Effects of exhaustive stretch-shortening cycle exercise on muscle blood flow during exercise

The Effect of High-Intensity Interval Training Type on Body Fat Percentage, Fat and Fat-Free Mass: A Systematic Review and Meta-Analysis of Randomized Clinical Trials

This systematic review and meta-analysis of randomized controlled trials (RCTs) compared body compositional changes, including fat mass (FM), body fat percentage (BF%), and fat-free mass (FFM), between different types of high-intensity interval training (HIIT) (cycling vs. overground running vs. treadmill running) as well as to a control (i.e., no exercise) condition. Meta-analyses were carried out using a random-effects model. The I2 index was used to assess the heterogeneity of RCTs. Thirty-six RCTs lasting between 3 to 15 weeks were included in the current systematic review and meta-analysis. RCTs that examined the effect of HIIT type on FM, BF%, and FFM were sourced from online databases including PubMed, Scopus, Web of Science, and Google Scholar up to 21 June 2022. HIIT (all modalities combined) induced a significant reduction in FM (weighted mean difference [WMD]: −1.86 kg, 95% CI: −2.55 to −1.18, p = 0.001) despite a medium between-study heterogeneity (I2 = 63.3, p = 0.001). Subgroup analyses revealed cycling and overground running reduced FM (WMD: −1.72 kg, 95% CI: −2.41 to −1.30, p = 0.001 and WMD: −4.25 kg, 95% CI: −5.90 to −2.61, p = 0.001, respectively); however, there was no change with treadmill running (WMD: −1.10 kg, 95% CI: −2.82 to 0.62, p = 0.210). There was a significant reduction in BF% with HIIT (all modalities combined) compared to control (WMD: −1.53%, 95% CI: −2.13, −0.92, p = 0.001). All forms of HIIT also decreased BF%; however, overground running induced the largest overall effect (WMD: −2.80%, 95% CI: −3.89 to −1.71, p = 0.001). All types of HIIT combined also induced an overall significant improvement in FFM (WMD: 0.51 kg, 95% CI: 0.06 to 0.95, p = 0.025); however, only cycling interventions resulted in a significant increase in FFM compared to other exercise modalities (WMD: 0.63 kg, 95% CI: 0.17 to 1.09, p = 0.007). Additional subgroup analyses suggest that training for more than 8 weeks, at least 3 sessions per week, with work intervals less than 60 s duration and separated by ≤90 s active recovery are more effective for eliciting favorable body composition changes. Results from this meta-analysis demonstrate favorable body composition outcomes following HIIT (all modalities combined) with overall reductions in BF% and FM and improved FFM observed. Overall, cycling-based HIIT may confer the greatest effects on body composition due to its ability to reduce BF% and FM while increasing FFM.

Relationships between resting blood flow and the indices of muscle damage after eccentric contractions

PURPOSE

The aim of the study is to examine the relationships between increments in resting blood flow and isometric maximal voluntary contraction (MVC) force reduction, muscle soreness, and swelling after eccentric contractions (ECs).

METHODS

Twenty-one young healthy men (age 20.8 ± 1.6 years; height 172.0 ± 5.3 cm; weight 64.9 ± 7.7 kg) were recruited for this study. All participants performed right arm ECs in five sets of 20 repetitions with 3 min of rest between the sets. The dumbbell weight corresponded to 60% MVC force of isometric contraction of elbow flexors with 90° elbow joint angle. Resting forearm blood flow (FBF), the MVC force, the muscle thickness (MT), and muscle soreness of elbow flexors, heart rate (HR), and blood pressure (BP) of brachial artery were measured before, 24 and 48 h after ECs.

RESULTS

Average and peak resting FBF after ECs significantly changed from the average values before ECs (21% and 39% increase, respectively, P < 0.01). However, resting HR and BP were not significantly different after ECs. Average increase in resting FBF showed a significant relationship with average isometric MVC force reduction (r =  - 0.45, P < 0.05), peak isometric MVC force reduction (r =  - 0.48, P < 0.05), average muscle soreness (r = 0.49, P < 0.05), and peak muscle soreness (r = 0.49, P < 0.05). Moreover, stepwise multiple regression analysis revealed that average increased resting FBF was explained by isometric MVC force reduction and muscle soreness (adjusted R² = 0.33).

CONCLUSIONS

These results suggested that increments in resting blood flow reflect muscle damage, and increased resting blood flow may be a result of acute inflammatory response induced by muscle damage.

Pathophysiology of exercise-induced muscle damage and its structural, functional, metabolic, and clinical consequences

Extreme or unaccustomed eccentric exercise can cause exercise-induced muscle damage, characterized by structural changes involving sarcomere, cytoskeletal, and membrane damage, with an increased permeability of sarcolemma for proteins. From a functional point of view, disrupted force transmission, altered calcium homeostasis, disruption of excitation-contraction coupling, as well as metabolic changes bring about loss of strength. Importantly, the trauma also invokes an inflammatory response and clinically presents itself by swelling, decreased range of motion, increased passive tension, soreness, and a transient decrease in insulin sensitivity. While being damaging and influencing heavily the ability to perform repeated bouts of exercise, changes produced by exercise-induced muscle damage seem to play a crucial role in myofibrillar adaptation. Additionally, eccentric exercise yields greater hypertrophy than isometric or concentric contractions and requires less in terms of metabolic energy and cardiovascular stress, making it especially suitable for the elderly and people with chronic diseases. This review focuses on our current knowledge of the mechanisms underlying exercise-induced muscle damage, their dependence on genetic background, as well as their consequences at the structural, functional, metabolic, and clinical level. A comprehensive understanding of these is a prerequisite for proper inclusion of eccentric training in health promotion, rehabilitation, and performance enhancement.

Impaired microvascular reactivity after eccentric muscle contractions is not restored by acute ingestion of antioxidants or dietary nitrate

Unaccustomed eccentric exercise leads to impaired microvascular function but the underlying mechanism is unknown. In this study, we evaluated the role of oxidative stress and of nitric oxide (NO) bioavailability. Thirty young men and women performed eccentric contractions of the tibialis anterior (TA) muscle (ECC), with the contralateral leg serving as nonexercising control (CON). Participants were randomized into three groups ingesting an antioxidant cocktail (AO), beetroot juice (BR) or placebo 46 h postexercise. At baseline and 48 h postexercise, hyperemic responses to brief muscle contractions and 5 min of cuff occlusion were assessed bilaterally in the TA muscles using blood oxygen level dependent (BOLD) magnetic resonance imaging. Eccentric contractions resulted in delayed time-to-peak (~22%; P < 0.001), blunted peak (~21%; P < 0.001) and prolonged time-to-half relaxation (~12%, P < 0.001) in the BOLD response to brief contractions, with no effects of AO or BR, and no changes in CON. Postocclusive time-to-peak was also delayed (~54%; P < 0.001) in ECC, with no effects of AO or BR, and no changes in CON. Impaired microvascular reactivity after eccentric contractions is confined to the exercised tissue, and is not restored with acute ingestion of AO or BR. Impairments in microvascular reactivity after unaccustomed eccentric contractions may result from structural changes within the microvasculature that can diminish muscle blood flow regulation during intermittent activities requiring prompt adjustments in oxygen delivery.

Following a Long-Distance Classical Race the Whole-Body Kinematics of Double Poling by Elite Cross-Country Skiers Are Altered

Introduction: Although short-term (approximately 10-min) fatiguing DP has been reported not to alter the joint kinematics or displacement of the centre of mass (COM) of high-level skiers, we hypothesize that prolonged DP does change these kinematics, since muscular strength is impaired following endurance events lasting longer than 2 h.

Methods: During the 58-km Marcialonga race in 2017, the fastest 15 male skiers were videofilmed (100 fps, FHD resolution in the sagittal plane) on two 20-m sections (inclines: 0.7 ± 0.1°) 48 km apart (i.e., 7 and 55 km from the start), approximating 50- km Olympic races. The cameras were positioned perpendicular to and about 40 m from the middle of each section and spatial dimensions adjusted for each individual track skied. Pole and joint kinematics, as well as displacement of the COM during two DP cycles were assessed.

Results: The 10 skiers who fulfilled our inclusion criteria finished the race in 2 h 09 min 19 s ± 28 s. Displacements of the joints and COM were comparable to previous observations on skiers roller skiing on a flat treadmill at similar speeds in the laboratory. 55 km after the start, cycle velocity and length were lower (P < 0.001 and P = 0.002, respectively) and the angular range of elbow joint flexion during the initial part of the poling phase reduced, while shoulder angle was greater during the first 35% of the DP cycle (all P < 0.05). Moreover, the ankle angle was increased and forward displacement of the COM reduced during the first 80% of the cycle.

Conclusion: Prolonged DP reduced the forward displacement of the COM and altered arm kinematics during the early poling phase. The inefficient utilization of COM observed after 2 h of competition together with potential impairment of the stretch-shortening of arm extensor muscles probably attenuated generation of poling force. To minimize these effects of fatigue, elite skiers should focus on maintaining optimal elbow and ankle kinematics and an effective forward lean during the propulsive phase of DP.

Eccentric exercise slows in vivo microvascular reactivity during brief contractions in human skeletal muscle

Unaccustomed exercise involving eccentric contractions results in muscle soreness and an overall decline in muscle function, however, little is known about the effects of eccentric exercise on microvascular reactivity in human skeletal muscle. Fourteen healthy men and women performed eccentric contractions of the dorsiflexor muscles in one leg, while the contralateral leg served as a control. At baseline, and 24 and 48 h after eccentric exercise, the following were acquired bilaterally in the tibialis anterior muscle: 1) transverse relaxation time (T2)-weighted magnetic resonance images to determine muscle cross-sectional area (mCSA) and T2; 2) blood oxygen level-dependent (BOLD) images during and following brief, maximal voluntary contractions (MVC) to monitor the hyperemic responses with participants positioned supine in a 3T magnet; 3) muscle strength; and 4) pain pressure threshold. Compared with the control leg, eccentric exercise resulted in soreness, decline in strength (∼20%), increased mCSA (∼7%), and prolonged T2 (∼7%) at 24 and 48 h ( P < 0.05). The BOLD response to a brief MVC was altered 24 and 48 h after eccentric exercise, such that time-to-peak (∼35%, P < 0.05) and time-to-half-recovery (∼23%, P < 0.05) were prolonged. The altered contraction-induced hyperemic response suggests slowed microvascular reactivity and altered matching of O2 delivery to O2 utilization within muscle tissue showing signs of muscle damage. These changes in microvascular regulation after eccentric exercise may impede rapid adjustments in muscle blood flow at exercise onset and during activities involving brief bursts of muscle activation, which may impair O2 delivery and contribute to reduced muscle function after eccentric exercise.

Adaptation of local muscle blood flow and surface electromyography to repeated bouts of eccentric exercise

The aim of this randomized controlled crossover study was to investigate the effect of a bout of unaccustomed eccentric exercise (ECC) followed by a consecutive bout of the same intensity on local muscle blood flow, amplitude, and frequency of the electromyographic (EMG) signal from the exercised tibialis anterior muscle. Sixteen healthy male participants (age, 25.7 (0.6) years; body mass index 24.8 (1) kg·m(-2) participated in this study. Two identical bouts of high-intensity ECC were performed on the tibialis anterior muscle 7 days apart. Control sessions involving no exercise were performed 4 weeks either before or after the exercise sessions. Changes in local total blood flow [ΔtHb], EMG root mean square, and median power frequency were recorded during isometric maximum voluntary contraction of ankle dorsiflexion. Measurements were performed before, immediately after, and the day after both ECCs (ECC1 and ECC2). The participants rested quietly in a chair in the control session. Eccentric exercise 1 led to a significant decrease in [ΔtHb] on the day after (p ≤ 0.05), whereas ECC2 did not. Median power frequency decreased significantly in ECC2 compared with ECC1 (p < 0.01). Root mean square was unchanged in all the instants. The present study showed that adaptation is depicted in the local muscle blood flow and the frequency contents of the EMG after an unaccustomed ECC inducing muscle soreness. These alterations provide a potential mechanism for a rapid adaptation, which decreases susceptibility of the muscle to develop further soreness in the subsequent ECC bout.

Serial assessment of local peripheral vascular function after eccentric exercise

Muscle damage is a common response to unaccustomed eccentric exercise; however, the effects of skeletal muscle damage on local vascular function and blood flow are poorly understood. This study examined serial local vascular responses to flow-mediated (endothelial-dependent) and nitroglycerin-mediated (endothelial-independent) dilation in the brachial artery after strenuous eccentric exercise and serially assessed resting blood flow. Ten healthy males performed 50 maximal eccentric unilateral arm contractions to induce muscle damage to the biceps brachii. Changes in maximal isometric strength and vascular responses were assessed 1, 24, 48, and 96 h after exercise. Mean blood velocities and arterial diameters, measured with Doppler ultrasound, were used to calculate blood flow and shear stress (expressed as area under the curve). Eccentric exercise resulted in impaired maximal isometric strength for up to 96 h (p < 0.001). Reductions in flow-mediated dilation (before exercise, 9.4% ± 2.6%; 1 h after exercise, 5.1% ± 2.2%) and nitroglycerin responses (before exercise, 26.3% ± 6.5%; 1 h after exercise, 20.7% ± 4.7%) were observed in the 1 h after exercise and remained lower for 96 h (p < 0.05). The shear stress response was attenuated immediately after exercise and remained impaired for 48 h (p < 0.05). Resting blood pressure and muscle blood flow remained similar throughout the study. Results suggest that muscle damage from eccentric exercise leads to impaired local endothelial and vascular smooth muscle function. Lower shear stress after exercise might contribute to the observed reduction in flow-mediated dilation responses, but the mechanism responsible for the attenuated endothelial-independent vasodilation remains unclear.

Squeezing the muscle: compression clothing and muscle metabolism during recovery from high intensity exercise

The purpose of this experiment was to investigate skeletal muscle blood flow and glucose uptake in m. biceps (BF) and m. quadriceps femoris (QF) 1) during recovery from high intensity cycle exercise, and 2) while wearing a compression short applying ∼37 mmHg to the thigh muscles. Blood flow and glucose uptake were measured in the compressed and non-compressed leg of 6 healthy men by using positron emission tomography. At baseline blood flow in QF (P = 0.79) and BF (P = 0.90) did not differ between the compressed and the non-compressed leg. During recovery muscle blood flow was higher compared to baseline in both compressed (P<0.01) and non-compressed QF (P<0.001) but not in compressed (P = 0.41) and non-compressed BF (P = 0.05; effect size = 2.74). During recovery blood flow was lower in compressed QF (P<0.01) but not in BF (P = 0.26) compared to the non-compressed muscles. During baseline and recovery no differences in blood flow were detected between the superficial and deep parts of QF in both, compressed (baseline P = 0.79; recovery P = 0.68) and non-compressed leg (baseline P = 0.64; recovery P = 0.06). During recovery glucose uptake was higher in QF compared to BF in both conditions (P<0.01) with no difference between the compressed and non-compressed thigh. Glucose uptake was higher in the deep compared to the superficial parts of QF (compression leg P = 0.02). These results demonstrate that wearing compression shorts with ∼37 mmHg of external pressure reduces blood flow both in the deep and superficial regions of muscle tissue during recovery from high intensity exercise but does not affect glucose uptake in BF and QF.

Muscle damage slows oxygen uptake kinetics during moderate-intensity exercise performed at high pedal rate

This study aimed to investigate the dependence of oxygen uptake (VO₂) kinetics on pedal cadence during moderate-intensity exercise following exercise-induced muscle damage (EIMD). Twenty untrained males were randomly assigned to a 50 revolution per minute (rpm) (age, 23.3 ± 1.8 years; VO₂(max), 38.9 ± 2.8 mL·kg⁻¹·min⁻¹) or 100 rpm group (age, 24.4 ± 3.5 years, VO₂(max), 42.9 ± 4.3 mL·kg⁻¹·min⁻¹). Participants completed "step" tests to moderate-intensity exercise from an unloaded baseline on a cycle ergometer before (baseline) and at 24 and 48 h after muscle-damaging exercise (10 sets of 10 eccentric contractions performed on an isokinetic dynamometer with a 2-min rest between each set). Pedal cadence was kept constant throughout each cycling trial (50 or 100 rpm). There were no changes in phase II pulmonary VO₂ kinetics following EIMD for the 50 rpm group (baseline = 35 ± 4 s; 24 h = 35 ± 7 s; and 48 h = 36 ± 9 s). However, the phase II VO₂ was significantly greater at 24 h (59 ± 27 s) compared with baseline (39 ± 6 s) and 48 h (40 ± 9 s) for the 100 rpm group. It is concluded that the effects of EIMD on phase II VO₂ kinetics during moderate-intensity cycling exercise is dependent on pedal cadence. The slower VO₂ kinetics after muscle damage suggests that type II fibers are involved during transition to moderate-intensity exercise at high pedal cadence.

The repeated-bout effect: influence on biceps brachii oxygenation and myoelectrical activity

This study investigated biceps brachii oxygenation and myoelectrical activity during and following maximal eccentric exercise to better understand the repeated-bout effect. Ten men performed two bouts of eccentric exercise (ECC1, ECC2), consisting of 10 sets of 6 maximal lengthening contractions of the elbow flexors separated by 4 wk. Tissue oxygenation index minimum amplitude (TOImin), mean and maximum total hemoglobin volume by near-infrared spectroscopy, torque, and surface electromyography root mean square (EMGRMS) during exercise were compared between ECC1 and ECC2. Changes in maximal voluntary isometric contraction (MVC) torque, range of motion, plasma creatine kinase activity, muscle soreness, TOImin, and EMGRMS during sustained (10-s) and 30-repeated isometric contraction tasks at 30% (same absolute force) and 100% MVC (same relative force) for 4 days postexercise were compared between ECC1 and ECC2. No significant differences between ECC1 and ECC2 were evident for changes in torque, TOImin, mean total hemoglobin volume, maximum total hemoglobin volume, and EMGRMS during exercise. Smaller ( P < 0.05) changes and faster recovery of muscle damage markers were evident following ECC2 than ECC1. During 30% MVC tasks, TOImin did not change, but EMGRMS increased 1–4 days following ECC1 and ECC2. During 100% MVC tasks, EMGRMS did not change, but torque and TOImin decreased 1–4 days following ECC1 and ECC2. TOImin during 100% MVC tasks and EMGRMS during 30% MVC tasks recovered faster ( P < 0.05) following ECC2 than ECC1. We conclude that the repeated-bout effect cannot be explained by altered muscle activation or metabolic/hemodynamic changes, and the faster recovery in muscle oxygenation and activation was mainly due to faster recovery of force.

Arterial stiffening following eccentric exercise-induced muscle damage

Acute inflammatory responses are linked to a transient increase in risk of a cardiovascular event, and this risk may be mediated by a concomitant reduction in vascular function. Humans experience an acute inflammatory response as a consequence of infection, injury, or muscle damage. We measured macrovascular function before and after eccentric exercise to determine whether muscle damage from unaccustomed exercise has an unfavorable effect on the large elastic arteries. A total of 27 healthy sedentary or recreationally active men (age 18–38 years) participated in either bilateral leg press eccentric exercise or unilateral elbow flexor eccentric exercise. Postexercise muscle damage was confirmed by significant reductions in isometric strength and increases in muscle soreness ( P < 0.05). Carotid-femoral pulse-wave velocity was significantly elevated 48 h after leg exercise (808 ± 31 vs. 785 ± 30 cm/s; P < 0.05) and arm exercise (790 ± 28 vs. 755 ± 24 cm/s; P < 0.05). There were no changes in mean arterial pressure. C-reactive protein was elevated after leg exercise but not after arm exercise. The increase in carotid-femoral pulse wave velocity 48 h after arm exercise was associated with muscle strength ( r = −0.47; P < 0.05) and creatine kinase concentrations ( r = 0.70; P < 0.01). We concluded that eccentric exercise in both small and large muscle mass translates to transient, unfavorable changes in central macrovascular function and that the increase in central arterial stiffness after small muscle eccentric exercise is associated with indicators of muscle damage.

Comparison between maximal lengthening and shortening contractions for biceps brachii muscle oxygenation and hemodynamics

Eccentric contractions (ECC) require lower systemic oxygen (O2) and induce greater symptoms of muscle damage than concentric contractions (CON); however, it is not known if local muscle oxygenation is lower in ECC than CON during and following exercise. This study compared between ECC and CON for changes in biceps brachii muscle oxygenation [tissue oxygenation index (TOI)] and hemodynamics [total hemoglobin volume (tHb) = oxygenated-Hb + deoxygenated-Hb], determined by near-infrared spectroscopy over 10 sets of 6 maximal contractions of the elbow flexors of 10 healthy subjects. This study also compared between ECC and CON for changes in TOI and tHb during a 10-s sustained and 30-repeated maximal isometric contraction (MVC) task measured immediately before and after and 1–3 days following exercise. The torque integral during ECC was greater ( P < 0.05) than that during CON by ∼30%, and the decrease in TOI was smaller ( P < 0.05) by ∼50% during ECC than CON. Increases in tHb during the relaxation phases were smaller ( P < 0.05) by ∼100% for ECC than CON; however, the decreases in tHb during the contraction phases were not significantly different between sessions. These results suggest that ECC utilizes a lower muscle O2 relative to O2 supply compared with CON. Following exercise, greater ( P < 0.05) decreases in MVC strength and increases in plasma creatine kinase activity and muscle soreness were evident 1–3 days after ECC than CON. Torque integral, TOI, and tHb during the sustained and repeated MVC tasks decreased ( P < 0.01) only after ECC, suggesting that muscle O2 demand relative to O2 supply during the isometric tasks was decreased after ECC. This could mainly be due to a lower maximal muscle mass activated as a consequence of muscle damage; however, an increase in O2 supply due to microcirculation dysfunction and/or inflammatory vasodilatory responses after ECC is recognized.

Physiological differences between cycling and running: lessons from triathletes

The purpose of this review was to provide a synopsis of the literature concerning the physiological differences between cycling and running. By comparing physiological variables such as maximal oxygen consumption (V O(2max)), anaerobic threshold (AT), heart rate, economy or delta efficiency measured in cycling and running in triathletes, runners or cyclists, this review aims to identify the effects of exercise modality on the underlying mechanisms (ventilatory responses, blood flow, muscle oxidative capacity, peripheral innervation and neuromuscular fatigue) of adaptation. The majority of studies indicate that runners achieve a higher V O(2max) on treadmill whereas cyclists can achieve a V O(2max) value in cycle ergometry similar to that in treadmill running. Hence, V O(2max) is specific to the exercise modality. In addition, the muscles adapt specifically to a given exercise task over a period of time, resulting in an improvement in submaximal physiological variables such as the ventilatory threshold, in some cases without a change in V O(2max). However, this effect is probably larger in cycling than in running. At the same time, skill influencing motor unit recruitment patterns is an important influence on the anaerobic threshold in cycling. Furthermore, it is likely that there is more physiological training transfer from running to cycling than vice versa. In triathletes, there is generally no difference in V O(2max) measured in cycle ergometry and treadmill running. The data concerning the anaerobic threshold in cycling and running in triathletes are conflicting. This is likely to be due to a combination of actual training load and prior training history in each discipline. The mechanisms surrounding the differences in the AT together with V O(2max) in cycling and running are not largely understood but are probably due to the relative adaptation of cardiac output influencing V O(2max) and also the recruitment of muscle mass in combination with the oxidative capacity of this mass influencing the AT. Several other physiological differences between cycling and running are addressed: heart rate is different between the two activities both for maximal and submaximal intensities. The delta efficiency is higher in running. Ventilation is more impaired in cycling than in running. It has also been shown that pedalling cadence affects the metabolic responses during cycling but also during a subsequent running bout. However, the optimal cadence is still debated. Central fatigue and decrease in maximal strength are more important after prolonged exercise in running than in cycling.

Effect of eccentric exercise-induced muscle damage on the dynamics of muscle oxygenation and pulmonary oxygen uptake

Unaccustomed eccentric exercise has a profound impact on muscle structure and function. However, it is not known whether associated microvascular dysfunction disrupts the matching of O2delivery (Q̇o2) to O2utilization (V̇o2). Near-infrared spectroscopy (NIRS) was used to test the hypothesis that eccentric exercise-induced muscle damage would elevate the muscle Q̇o2:V̇o2ratio during severe-intensity exercise while preserving the speed of the V̇o2kinetics at exercise onset. Nine physically active men completed “step” tests to severe-intensity exercise from an unloaded baseline on a cycle ergometer before (Pre) and 48 h after (Post) eccentric exercise (100 squats with a load corresponding to 70% of body mass). NIRS and breath-by-breath pulmonary V̇o2were measured continuously during the exercise tests and subsequently modeled using standard nonlinear regression techniques. There were no changes in phase II pulmonary V̇o2kinetics following the onset of exercise (time constant: Pre, 25 ± 4 s; Post, 24 ± 2 s; amplitude: Pre, 2.36 ± 0.23 l/min; Post, 2.37 ± 0.23 l/min; all P > 0.05). However, the primary (Pre, 14 ± 3 s; Post, 19 ± 3 s) and overall (Pre, 16 ± 4 s; Post, 21 ± 4 s) mean response time of the [HHb] response was significantly slower following eccentric exercise ( P < 0.05). The slower [HHb] kinetics observed following eccentric exercise is consistent with an increased Q̇o2:V̇o2ratio during transitions to severe-intensity exercise. We propose that unchanged primary phase V̇o2kinetics are associated with an elevated Q̇o2:V̇o2ratio that preserves blood-myocyte O2flux.

Monitoring muscle oxygenation after eccentric exercise-induced muscle damage using near-infrared spectroscopy

Eccentric exercise (EE), a common type of muscular activity whereby muscles lengthen and contract simultaneously, is associated with higher levels of force but may also evoke muscle damage. We investigated the hypothesis that unaccustomed EE might impair muscle oxygenation and muscle blood flow in healthy adults. Ten healthy males performed a bout of 70 maximal eccentric contractions of the elbow flexors. Before and after EE on day 1 and over the next 6 days, maximum voluntary isometric torque (MVT), serum creatine kinase (CK), and the changes in muscle oxygen saturation, blood flow, and oxygen uptake (using near-infrared spectroscopy) within the biceps brachii were assessed. MVT decreased, whereas muscle soreness and CK increased after EE (p < 0.05). Mean resting oxygen saturation increased by 22% after acute EE, and remained elevated by 5%–9% for the following 6 days. During isometric contractions, significant decreases were observed in oxygen desaturation and re-saturation kinetics after EE and these declines were also significantly prevalent over the following 6 days. Both muscle blood flow and oxygen uptake increased significantly after acute EE, but recovered on the next day. This study revealed some prolonged alterations in muscle oxygenation at rest and during exercise after EE, which might be due to a decrease in muscle oxygen consumption, an increase in oxygen delivery, and (or) a combination of both. However, both oxygen consumption and blood flow recovered within 24 h after the eccentric exercise session, and therefore, the reason(s) for the changes in tissue oxygen saturation remain unknown.

Muscle oxygenation after downhill walking-induced muscle damage

The purpose of this study was to investigate changes in muscle oxygenation and blood flow within vastus lateralis after an exhaustive session of downhill walking (DW). Nine healthy males performed 40-min DW on a treadmill with a gradient of -25% and at walking velocity of 6.4 km h(-1). To increase the likelihood that DW would induce muscle damage, subjects were loaded with 5% of their body weight carried in a back pack. Before and after DW exercise on day 1 and over the next 4 days, maximum voluntary contractions (MVCs), subjects' perception of muscle soreness (SOR), plasma creatine kinase (CK) activity and myoglobin (Mb) concentrations, and muscle oxygenation (using near infrared spectroscopy; NIRS) within vastus lateralis were assessed. Repeated-measures anova revealed that MVC decreased while SOR and Mb concentration increase significantly (P<0.05) after DW, consistent with its effectiveness to evoke muscle damage. Resting tissue oxygen saturation increased immediately after DW, but recovered within 24 h. During isometric contractions at 30%, 50% and 80% of MVC, oxygen desaturation and re-saturation kinetics became significantly faster than pre-exercise values. The possible mechanism responsible for these changes might be increased resting muscle oxygen utilization after muscle damage because of an increased requirement for aerobic energy-demanding repair processes.