Acute performance fatigability following continuous vs. intermittent cycling protocols is not proportional to total work done.

Classical training theory postulates that performance fatigability following a training session should be proportional to the total work done (TWD); however, this notion has been questioned. This study investigated indices of performance and perceived fatigability after primary sessions of high-intensity interval (HIIT) and constant-work rate (CWR) cycling, each followed by a cycling time-to-task-failure (TTF) bout. On separate days, 16 participants completed an incremental cycling test, and, in a randomized order, i) a TTF trial at 80% of peak power output (PPO), ii) a HIIT session and iii) a CWR session, both of which were immediately followed by a TTF trial at 80% PPO. Central and peripheral aspects of performance fatigability were measured using interpolated twitch technique, and perceptual measures were assessed prior to and following the HIIT and CWR trials, and again following the TTF trial. Despite TWD being less following HIIT (P=0.029), subsequent TTF trial was an average of 125 s shorter following HIIT vs. CWR (P<0.001), and this was accompanied by greater impairments in voluntary and electrically evoked forces (P<0.001), as well as exacerbated perceptual measures (P<0.001); however, there were no differences in any fatigue measure following the TTF trial (P≥0.149). There were strong correlations between the decline in TTF and indices of peripheral (r=0.70) and perceived fatigability (r≥0.80) measured at the end of HIIT and CWR. These results underscore the dissociation between TWD and performance fatigability and highlight the importance of peripheral components of fatigability in limiting endurance performance during high-intensity cycling exercise.

Validity of the Entralpi force plate in the assessment of finger flexor performance metrics in rock climbers.

This study assessed the validity of the Entralpi force plate in the assessment of finger flexor performance in rock climbers. In addition to a static force evaluation, peak force, peak impulse, and total impulse were measured during 30 all-out performance trials by 15 participants, in which force during the trials was recorded simultaneously by the Entralpi and a Pasco force plate. Agreement between devices was assessed by a variety of statistical analyses, including intraclass correlation coefficient (ICC), coefficient of variation (CV), and Bland-Altman analyses. The static force evaluation showed a mean relative error of 0.41% and excellent day-to-day reliability (ICC = 1; CV = 0.03%). Peak force, peak impulse, and total impulse from the performance trials demonstrated strong agreement (ICC ≥ 0.991, CV ≤ 1.9%, Bland-Altman mean bias ≤ 0.5%). These results illustrate that the Entralpi force plate provides accurate and reliable data for rock climbing related tasks at an affordable cost.

Test-Retest Reliability of a 4-Minute All-Out Critical Force Test in Rock Climbers.

The purpose of this study was to assess the test-retest reliability of a 4-minute all-out critical force test in well-trained rock climbers. Thirteen rock climbers (n=4 females) completed a familiarization session and two all-out critical force tests on different days. During each trial, participants completed 24 repetitions of 7s right-handed, maximal effort hangs from a 20mm edge interspersed with 3 s rest. The end-test force (EF; i.e., critical force), impulse above EF (IEF), and peak force achieved during the test were analyzed with paired t-tests to determine differences between trials. Intraclass correlation coefficient (ICC), coefficient of variation (CV), and Bland-Altman analysis were performed to quantify the relative and absolute reliability of the measure, respectively. The level of significance for this study was set at p<0.05. There were no significant differences between trials for any of the reported variables (P≥0.455). For EF, IEF, and peak force, ICC was 0.848, 0.820, and 0.938, respectively; and CV was 21.0%, 13.2%, and 5.6%, respectively. Bland-Altman analyses showed a mean relative bias of -2.3%, -2.8%, and -1.3%, with 95% limits of agreement (LoA) of -62.6% to 58.1%, -40.5% to 30.9%, and -17.2% to 14.6% for EF, IEF, and peak force, respectively, however linear regression revealed a significant proportional bias for EF (p = 0.026, R2 = 0.377). The reliability of this protocol was good to excellent for all parameters; however, there was larger intra-individual variability for EF and IEF. This study suggests that when using the 4-min all-out critical force test in rock climbers, coaches and athletes should be aware that there may be a trade-off between the test's practicality and the precision of its results.

Ischemic Preconditioning, But Not Priming Exercise, Improves Exercise Performance in Trained Rock Climbers.

ABSTRACT: MacDougall, KB, McClean, ZJ, MacIntosh, BR, Fletcher, JR, and Aboodarda, SJ. Ischemic preconditioning, but not priming exercise, improves exercise performance in trained rock climbers. J Strength Cond Res 37(11): 2149-2157, 2023-To assess the effects of ischemic preconditioning (IPC) and priming exercise on exercise tolerance and performance fatigability in a rock climbing-specific task, 12 rock climbers completed familiarization and baseline tests, and constant-load hangboarding tests (including 7 seconds on and 3 seconds off at an intensity estimated to be sustained for approximately 5 minutes) under 3 conditions: (a) standardized warm-up (CON), (b) IPC, or (c) a priming warm-up (PRIME). Neuromuscular responses were assessed using the interpolated twitch technique, including maximum isometric voluntary contraction (MVC) of the finger flexors and median nerve stimulation, at baseline and after the performance trial. Muscle oxygenation was measured continuously using near-infrared spectroscopy (NIRS) across exercise. Time to task failure (T lim ) for IPC (316.4 ± 83.1 seconds) was significantly greater than CON (263.6 ± 69.2 seconds) ( p = 0.028), whereas there was no difference between CON and PRIME (258.9 ± 101.8 seconds). At task failure, there were no differences in MVC, single twitch force, or voluntary activation across conditions; however, recovery of MVC and single twitch force after the performance trial was delayed for IPC and PRIME compared with CON ( p < 0.05). Despite differences in T lim , there were no differences in any of the NIRS variables assessed. Overall, despite exercise tolerance being improved by an average of 20.0% after IPC, there were no differences in neuromuscular responses at task failure, which is in line with the notion of a critical threshold of peripheral fatigue. These results indicate that IPC may be a promising precompetition strategy for rock climbers, although further research is warranted to elucidate its mechanism of action.

Efficiency of cycling exercise: Quantification, mechanisms, and misunderstandings.

The energetics of cycling represents a well-studied area of exercise science, yet there are still many questions that remain. Efficiency, broadly defined as the ratio of energy output to energy input, is one key metric that, despite its importance from both a scientific as well as performance perspective, is commonly misunderstood. There are many factors that may affect cycling efficiency, both intrinsic (e.g., muscle fiber type composition) and extrinsic (e.g., cycling cadence, prior exercise, and training), creating a complex interplay of many components. Due to its relative simplicity, the measurement of oxygen uptake continues to be the most common means of measuring the energy cost of exercise (and thus efficiency); however, it is limited to only a small proportion of the range of outputs humans are capable of, further limiting our understanding of the energetics of high-intensity exercise and any mechanistic bases therein. This review presents evidence that delta efficiency does not represent muscular efficiency and challenges the notion that the slow component of oxygen uptake represents decreasing efficiency. It is noted that gross efficiency increases as intensity of exercise increases in spite of the fact that fast-twitch fibers are recruited to achieve this high power output. Understanding the energetics of high-intensity exercise will require critical evaluation of the available data.

What Is Moderate to Vigorous Exercise Intensity?

A variety of health benefits associated with physical activity depends upon the frequency, intensity, duration, and type of exercise. Intensity of exercise is the most elusive of these elements and yet has important implications for the health benefits and particularly cardiovascular outcomes elicited by regular physical activity. Authorities recommend that we obtain 150min of moderate to vigorous intensity physical activity (MVPA) each week. The current descriptions of moderate to vigorous intensity are not sufficient, and we wish to enhance understanding of MVPA by recognition of important boundaries that define these intensities. There are two key thresholds identified in incremental tests: ventilatory and lactate thresholds 1 and 2, which reflect boundaries related to individualized disturbance to homeostasis that are appropriate for prescribing exercise. VT2 and LT2 correspond with critical power/speed and respiratory compensation point. Moderate intensity physical activity approaches VT1 and LT1 and vigorous intensity physical activity is between the two thresholds (1 and 2). The common practice of prescribing exercise at a fixed metabolic rate (# of METs) or percentage of maximal heart rate or of maximal oxygen uptake (V̇O2max) does not acknowledge the individual variability of these metabolic boundaries. As training adaptations occur, these boundaries will change in absolute and relative terms. Reassessment is necessary to maintain regular exercise in the moderate to vigorous intensity domains. Future research should consider using these metabolic boundaries for exercise prescription, so we will gain a better understanding of the specific physical activity induced health benefits.

Age-related reductions in the number of serial sarcomeres contribute to shorter fascicle lengths but not elevated passive tension.

We investigated age-related changes to fascicle length, sarcomere length and serial sarcomere number (SSN), and how this affects passive force. Following mechanical testing to determine passive force, the medial gastrocnemius muscle of young (n=9) and old (n=8) Fisher 344BN hybrid rats was chemically fixed at the optimal muscle length for force production; individual fascicles were dissected for length measurement, and laser diffraction was used to assess sarcomere length. Old rats had ∼14% shorter fascicle lengths than young rats, which was driven by a ∼10% reduction in SSN, with no difference in sarcomere length (∼4%). Passive force was greater in the old than in the young rats at long muscle lengths. Shorter fascicle lengths and reduced SSN in the old rats could not entirely explain increased passive forces for absolute length changes, owing to a slight reduction in sarcomere length in old rats, resulting in similar sarcomere length at long muscle lengths.

Calcium sensitivity during staircase with sequential incompletely fused contractions.

Activity dependent potentiation is thought to result from phosphorylation of the regulatory light chains of myosin, increasing Ca2+ sensitivity. Yet, Ca2+ sensitivity decreases early in a period of intermittent contractions. The purpose of this study was to investigate the early change in Ca2+ sensitivity during intermittent submaximal tetanic contractions. Flexor digitorum brevis muscle fibres were dissected from mice after cervical disarticulation. Fibres were superfused with Tyrode solution at 32 °C. Length was set to yield maximal tetanic force. Indo-1 was microinjected into fibres and allowed to dissipate for 30 min. Fluorescence was measured at 405 and 495 nm wavelength and the ratio was used to estimate [Ca2+]. A control force-Ca2+ relationship was determined with stimulation over a range of frequencies, yielding constants for slope, max force, and half-maximal [Ca2+] (pCa2 +50). Data were collected for sequential contractions at 40 Hz at 2 s intervals. Active force decreased over the first 1-4 contractions then increased. A force-pCa2+ curve was fit to each contraction, using the control values for the Hill slope and max force by adjusting pCa2+50 until the curve passed through the target contraction. Data are presented for three contractions for each fibre: first, maximum shift to the right, and last contraction. There was a significant shift to the right for pCa2+50 (decreased Ca2+ sensitivity), usually early in the series of intermittent contractions, then pCa2 +50 shifted to the left, but remained significantly different from the control value. Although potentiation is associated with increased Ca2+ sensitivity, this increase begins only after Ca2+ sensitivity has decreased and, in most cases, Ca2+ sensitivity does not increase above the control level.

Additional in-series compliance does not affect the length dependence of activation in rat medial gastrocnemius.

NEW FINDINGS: What is the central question of this study? The length dependence of activation (LDA) is typically explained by a length-dependent increase in calcium sensitivity, but recently calcium-independent mechanisms have been suggested: does active muscle shortening provided by a compliant in-series component impact the muscle length at which force output is maximized, thus contributing to LDA? What is the main finding and its importance? Using an in situ rat medial gastrocnemius set-up and varying the magnitude of muscle shortening via an artificial compliant series-elastic component, we were unable to observe any change in optimal length between conditions, contrary to some previous findings. More research is therefore required to explain these discrepancies. ABSTRACT: The force-length relationship dictates the amount of force a muscle can produce as a function of its length, during maximal isometric contractions. When activation is submaximal, it has been shown that the length at which force production is highest (the optimal length) is longer. This is typically explained by a length-dependent increase in Ca2+ sensitivity, known as the 'length dependence of activation'. Recent reports have implicated shortening against in-series compliance to be a potential factor in the observed optimal length (L0 ) of muscle, via the phenomenon of shortening-induced force depression (a phenomenon which describes the relative reduction in muscle force when a muscle is actively shortening to a given length compared to contracting isometrically at that same length). In the current study, rat medial gastrocnemius was stimulated with single and triple pulses (200 Hz) over a range of lengths, both with and without additional in-series compliance provided by a small piece of silicon tubing in series with the muscle, which allowed greater fascicle shortening upon activation. Fascicle length was measured using sonomicrometry crystals, and peak force (Fpeak ) and L0 were estimated by curve-fitting of the force-length data. The additional in-series compliance significantly reduced Fpeak by approximately 14% and 25% for the single and triple pulses, respectively (P = 0.003, P < 0.001), yet L0 remained unchanged (P = 0.405), suggesting that in our model, shortening against in-series compliance does not affect L0 . We offer potential explanations for the discrepancies seen and discuss whether the velocity of shortening may have a role in the length dependence of force.

Mechanisms of reduced plantarflexor function in Cerebral palsy: smaller triceps surae moment arm and reduced muscle force.

Both muscle forces, and moment arm (MA) could contribute to reduced muscle moment in people with Cerebral Palsy (CP). Current reports in CP are conflicting. The tendon travel method of estimating MA requires constant force, but passive force is high and variable in CP, and range of motion is limited. Therefore, the purpose of this study was to examine triceps surae muscle MA in 12 subjects with mild to moderate CP (15-32 years) and 10 typically developing peers (TD, 17-26 years) by tendon travel and by visually measuring the apparent MA. MA was calculated at 90° and at a reference angle (∼106°) with zero net passive moment. The tendon travel (28.8 ±â€¯5.6 mm) and visual methods (29.1 ±â€¯5.5 mm) yielded similar MA in CP (p = 0.94) at the reference angle. TD had significantly larger triceps surae muscle MA than CP subjects (p = 0.002), 35.4 ±â€¯4.1 mm at the reference angle for tendon travel and 35.4 ±â€¯3.6 mm by the visual method. Test/retest revealed less bias (0.8 mm) using the visual method. Calculated active peak isometric force was significantly less in CP (1983.8 ±â€¯887.0 N) than TD (4104.9 ±â€¯1154.9 N, p < 0.001). There are challenges in estimating MA in CP, but the visual method is more reliable. Although a shorter moment arm would reduce the joint moment, joint angular velocity for a given velocity of muscle shortening would be enhanced. Strength training may mitigate the effects of the smaller moment arm and reduced joint moment generated in those with CP.

Force-frequency relationship during fatiguing contractions of rat medial gastrocnemius muscle.

The force-frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. During repetitive muscular contractions, fatigue and potentiation may both impact the resultant contractile response. However, both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. Thus, we wanted to explore the effects that repetitive stimulation had on the force-frequency relationship. The force-frequency relationship of the rat medial gastrocnemius muscle was investigated during consecutive bouts of increasing fatigue with 20 to 100 Hz stimulation. Force was measured prior to the fatiguing protocol, during each of three levels of fatigue, and after 30 min of recovery. Force at each frequency was quantified relative to the pre-fatigued 100 Hz contractions, as well as the percentage reduction of force from the pre-fatigued level at a given frequency. We observed less reduction in force at low frequencies compared to high frequencies, suggesting an interplay of fatigue and potentiation, in which potentiation can "protect" against fatigue in a frequency-dependent manner. The exact mechanism of fatigue is unknown, however the substantial reduction of force at high frequency suggests a role for reduced force per cross-bridge.

Greater Short-Time Recovery of Peripheral Fatigue After Short- Compared With Long-Duration Time Trial.

The kinetics of recovery from neuromuscular fatigue resulting from exercise time trials (TTs) of different durations are not well-known. The aim of this study was to determine if TTs of three different durations would result in different short-term recovery in maximal voluntary contraction (MVC) and evoked peak forces. Twelve trained subjects performed repetitive concentric right knee extensions on an isokinetic dynamometer self-paced to last 3, 10, and 40 min (TTs). Neuromuscular function was assessed immediately (<2 s) and 1, 2, 4, and 8 min after completion of each TT using MVCs and electrical stimulation. Electrical stimulations consisted of single stimulus (SS), paired stimuli at 10 Hz (PS10), and paired stimuli at 100 Hz (PS100). Electrically evoked forces including the ratio of low- to high-frequency doublets were similar between trials at exercise cessation but subsequently increased more (P < 0.05) after the 3 min TT compared with either the 10 or 40 min TT when measured at 1 or 2 min of recovery. MVC force was not different between trials. The results demonstrate that recovery of peripheral fatigue including low-frequency fatigue depends on the duration and intensity of the preceding self-paced exercise. These differences in recovery probably indicate differences in the mechanisms of fatigue for these different TTs. Because recovery is faster after a 3 min TT than a 40 min TT, delayed assessment of fatigue will detect a difference in peripheral fatigue between trials that was not present at exercise cessation.

Does postactivation potentiation (PAP) increase voluntary performance?

The transient increase in torque of an electrically evoked twitch following a voluntary contraction is called postactivation potentiation (PAP). Phosphorylation of myosin regulatory light chains is the most accepted mechanism explaining the enhanced electrically evoked twitch torque. While many authors attribute voluntary postactivation performance enhancement (PAPE) to the positive effects of PAP, few actually confirmed that contraction was indeed potentiated using electrical stimulation (twitch response) at the time that PAPE was measured. Thus, this review aims to investigate if increases in voluntary performance after a conditioning contraction (CC) are related to the PAP phenomenon. For this, studies that confirmed the presence of PAP through an evoked response after a voluntary CC and concurrently evaluated PAPE were reviewed. Some studies reported increases in PAPE when PAP reaches extremely high values. However, PAPE has also been reported when PAP was not present, and unchanged/diminished performance has been identified when PAP was present. This range of observations demonstrates that mechanisms of PAPE are different from mechanisms of PAP. These mechanisms of PAPE still need to be understood and those studying PAPE should not assume that regulatory light chain phosphorylation is the mechanism for such enhanced voluntary performance. Novelty The occurrence of PAP does not necessarily mean that the voluntary performance will be improved. Improvement in voluntary performance is sometimes observed when the PAP level reaches extremely high values. Other mechanisms may be more relevant than that for PAP in the manifestation of acute increases in performance following a conditioning contraction.

Fatigue and recovery measured with dynamic properties versus isometric force: effects of exercise intensity.

Although fatigue can be defined as an exercise-related decrease in maximal power or isometric force, most studies have assessed only isometric force. The main purpose of this experiment was to compare dynamic measures of fatigue [maximal torque (Tmax), maximal velocity (Vmax) and maximal power (Pmax)] with measures associated with maximal isometric force [isometric maximal voluntary contraction (IMVC) and maximal rate of force development (MRFD)] 10 s after different fatiguing exercises and during the recovery period (1-8 min after). Ten young men completed six experimental sessions (3 fatiguing exercises×2 types of fatigue measurements). The fatiguing exercises were: 30 s all-out intensity (AI), 10 min at severe intensity (SI) and 90 min at moderate intensity (MI). Relative Pmax decreased more than IMVC after AI exercise (P=0.005) while the opposite was found after SI (P=0.005) and MI tasks (P<0.001). There was no difference between the decrease in IMVC and Tmax after the AI exercise, but IMVC decreased more than Tmax immediately following and during the recovery from the SI (P=0.042) and MI exercises (P<0.001). Depression of MRFD was greater than Vmax after all fatiguing exercises and during recovery (all P<0.05). Despite the general definition of fatigue, isometric assessment of fatigue is not interchangeable with dynamic assessment following dynamic exercises with large muscle mass of different intensities, i.e. the results from isometric function cannot be used to estimate dynamic function and vice versa. This implies different physiological mechanisms for the various measures of fatigue.

A stochastic simulation of skeletal muscle calcium transients in a structurally realistic sarcomere model using MCell.

Skeletal muscle contraction is initiated when an action potential triggers the release of Ca2+ into the sarcomere in a process referred to as excitation-contraction coupling. The speed and scale of this process makes direct observation very challenging and invasive. To determine how the concentration of Ca2+ changes within the myofibril during a single activation, several simulation models have been developed. These models follow a common pattern; divide the half sarcomere into a series of compartments, then use ordinary differential equations to solve reactions occurring within and between the compartments. To further develop this type of simulation, we have created a realistic structural model of a skeletal muscle myofibrillar half-sarcomere using MCell software that incorporates the myofilament lattice structure. Using this simulation model, we were successful in reproducing the averaged calcium transient during a single activation consistent with both the experimental and previous simulation results. In addition, our simulation demonstrated that the inclusion of the myofilament lattice within our model produced an asymmetric distribution of Ca2+, with more Ca2+ accumulating near the Z-disk and less Ca2+ reaching the m-line. This asymmetric distribution of Ca2+ is also apparent when we examine how the Ca2+ are bound to the troponin-C proteins along the actin filaments. Our simulation model also allowed us to produce advanced visualizations of this process, including two simulation animations, allowing us to view Ca2+ release, diffusion, binding and uptake within the myofibrillar half-sarcomere.

The effect of torsional shoe sole stiffness on knee moment and gross efficiency in cycling.

Altering torsional stiffness of cycling shoe soles may be a novel approach to reducing knee joint moments and overuse injuries during cycling. We set out to determine if the magnitude of three-dimensional knee moments were different between cycling shoe soles with different torsional stiffnesses. Eight trained male cyclists cycled at 90% lactate threshold power output in one of two cycling shoe conditions in a randomized crossover design. The shoe sole was considered torsionally flexible (FLEX) compared to a relatively stiffer (STIFF) sole. Gross efficiency (GE) and knee joint moments were quantified. No significant effect of shoe condition was seen in GE (21.4 ± 1.1% and 20.9 ± 1.6% for FLEX and STIFF, respectively, P = 0.12), nor in three-dimensional knee moments. 4 of the 8 subjects had reduced knee moments in at least 2 of the 3 moment directions. These "responders" were significantly shorter (1.73 ± 0.02 m vs 1.81 ± 0.04 m, P = 0.017) and had a higher relative maximal aerobic power (MAP) (4.6 ± 0.3 W∙kg-1 vs 3.9 ± 0.3 W∙kg-1, P = 0.024) compared to non-responders. These results suggest that certain shoe characteristics may influence certain individuals differently because these participants belong to different "functional groups"; certain individuals may respond positively to FLEX, while others may not. Further studies should test this proposed hypothesis.