Isometric Plantar Flexion Exercises Research Articles

A 10-mg dose of amiloride increases time to failure during blood-flow-restricted plantar flexion in healthy adults without influencing blood pressure.

Amiloride has been shown to inhibit acid-sensing ion channels (ASICs), which contribute to ischemia-related muscle pain during exercise. The purpose of this study was to determine if a single oral dose of amiloride would improve exercise tolerance and attenuate blood pressure during blood-flow-restricted (BFR) exercise in healthy adults. Ten subjects (4 females) performed isometric plantar flexion exercise with BFR (30% maximal voluntary contraction) after ingesting either a 10-mg dose of amiloride or a volume-matched placebo (random order). Time to failure, time-tension index (TTI), and perceived pain (visual analog scale) were compared between the amiloride and placebo trials. Mean blood pressure, heart rate, blood pressure index (BPI), and BPI normalized to TTI (BPInorm) were also compared between trials using both time-matched (TM50 and TM100) and effort-matched (T50 and T100) comparisons. Time to failure (+69.4 ± 63.2 s, P < 0.01) and TTI (+1,441 ± 633 kg·s, P = 0.02) were both significantly increased in the amiloride trial compared with placebo, despite no increase in pain (+0.4 ± 1.7 cm, P = 0.46). In contrast, amiloride had no significant influence on the mean blood pressure or heart rate responses, nor were there any significant differences in BPI or BPInorm between trials when matched for time (all P ≥ 0.13). When matched for effort, BPI was significantly greater in the amiloride trial (+5,300 ± 1,798 mmHg·s, P = 0.01), likely owing to an increase in total exercise duration. In conclusion, a 10-mg oral dose of amiloride appears to significantly improve the tolerance to BFR exercise in healthy adults without influencing blood pressure responses.

Toward a Better Understanding of Muscle Microvascular Perfusion During Exercise in Patients With Peripheral Artery Disease: The Effect of Lower-Limb Revascularization

Purpose: Leg muscle microvascular blood flow (perfusion) is impaired in response to maximal exercise in patients with peripheral artery disease (PAD); however, during submaximal exercise, microvascular perfusion is maintained due to a greater increase in microvascular blood volume compared with that seen in healthy adults. It is unclear whether this submaximal exercise response reflects a microvascular impairment, or whether it is a compensatory response for the limited conduit artery flow in PAD. Therefore, to clarify the role of conduit artery blood flow, we compared whole-limb blood flow and skeletal muscle microvascular perfusion responses with exercise in patients with PAD (n=9; 60±7 years) prior to, and following, lower-limb endovascular revascularization. Materials and Methods: Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after a 5 minute bout of submaximal intermittent isometric plantar-flexion exercise using contrast-enhanced ultrasound imaging. Exercise contraction-by-contraction whole-leg blood flow and vascular conductance were measured using strain-gauge plethysmography. Results: With revascularization there was a significant increase in whole-leg blood flow and conductance during exercise (p<0.05). Exercise-induced muscle microvascular perfusion response did not change with revascularization (pre-revascularization: 3.19±2.32; post-revascularization: 3.89±1.67 aU.s−1; p=0.38). However, the parameters that determine microvascular perfusion changed, with a reduction in the microvascular volume response to exercise (pre-revascularization: 6.76±3.56; post-revascularization: 2.42±0.69 aU; p<0.01) and an increase in microvascular flow velocity (pre-revascularization: 0.25±0.13; post-revascularization: 0.59±0.25 s−1; p=0.02). Conclusion: These findings suggest that patients with PAD compensate for the conduit artery blood flow impairment with an increase in microvascular blood volume to maintain muscle perfusion during submaximal exercise. Clinical Impact The findings from this study support the notion that the impairment in conduit artery blood flow in patients with PAD leads to compensatory changes in microvascular blood volume and flow velocity to maintain muscle microvascular perfusion during submaximal leg exercise. Moreover, this study demonstrates that these microvascular changes are reversed and become normalized with successful lower-limb endovascular revascularization.

Immediate Effects of two Isometric Calf Muscle Exercises on Mid-portion Achilles Tendon Pain

The objectives of this randomized, cross-over pilot study were to determine whether isometric plantarflexion exercises resulted in an immediate change in Achilles tendon pain during a loading task, and whether this differed in knee extension or flexion. Eleven participants with mid-portion Achilles tendinopathy were recruited from NHS community physiotherapy services and local running clubs. Participants were then randomized to complete an isometric calf muscle exercise with the knee fully extended or flexed to 80°. Participants switched to the alternate exercise after a minimum seven-day period. Achilles tendon pain during a specific, functional load test was measured on a 11-point numeric pain rating scale (NPRS) pre- and post-intervention. There was a small, immediate, mean reduction in pain following isometric plantar flexion performed in both knee extension (1.6, 95%CI 0.83 to 2.45, p=0.001) and knee flexion (1.3, 95%CI 0.31 to 2.19, p=0.015). There were no significant differences between the two positions. A non-significant, potentially clinically relevant finding was a 20% larger reduction in symptoms in knee extension versus flexion (p=0.110). In conclusion, isometric plantarflexion holds gave an approximately 50% immediate reduction in Achilles tendon pain with a functional load test. There were no significant differences between the two positions and both were well tolerated.

Diffusion Tensor Imaging of the Calf Muscles in Subjects With and Without Diabetes Mellitus.

Diffusion tensor imaging (DTI) has been used to characterize calf skeletal muscle architecture. To assess the diffusional properties of the calf muscles of subjects with and without diabetes, at rest and during isometric plantarflexion exercise. Prospective. Twenty-six subjects in two groups: 13 healthy and 13 subjects with type 2 diabetes (DM); each group consisted of seven females and six males. 3T/2D single-shot spin echo planar imaging. Fractional anisotropy (FA), mean diffusivity (MD), diffusion eigenvalues, and fiber tracking indices were obtained from the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) muscles of the calf at rest and during isometric plantarflexion exercise. We used a combination of nonparametric (Wilcoxon) and parametric (t-test) statistical assessments. The medial gastrocnemius muscle had more indices with significant differences between the two groups (six indices with P < 0.05) than did the lateral gastrocnemius (three indices with P < 0.05) and soleus muscles (only one index with P < 0.05). While the healthy group showed elevated MD values from rest to exercise (MG = 5.83%, LG = 13.45%, and SOL = 11.68%), the diabetic MD showed higher increases (MG = 19.74%, LG = 29.31%, and SOL = 20.84%) that were different between groups (MG: P = 0.009, LG: P = 0.037, and SOL: P = 0.049). Our results indicate considerable diffusional changes between healthy subjects and subjects with diabetes at rest and during isometric plantarflexion exercise in the calf muscles. The medial gastrocnemius muscle displayed the most diffusion sensitivity to diabetes-related microstructural changes. 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:1285-1295.

Leg blood flow and skeletal muscle microvascular perfusion responses to submaximal exercise in peripheral arterial disease.

Peripheral arterial disease (PAD) is characterized by stenosis and occlusion of the lower limb arteries. Although leg blood flow is limited in PAD, it remains unclear whether skeletal muscle microvascular perfusion is affected. We compared whole leg blood flow and calf muscle microvascular perfusion after cuff occlusion and submaximal leg exercise between patients with PAD ( n = 12, 69 ± 9 yr) and healthy age-matched control participants ( n = 12, 68 ± 7 yr). Microvascular blood flow (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after the following: 1) 5 min of thigh-cuff occlusion, and 2) a 5-min bout of intermittent isometric plantar-flexion exercise (400 N) using real-time contrast-enhanced ultrasound. Whole leg blood flow was measured after thigh-cuff occlusion and during submaximal plantar-flexion exercise using strain-gauge plethysmography. Postocclusion whole leg blood flow and calf muscle microvascular perfusion were lower in patients with PAD than control participants, and these parameters were strongly correlated ( r = 0.84, P < 0.01). During submaximal exercise, total whole leg blood flow and vascular conductance were not different between groups. There were also no group differences in postexercise calf muscle microvascular perfusion, although microvascular blood volume was higher in patients with PAD than control participants (12.41 ± 6.98 vs. 6.34 ± 4.98 arbitrary units, P = 0.03). This study demonstrates that the impaired muscle perfusion of patients with PAD during postocclusion hyperemia is strongly correlated with disease severity and is likely mainly determined by the limited conduit artery flow. In response to submaximal leg exercise, microvascular flow volume was elevated in patients with PAD, which may reflect a compensatory mechanism to maintain muscle perfusion and oxygen delivery during recovery from exercise. NEW & NOTEWORTHY This study suggests that peripheral arterial disease (PAD) has different effects on the microvascular perfusion responses to cuff occlusion and submaximal leg exercise. Patients with PAD have impaired microvascular perfusion after cuff occlusion, similar to that previously reported after maximal exercise. In response to submaximal exercise, however, the microvascular flow volume response was elevated in patients with PAD compared with control. This finding may reflect a compensatory mechanism to maintain perfusion and oxygen delivery during recovery from exercise.

Magnetic Resonance Diffusion Tensor Imaging of the Human Calf in Subjects With and Without Type 2 Diabetes during Rest and Isometric Plantarflexion Exercise

Aim: This project aims to study differences in diffusion properties of the human calf muscle in subjects with and without type 2 diabetes mellitus (DM) using diffusional properties of water molecule. Methods: Twenty-one subjects were examined in a 3T MRI scanner after obtaining consents. Two groups were tested: 8 participants without DM (WDM) and 13 participants with DM. We used a custom-made MRI compatible ergometer that allows a constant exercise load. Finally, we calculated diffusion indices including fractional anisotropy (FA), mean diffusivity (MD), and eigenvalues (λ1, λ2, λ3) for further analysis. Results: Statistical analysis revealed no significant differences (P&amp;gt;0.05) in diffusion indices of muscles between WDM and DM at the rest position. However, all the muscles in DM showed (Figure) an overall statistically significant increase (P&amp;lt;0.05) in MD, λ2, and λ3 in response to exercise, in comparison with the WDM (MD= 15.02%; λ2= 16.50%; λ3= 14.22% for the MG). Conclusion: There were considerable changes in diffusion indices between WDM and DM groups during the calf exercise. This study demonstrated that structural muscle damage due to the diabetes can be readily distinguished from nondiabetes using DTI. Disclosure M. Edalati: None. C.J. Sorensen: None. M. Hastings: None. M.A. Zayed: None. M.J. Mueller: None. J. Zheng: None.

Enhanced Local Skeletal Muscle Oxidative Capacity and Microvascular Blood Flow Following 7-Day Ischemic Preconditioning in Healthy Humans.

Ischemic preconditioning (IPC), which involves intermittent periods of ischemia followed by reperfusion, is an effective clinical intervention that reduces the risk of myocardial injury and confers ischemic tolerance to skeletal muscle. Repeated bouts of IPC have been shown to stimulate long-term changes vascular function, however, it is unclear what metabolic adaptations may occur locally in the muscle. Therefore, we investigated 7 days of bilateral lower limb IPC (4 × 5 min) above limb occlusion pressure (220 mmHg; n = 10), or sham (20 mmHg; n = 10), on local muscle oxidative capacity and microvascular blood flow. Oxidative capacity was measured using near-infrared spectroscopy (NIRS) during repeated short duration arterial occlusions (300 mmHg). Microvascular blood flow was assessed during the recovery from submaximal isometric plantar flexion exercises at 40 and 60% of maximal voluntary contraction (MVC). Following the intervention period, beyond the late phase of protection (72 h), muscle oxidative recovery kinetics were speeded by 13% (rate constant pre 2.89 ± 0.47 min-1 vs. post 3.32 ± 0.69 min-1; P < 0.05) and resting muscle oxygen consumption (mO2) was reduced by 16.4% (pre 0.39 ± 0.16%.s-1 vs. post 0.33 ± 0.14%.s-1; P < 0.05). During exercise, changes in deoxygenated hemoglobin (HHb) from rest to steady state were reduced at 40 and 60% MVC (16 and 12%, respectively, P < 0.05) despite similar measures of total hemoglobin (tHb). At the cessation of exercise, the time constant for recovery in oxygenated hemoglobin (O2Hb) was accelerated at 40 and 60% MVC (by 33 and 43%, respectively) suggesting enhanced reoxygenation in the muscle. No changes were reported for systemic measures of resting heart rate or blood pressure. In conclusion, repeated bouts of IPC over 7 consecutive days increased skeletal muscle oxidative capacity and microvascular muscle blood flow. These findings are consistent with enhanced mitochondrial and vascular function following repeated IPC and may be of clinical or sporting interest to enhance or offset reductions in muscle oxidative capacity.

Effects of muscle fiber motion on diffuse correlation spectroscopy blood flow measurements during exercise

The influence of muscle fiber motion during exercise on diffuse correlation spectroscopy (DCS) measurements of skeletal muscle blood flow is explored. Isotonic (with muscle fiber motion) and isometric (without muscle fiber motion) plantar flexion exercises were performed at 30% of maximal force on a dynamometer, and muscle blood flow was continuously monitored on the medial gastrocnemius (calf) muscle of a healthy volunteer using DCS. During exercise, dynamometer recordings including footplate position, footplate angular velocity, and plantar flexion torque were obtained. Muscle fiber motions introduced artifacts into the DCS signals, causing an overestimation of blood flow changes. We show how proper co-registration of dynamometer recordings and DCS measurements enables separation of the true blood flow responses during exercise from those affected by the motion artifacts.

The blood flow and fatigue response to isometric plantar-flexion exercise in patients with peripheral arterial disease

Introduction: The aim of the present study was to assess the relationship between blood flow and fatigue during lower limb plantar-flexion exercise in peripheral arterial disease (PAD) and age matched control subjects. Methods: Subjects performed isometric plantar-flexion exercise tests. The protocol commenced with a series of maximum voluntary contractions (MVC) to determine muscle strength. This was followed by a continuous bout of intermittent submaximal exercise (12 contractions/min) to determine muscle endurance.

Energy metabolism in human calf muscle performing isometric plantar flexion superimposed by 20-Hz vibration

Vibration training is commonly expected to induce an active muscle contraction via a complex reflex mechanism. In calf muscles of 20 untrained subjects, the additional energy consumption in response to vibration superimposed on an isometric contraction was examined by (31)P magnetic resonance spectroscopy and by near infrared spectroscopy. Subjects performed 3 min of isometric plantar flexion exercise at 40% MVC under four conditions: with (VIB) and without (CON) superimposed 20 Hz vibration at +/-2 mm amplitude, both combined with or without arterial occlusion (AO). After contraction under all conditions, the decreases in oxygenated haemoglobin were not significantly different. After VIB + AO consumption of ATP was increased by 60% over CON + AO, visible by significant decreases in [PCr] and intracellular pH (P < 0.05). The additional energy consumption by vibration was not detectable under natural perfusion. Probably without AO the additional energy consumption by vibration was compensated by oxidative phosphorylation enabled by additional perfusion.

Noninvasive measures of blood volume and oxygen saturation of human Achilles tendon by red laser lights

Spectroscopic measurement using near-infrared spectroscopy and red laser lights was performed to investigate the blood volume (total haemoglobin; THb) and oxygen saturation (oxygenated haemoglobin saturation; StO2) of the human Achilles tendon during single and repetitive isometric contractions. Twelve men performed isometric plantar flexion exercises at intensities of 10% (3 min), 30% (2 min), 50% (1 min) and 70% (30 s) of the maximum voluntary contraction (MVC). In addition, they performed 50 repetitions at 70% of the MVC with 5-s contractions and a 5-s gap between repetitions for 10 min. The probes were positioned on the Achilles tendon and medial gastrocnemius muscle. THb and StO2 of the Achilles tendon decreased during 'single' contraction at all force levels. On the other hand, after the end of the repetitive exercises, THb and StO2 of the Achilles tendon increased gradually, and remained at this level until the end of the recovery period. The blood volume and oxygen saturation within the human Achilles tendon increased after the 'repetitive' muscle contractions, while they decreased after the 'single' contraction. Furthermore, the increases in blood volume and oxygen saturation within the tendon after repetitive contractions may be related to tendon repair after exercises involving mechanical loading of the tendon.

351 Strength exercise results in an acute decrease in achilles tendon thickness

observed a 27% decrease in the Achilles tendon stiffness immediately following five minutes of isometric plantarflexion exercise and hypothesized that post-exercise remodelling resulted in an initial transient period of mechanical weakness. While the study did not include measures of tendon cross-section to support their claim, a recent investigation employing magnetic resonance imaging (MRI) has shown an increase in Achilles tendon volume (12%-17%) and intratendinous signal 30 minutes after exercise.

Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BackgroundIt has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption (o2mus) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise.MethodsFive male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by 31P-magnetic resonance spectroscopy. o2mus was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake (o2pul ) was measured by the breath-by-breath gas analysis.ResultsEMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, o2mus, and o2pul did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, o2mus, and o2pul increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, o2mus, and o2pul accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, o2mus, and o2pul were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001).ConclusionWith decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.

MUSCLE ENERGETICS AND PULMONARY OXYGEN UPTAKE DURING INCREMENTAL ISOMETRIC PLANTAR FLEXION EXERCISE

The purpose of this study was to clarify the relationship between muscle energetics and pulmonary oxygen uptake (VO2) during incremental exercise. Five male subjects performed a repetitive intermittent (6-s contraction / 4-s relaxation) isometric plantar flexion exercise. Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC) starting from 10% MVC to the intensity where the subject could no longer maintain the prescribed force. The measurement site was at the medial gastrocnemius muscle. Changes in phosphocreatine (PCr), inorganic phosphate (Pi) and pH in the muscle were measured by 31P-magnetic resonance spectroscopy. The muscle oxygen kinetics was measured by near infrared spectroscopy. Also, the muscle oxygen consumption was determined from the rate of deoxygenation during transient arterial occlusion (Hamaoka et al. J. Appl. Physiol. 1996). The electromyogram (EMG) was also recorded. Pulmonary VO2 was measured by the breath-by-breath gas analysis method. During exercise below 30% MVC, although the EMG amplitude increased and muscle oxygenation gradually decreased, the muscle PCr and oxygen consumption did not change significantly. However, an exercise above 30% MVC muscle PCr decreased and oxygen consumption increased with exercise intensity. Pulmonary VO2 also changed slightly, and no significant difference between exercise intensity was observed below 40% MVC. When exercise intensity was raised above 40% MVC, pulmonary VO2 increased with exercise intensity. There was a significant linear correlation between muscle PCr and muscle oxygen consumption (r = 0.975, p < 0.0001), muscle oxygen consumption and pulmonary VO2 (r = 0.962, p < 0.0001), and muscle PCr and pulmonary VO2 (r = 0.977, p < 0.0001). Furthermore, the decrease in PCr and increase in muscle oxygen consumption preceded the increase in pulmonary VO2. These results suggest that the breakdown of intramuscular PCr is a factor to stimulate muscle oxygen consumption resulting in the increase in pulmonary VO2. The results of this study indicated that muscle PCr, muscle oxygen consumption, and pulmonary VO2 are closely related during an incremental plantar flexion exercise.

31P MRS measurement of mitochondrial function in skeletal muscle: reliability, force-level sensitivity and relation to whole body maximal oxygen uptake.

The reliability, relation to whole-body maximal oxygen uptake (VO(2max)), and force-level sensitivity of (31)P MRS markers of mitochondrial function were studied in 39 normal-weight women. Following 90 s isometric plantar-flexion exercises at 45, 70 and 100% of maximum voluntary contraction, skeletal muscle mitochondrial function was determined from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). VO(2max) was measured on a treadmill. Test-retest measurements were obtained in a subset of seven women. Overall, TC(PCr), TC(ADP) and OxPhos were reproducible for all exercises (coefficients of variation = 2.3-19.3%). With increasing force-level, TC(PCr) was prolonged (29.0 +/- 8.2, 31.9 +/- 9.0 and 35.4 +/- 9.5 s), OxPhos was increased (0.159 +/- 0.081, 0.247 +/- 0.090 and 0.310 +/- 0.114), and TC(ADP) was shortened (22.4 +/- 7.9, 21.3 +/- 6.2, and 19.5 +/- 6.7; p < 0.01). All MRS markers of mitochondrial function were correlated with VO(2max) (r = 0.41-0.72; p < 0.05). These results suggest that measurements of TC(PCr), TC(ADP) and OxPhos yield reproducible results that correlate with whole-body VO(2max) and that vary in force-level sensitivity.

Muscle Injury's Effects on Energy Metabolism in a Trained Individual

182 Using 31P magnetic resonance spectroscopy (MRS) we have recently shown that acute increases in volume and/or intensity of exercise have elicited long-term effects on the ability of muscle to generate ATP from oxidative phosphorylation (OxPhos). OxPhos was measured from the initial PCr recovery rate during the first 14-seconds of recovery following 90-seconds isometric plantar flexion exercises using 31P MRS. The following (figure) shows fluctuations in OxPhos consequent to three acute increases in volume/intensity of exercise that produced delayed onset muscle soreness (EXMS). OxPhos was decreased to less than 50% of pre-injury stress values even after 6 weeks post injury. Furthermore, it took over 14-weeks to recover to pre-EXMS levels. The reductions in OxPhos were also accompanied by an increased rate of fatigue and indicators of exercise difficulty. These decreases in OxPhos can not be attributed to random fluctuations. Our lab has previously shown that these types of measurements of OxPhos have a test-retest reliability of 5.6% CV and/or an SEP of 0.033 mM/sec. In conclusion, our initial results indicate that our method of measuring OxPhos may be a very specific marker of compromised mitochondria and/or muscle capillary bed function.FigureSupported by NIH NCRR RR 11811

Adenosine triphosphate production rates, metabolic economy calculations, pH, phosphomonoesters, phosphodiesters, and force output during short-duration maximal isometric plantar flexion exercises and repeated maximal isometric plantar flexion exercises.

Measurements of adenosine triphosphate (ATP) production rates, metabolic economy, intracellular pH, phosphodiesters, and phosphomonoesters along with the force output were used to study 90-s maximum voluntary contractions and two new exercise protocols (20-10 and 30-16 exercises). The 20-10 exercise consisted of thirty-one 20-s maximal voluntary contractions separated by 10-s rest periods. The 30-16 exercise consisted of twenty 30-s maximal voluntary contractions separated by 16-s rest periods. There were no differences in ATP production rates, metabolic economy, intracellular pH, or force output between the 20-10 and 30-16 exercises. The 20-10 exercises accumulated more phosphomonoesters than the 30-16 exercises. These increases in phosphomonoesters may be attributed to increased accumulations of glucose-6-phosphate and/or inosine monophosphate. The increased perception of effort reported during and after the 20-10 exercises was not present during the 30-16 or 90-s exercises. This increased perception of effort may be related to increases in lactate, glucose-6-phosphate, inosine monophosphate, and/or NH3.

Adenosine triphosphate production rates, metabolic economy calculations, pH, phosphomonoesters, phosphodiesters, and force output during short‐duration maximal isometric plantar flexion exercises and repeated maximal isometric plantar flexion exercises

Adenosine triphosphate production rates, metabolic economy calculations, pH, phosphomonoesters, phosphodiesters, and force output during short‐duration maximal isometric plantar flexion exercises and repeated maximal isometric plantar flexion exercises