in PCr during the transition to moderate-intensity exercise and 2) a similar time course of adaptation of HHb but greater increase in HHb signal during the exercise transition.

Kimura et al., 2006 [38]

7, 24–30, male

The measurements of 1) resting metabolic rate using the arterial occlusion method, 2) exercise under intramuscular condition, and 3) exercise under anaerobic condition were performed. An isometric wrist flexion was performed at a constant load.

No differences in pH at each %MVC between condition 2) and 3).

1) After 2 min of rest, forearm arterial blood flow was arrested by a pneumatic cuff. AO was maintained for 12 min at rest. 2) Under intramuscular condition, exercise was performed just after the beginning of the arterial occlusion. 3) For anaerobic exercise, 6 min of resting arterial occlusion was carried out before starting with the exercise protocol. Exercise was performed with 1 min for 30 and 50% MVC, and until exhaustion for 70% MVC.

Flexor digitorum superficialis

Otsuka Electronic Co. Ltd., Osaka (PCr); HEO-200, Omron Co. Ltd., Kyoto (HbO₂)

At 30% MVC, PCr breakdown of 2) was significant lower (p < .05) compared to 3). Oxidative metabolism using intramuscular O₂  above 50% MVC did not contribute to the decrease in PCr breakdown. The availability of O₂ at higher intensities was not enhanced in spite of the significantly increased ADP and adequate stored intramuscular O₂.

Jones et al., 2008 [39]

6, 28 ± 6, male

Single-legged knee-extension exercise in a prone position

Shift of pH in first exercise bout: 7.05 ± .01 to 6.89 ± .12. It recovered to 6.97 ± .06. Shift of pH in second bout to 6.84 ± .22. The pH value was significant higher in the third minute of the second bout compared to the first bout.

Two 6 min bouts of exercise with 6 min of resting recovery in between. 80% of the peak work rate.

Quadriceps

Intera, Philips, Eindhoven, The Netherlands (PCr); NIRO 300, Hamamatsu Photonics KK, Japan (O₂Hb, HHb, and Hb_tot)

O₂Hb and Hb_tot were significantly higher in the baseline period prior to the second compared with the first exercise bout. Priming exercise did not alter tau for PCr, indicating that muscle oxygen availability was not limiting the initial rate of PCr hydrolysis within the rest-to-exercise transient. PCr dynamics were not different between bouts (51 ± 8 s vs. 52 ± 17 s) but over the entire response were faster in the second bout (mean response time 72 ± 16 s vs. 57 ± 8 s), as a consequence of a greater decrease in PCr in the beginning phase and a reduction in the magnitude of the ‘slow component’ in PCr beyond 3 min of exercise (10 ± 6% vs. 5 ± 3%; p > .05).

Forbes et al., 2008 [40]

7, 24 ± 4, male

Three Single-leg plantar flexion exercise was performed. Dynamic contractions were performed at the rate of .5 Hz.

–

6 min of resting measurements, followed by two 6 min bouts of heavy-intensity exercise separated by either 3-,6-, or 15-min resting recovery. The order of the three experimental trials was randomized. Additionally, each condition was performed three times to improve the confidence of the parameter estimations for PCr kinetics. Work-rate was set between pH threshold and the maximum work-rate achieved in a previous ramp test.

Gastrocnemius

IMRIS, Winnipeg, MB, Canada and Surrey Medical Imaging Systems, Guilford, UK (PCr); Wright Instruments, Enfield, Middlesex, UK (HHb, Hb_tot)

Hb_tot was elevated after 3 min and the HHb onset kinetics were slowed in the bout after 3 min recovery. PCr slow component was similarly reduced in all subsequent bouts. O₂ delivery at exercise onset is not directly related to the time course of adaptation of PCr breakdown at the onset of subsequent bouts of heavy-intensity exercise.

Zange et al., 2008 [41]

20, 21 ± 3, male

3 min of plantar flexion isometric contraction at 40% of MVC, CON performed under an AO, 3 min of isometric contraction at 40% MVC superimposed by a 20 Hz VIB of the pedal, and VIB under AO.

At the end of isometric contraction, pH decreased by .09. Only under AO vibration caused a decrease in pH of .16.

1 min rest, 3 min exercise, 10 min recovery at rest, 3 min exercise, and further 10 min recovery at rest. Per examination only one period of VIB and one period of AO was performed. Intensity was set at 40% MVC.

Gastrocnemius

Bruker-Biospec 47/40, Bruker-Medical, Ettlingen, Germany (PCr, ATP); Oxymon, Artinis Medical Systems, Andelst, The Netherlands (HbO₂)

The average level of ATP did not change during exercise and recovery. After 3 min of exercise muscles had consumed in average 27% of their initial PCr. Under AO, contraction with VIB caused a larger decrease in PCr of 35%. Under AO, contraction plus VIB caused a significantly decrease in PCr compared to contraction without VIB. Natural perfusion did not change the decrease in PCr.

Layec et al., 2009 [42]

15, 32 ± 6, 1 female, 14 males

Two-leg knee-extension exercise

The pH value was 7.02–7.08 at the beginning of the exercise sessions and 6.84–6.9 at end of exercise.

Two 6 min bouts of exercise with 6 min of resting recovery in between. Exercises were performed at 35% MVC. If slow phase of oxygen uptake kinetics did not occur, intensity was increased.

Vastus lateralis

Siemens Vision Plus system, Siemens AG, Erlangen, Germany (PCr, ATP); NIRO-300, Hamamatsu Photonics, Hamamatsu City, Japan (HHb)

A reduced deoxygenation time delay and an increased deoxygenation amplitude coupled to an increased oxidative ATP cost in the early part of exercise was observed. ATP production from PCr breakdown and glycolysis were reduced. The priming exercise increased the PCr Tc within the fundamental phase of the response but did not alter the subsequent PCr slow component.

Willcocks et al., 2010 [43]

11, 24 ± 4, 5 female, 6 males

Two constant work rate exercise tests.

There was no significant change in pH in men (.06 ± .07, p = .11) or women (.03 ± .04, p = .18)

Two minutes of resting data collection and seven minutes of work. Work was performed at an intensity corresponding to 20% of the difference between the workload at the intracellular P/PCr threshold and the maximal workload.

Rectus femoris

Intera, Philips, The Netherlands (PCr); NIRO-300, Hamamatsu Photonics KK (HHb)

6 adults had an upward HHb, 4 maintained a steady-state plateau and 1 displayed a falling HHb whereas PCr decreased from rest to end-exercise by 56 and 44% in men and women, respectively.

Layec et al., 2012 [44]

13, 30 ± 6, male

Two-leg knee-extension exercise. After 2 min resting period, a series of ‘step’ exercise tests to the work rate which induces slow phase of oxygen uptake kinetics.

Delta pH was 0.45 in sedentary and − 0.69 in trained population

One 6 min bout at 35% MVC.

Vastus lateralis

Siemens Vision Plus system; Siemens AG, Erlangen, Germany (PCr); NIRO-300; Hamamatsu Photonics, Hamamatsu, Japan (HbO₂, HHb, Hb_tot)

HbO₂ increased progressively after 120 s. As a result of increased O₂ availability, the PCr slow component can be minimized in athletes.

Layec et al., 2013 [45]

9, 23, male

Constant-load submaximal plantar flexion at 40% of MVC either under conditions of free flow or reactive hyperemia, induced by a period of brief ischemia during the last minute of exercise.

No significant change in pH Value

After 2 min of rest, subjects exercised for 5 min at 40% of MVC followed by 5 min of recovery.

Calf muscle

Tim-Trio, Siemens Medical Solutions (PCr); Oxiplex TS, ISS (HHb, TOI)

A release of brief circulatory occlusion at the offset of plantar flexion exercise 1) led to a substantially increased convective O₂ delivery and capillary blood flow in muscle, 2) was associated with a faster tissue reoxygenation, and 3) increased muscle PCr synthesis and estimated peak mitochondrial respiration rates.

Ryan et al., 2013 [6]

16, 23 ± 3, 6 female, 10 males

A plantar flexion exercise was conducted, following 10–18 brief (5–10s) AO.

Minimum ph value during resting was 7.05 ± .03 and at end of exercise at 6.98 ± .06.

10 sec of plantar flexion was performed. Intensity was set with pneumatic resistance (psi) which allowed for a minimum of two contractions. Between bouts 5–7 min recovery period was added.

Gastrocnemius

GE Healthare, Waukesha, WI (PCr); Oxymon MKIII, Artinis Medical Systems (mVO₂)

Recovery kinetics of mVO₂ were not statistically different from and correlated well with recovery kinetics of PCr after short-duration exercise. Rate of recovery of mV measured by NIRS correlated well with the rate of recovery of PCR (Pearson’s r = .88, p < .001 for Channel 1 and Pearson’s r = .95, p < .001 for Channel 2).

Fulford et al., 2014 [46]

10, 19–25, 5 female, 5 male

Participants were asked to maintain their back extended in a prone position until the upper body fatigued.

The pH value was not measured.

First exercise was performed till fatigue, followed by 30 min break. Next five repetitions were performed for 24 s with a 216 s break in between. Participants hold upwards against gravity in a prone position.

Lumbar muscles

Intera, Philips, The Netherlands (PCr); NIRO-200, Hamamatsu Photonics KK (HHb, TOI)

During fatigue exercise PCr and TOI decreased whilst HHb increased. In the recovery period after 24 s of exercise, HHb levels decreased and PCr increased. The single measures reliability of the measurements of TOI and changes in TOI, PCr, and HHb ranged from good to excellent. The Tc had poor to fair reliability.

Hart et al., 2014 [47]

20, 22 ± 2, 10 female, 10 male

Constant-load submaximal plantar flexion

Decrease in pH from rest to end-exercise was 7.03 ± .08 to 6.86 ± .16

5 min exercise followed by 5 min recovery. 40% of individual maximum plantar flexion work-rate.

Gastrocnemius

Tim-Trio, Siemens Medical Solutions (PCr, ATP); Oxiplex TS, ISS (HHb)

PCr Tau: 33 ± 16 seconds; maximal ATP synthesis rate: 25 ± 9 mM/min; muscle reoxygenation rate: 48 ± 5 seconds.

Layec et al., 2014 [48]

20, 22 ± 2, 10 female, 10 male

Constant-load sub-maximal plantar flexion

pH dropped from 7.03 ± .08 at rest to 6.86 ± 16 at the end of exercise

After 1 min of data collection at rest, subjects exercised for 5 min followed by 5 min of recovery. Exercise intensity was set at 40% of MVC.

Gastrocnemius

Tim-Trio, Siemens Medical Solutions (PCr, ATP); Oxiplex TS, ISS (HHb)

At rest: PCr 32.6 ± 5.2 mM, ADP 9.5 ± 1.6 μM, TOI 68 ± 1%. At exercise end: PCr 18 ± 4 mM, ADP 43 ± 14 μM, TOI: 60 ± 8%

Bendahan et al., 2017 [49]

4, 37 ± 13, male

Protocol consisted of ramp, repeated isometric finger flexor performed at three different intensities (10, 20, 30% MVC). No resting period was performed. After a recovery period, a pressure cuff around the arm was rapidly inflated and was maintained for 6-min period.

Delta pH ranged from .02 ± .02 at 10% MVC to .21 ± .14 at 30% MVC.

Each step was performed in a 2-min trial and consisted of 80 total isometric contractions. An 8 min recovery followed the 3 exercise increments (10, 20, 30% MVC) Finally an ischemic period for 6 min was performed.

Superficial finger flexor

Bruker-Biospec 47/30, Bruker-Medical, Ettlingen, Germany (PCr, ATP); Oxymon, Artinis Medical Systems, Andelst, The Netherlands (HbO₂)

Postischemic returning O₂ does not impact any PCr recovery, since during the 6-min ischemia, PCr level has remained constant. During recovery, O₂ recovery rate also does not appear to limit PCr recovery rate. NIRS shows O₂ recovering with t1/2 of 0.4 min (24 s). PCr recovers after 30% MVC with a t1/2 of .27 min (16 s). Within the measurement errors, O₂ and PCr show matching recovery kinetics.

Yanagisawa et al., 2017 [50]

7, 22 ± 2, male

Low-load ankle plantar flexion exercise (120 repetitions, 30% of one-repetition maximum) with and without BFR (130% of systolic blood pressure)

There was no difference in pH between the control and BFR conditions in the pre-exercise values, but differences in pH between the two conditions were observed at 2–4 min during exercise (at 2 min, P < 0.05; at 3 and 4 min, P < 0.01).

120 repetitions at 30% MVC.

Gastrocnemius

Signa Excite XIV, GE Healthcare, Milwaukee, WI, USA (PCr); HEM7420, OMRON, Kyoto, Japan (TOI)

TOI of both exercise conditions decreased during each exercise. BFR condition showed lower O₂Hb and higher HHb values during exercise compared with the control condition. BFR condition showed greater Pi/PCr ratios during exercise than the control condition. In total, low-load BFR exercise stressed intramuscular high-energy phosphate metabolism greatly and created greater hypoxic and acidic environments within the exercising muscle in comparison with low-load non-restricted exercise.

Heskamp et al., 2020 [51]

20, 18–35, male

15 subjects performed continuous ankle dorsiflexion. 5 subjects performed intermitted isometric ankle dorsiflexion.

At the end of exercise pH range from 6.87–6.75

Continuous ankle dorsiflexion was performed at 30% MVC until exhaustion (2–5 min). Intermittent isometric ankle dorsiflexion started at 10% MVC and step wisely increased by 10% MVC every 30 sec until exhaustion (60–70% MVC).

Tibialis anterior

Sauter FL 500, Balingen, Germany (PCr); OxyMon MK III, Artinis Medical System, Elst, The Netherlands (O₂Hb).

Following continuous isometric ankle dorsiflexion at 30% MVC until exhaustion, the recovery rate constants of O₂Hb and of PCr, and the absolute rate of PCr recovery increased from distal to proximal along the length of the tibialis anterior. As PCr resynthesis is proportional to the oxygen-dependent suprabasal ATP synthesis, and as additionally Hb recovers faster proximally than distally, this indicates that O₂ supply also exhibits a proximo-distal gradient with the highest O₂ supply proximally. Correlation analysis revealed that PCr recovery constant correlated strongly with O₂Hb recovery constant (r = .956, p = .011).