Brian R. MacIntosh

Coexistence of potentiation and fatigue in skeletal muscle

Twitch potentiation and fatigue in skeletal muscle are two conditions in which force production is affected by the stimulation history. Twitch potentiation is the increase in the twitch active force observed after a tetanic contraction or during and following low-frequency stimulation. There is evidence that the mechanism responsible for potentiation is phosphorylation of the regulatory light chains of myosin, a Ca2+-dependent process. Fatigue is the force decrease observed after a period of repeated muscle stimulation. Fatigue has also been associated with a Ca2+-related mechanism: decreased peak Ca2+ concentration in the myoplasm is observed during fatigue. This decrease is probably due to an inhibition of Ca2+ release from the sarcoplasmic reticulum. Although potentiation and fatigue have opposing effects on force production in skeletal muscle, these two presumed mechanisms can coexist. When peak myoplasmic Ca2+ concentration is depressed, but myosin light chains are relatively phosphorylated, the force response can be attenuated, not different, or enhanced, relative to previous values. In circumstances where there is interaction between potentiation and fatigue, care must be taken in interpreting the contractile responses.

Cadence, power, and muscle activation in cycle ergometry

PURPOSE Based on the resistance-rpm relationship for cycling, which is not unlike the force-velocity relationship of muscle, it is hypothesized that the cadence which requires the minimal muscle activation will be progressively higher as power output increases. METHODS To test this hypothesis, subjects were instrumented with surface electrodes placed over seven muscles that were considered to be important during cycling. Measurements were made while subjects cycled at 100, 200, 300, and 400 W at each cadence: 50, 60, 80, 100, and 120 rpm. These power outputs represented effort which was up to 32% of peak power output for these subjects. RESULTS When all seven muscles were averaged together, there was a proportional increase in EMG amplitude each cadence as power increased. A second-order polynomial equation fit the EMG:cadence results very well (r2 = 0.87- 0.996) for each power output. Optimal cadence (cadence with lowest amplitude of EMG for a given power output) increased with increases in power output: 57 +/- 3.1, 70 +/- 3.7, 86 +/- 7.6, and 99 +/- 4.0 rpm for 100, 200, 300, and 400 W, respectively. CONCLUSION The results confirm that the level of muscle activation varies with cadence at a given power output. The minimum EMG amplitude occurs at a progressively higher cadence as power output increases. These results have implications for the sense of effort and preferential use of higher cadences as power output is increased.

Economy of running: beyond the measurement of oxygen uptake

The purpose of this study was to compare running economy across three submaximal speeds expressed as both oxygen cost (mlxkg(-1)xkm(-1)) and the energy required to cover a given distance (kcalxkg(-1)xkm(-1)) in a group of trained male distance runners. It was hypothesized that expressing running economy in terms of caloric unit cost would be more sensitive to changes in speed than oxygen cost by accounting for differences associated with substrate utilization. Sixteen highly trained male distance runners [maximal oxygen uptake (Vo(2max)) 66.5 +/- 5.6 mlxkg(-1)xmin(-1), body mass 67.9 +/- 7.3 kg, height 177.6 +/- 7.0 cm, age 24.6 +/- 5.0 yr] ran on a motorized treadmill for 5 min with a gradient of 0% at speeds corresponding to 75%, 85%, and 95% of speed at lactate threshold with 5-min rest between stages. Oxygen uptake was measured via open-circuit calorimetry. Average oxygen cost was 221 +/- 19, 217 +/- 15, and 221 +/- 13 mlxkg(-1)xkm(-1), respectively. Caloric unit cost was 1.05 +/- 0.09, 1.07 +/- 0.08, and 1.11 +/- 0.07 kcalxkg(-1)xkm(-1) at the three trial speeds, respectively. There was no difference in oxygen cost with respect to speed (P = 0.657); however, caloric unit cost significantly increased with speed (P < 0.001). It was concluded that expression of running economy in terms of caloric unit cost is more sensitive to changes in speed and is a more valuable expression of running economy than oxygen uptake, even when normalized per distance traveled.

Abdominal muscle activation of elite male golfers with chronic low back pain.

PURPOSE The purpose of this study was twofold: 1) to determine whether elite male golfers with chronic low back pain (CLBP) exhibit different abdominal muscle activity patterns during the golf swing than asymptomatic control (AC) golfers and 2) to determine whether elite male golfers with CLBP experience greater fatigue in the abdominal muscles than AC golfers after a typical practice session. METHODS Surface EMG data were collected bilaterally from the rectus abdominis (RA), external oblique (EO), and internal oblique (IO) muscles. Muscle activity during the golf swing was measured using the root mean square (RMS) of the EMG signal in various phases of the golf swing. Fatigue was assessed using the median frequency (MF) and RMS of the EMG signal during a 10-s submaximal isometric contraction. Low back pain was quantified with the McGill Pain Questionnaire before and after the practice session. RESULTS No differences in the RMS of abdominal muscle activity were noted during the golf swing between AC and CLBP subjects. However, EO (lead) onset times were significantly delayed with respect to the start of the backswing in CLBP subjects. Low back pain in CLBP golfers increased significantly after the practice session. Abdominal muscle fatigue, as measured with MF or RMS, was not evident after the practice session for either AC or CLBP subjects. CONCLUSION Abdominal muscle activity and muscle fatigue characteristics were quite similar between AC and CLBP subjects after repetitive golf swings. Despite this, it was clear that repetitive golf swings were aggravating some part of the musculoskeletal system in CLBP subjects, which resulted in increased pain in the low back area.

Skeletal muscle fatigue – regulation of excitation–contraction coupling to avoid metabolic catastrophe

ATP provides the energy in our muscles to generate force, through its use by myosin ATPases, and helps to terminate contraction by pumping Ca2+ back into the sarcoplasmic reticulum, achieved by Ca2+ ATPase. The capacity to use ATP through these mechanisms is sufficiently high enough so that muscles could quickly deplete ATP. However, this potentially catastrophic depletion is avoided. It has been proposed that ATP is preserved not only by the control of metabolic pathways providing ATP but also by the regulation of the processes that use ATP. Considering that contraction (i.e. myosin ATPase activity) is triggered by release of Ca2+, the use of ATP can be attenuated by decreasing Ca2+ release within each cell. A lower level of Ca2+ release can be accomplished by control of membrane potential and by direct regulation of the ryanodine receptor (RyR, the Ca2+ release channel in the terminal cisternae). These highly redundant control mechanisms provide an effective means by which ATP can be preserved at the cellular level, avoiding metabolic catastrophe. This Commentary will review some of the known mechanisms by which this regulation of Ca2+ release and contractile response is achieved, demonstrating that skeletal muscle fatigue is a consequence of attenuation of contractile activation; a process that allows avoidance of metabolic catastrophe.

Muscle fiber type distribution as estimated by Cybex testing and by muscle biopsy.

The purpose of the present study was to derive a regression equation relating variables obtained from a series of noninvasive functional tests in a normal subject population to the fiber type distribution of vastus lateralis muscle (VL) determined using muscle biopsy. All functional tests were designed to distinguish between basic properties of Type II fibers (fast twitch fibers) and Type I fibers (slow twitch fibers) and included assessment of peak torque, power output at nine different angular velocities (60 degrees.s-1 to 300 degrees.s-1), as well as a fatigue test consisting of 60 consecutive contractions at 90 degrees.s-1 to establish fatigue resistance of the knee extensor muscles. Using a stepwise multiple regression procedure, relative torque after 53-55 contractions (T55) in the fatigue test and power output at an angular velocity of 280 degrees.s-1 normalized for fat free mass of the thigh (FFMT) were the best predictors for fiber type distribution, explaining 51.8% of the variance in the proportion of Type II fibers in VL. No other measured variable met entering criteria. Subgroup analyses revealed a higher peak torque/FFMT, higher power/FFMT values at angular velocities of 200 degrees.s-1 and higher, and lower relative torque beyond 30 contractions in the fatigue test for the fast twitch group, FTG (subjects with > 60% Type II fibers, N = 8) as compared with the STG (subjects with < 45% Type II fibers, N = 9). Results from the present study suggest that two simple functional tests on a Cybex dynamometer yield reasonable estimates of the fiber type distribution in VL.

Myosin light chain phosphorylation and posttetanic potentiation in fatigued skeletal muscle

Myosin light chain (P-LC) phosphorylation, which is thought to be the principle mechanism for twitch potentiation in skeletal muscle, is significantly decreased during staircase in fatigued muscle. Attenuated phosphorylation of P-LC could be due to either depressed Ca2+ transients in fatigue, or to some inhibitory influence of contractile activity on myosin light chain kinase (MLCK). Tetanic stimulation, which would presumably result in maximal activation of MLCK, could be used to evaluate these potential mechanisms. P-LC phosphorylation and twitch developed tension (DT) were assessed at 20 and 120 s following a tetanic contraction in either rested or fatigued rat gastrocnemius muscle in situ. P-LC phosphorylation was significantly lower in fatigued muscles (39.7 ± 3.2% vs 54.8±3.5%, 20 s after a 2-s tetanic contraction), while posttetanic potentiation (PTP) was similar in fatigued (189.1 ± 6.5%) versus rested muscle (169.5 ± 2.6%). Tetanic DT was reduced following the fatigue protocol and, thus, the assumption that the MLCK system was fully activated by Ca2+ may not be valid. The potentiation-phosphorylation relationships were linear for both rested and fatigued muscles; however this relationship was shifted markedly leftward in fatigued muscles. It appears that during PTP, equivalent potentiation is attained with correspondingly lower levels of P-LC phosphorylation in fatigued muscle. This enhanced relative potentiation for a given level of phosphorylation could be expected if Ca2+ transients were attenuated in the fatigued muscle. However the results do not rule out the possibility that other factors contribute to potentiation under these circumstances.

Optimal pacing strategy: from theoretical modelling to reality in 1500-m speed skating

Purpose Athletes are trained to choose the pace which is perceived to be correct during a specific effort, such as the 1500-m speed skating competition. The purpose of the present study was to “override” self-paced (SP) performance by instructing athletes to execute a theoretically optimal pacing profile. Methods Seven national-level speed-skaters performed a SP 1500-m which was analysed by obtaining velocity (every 100 m) and body position (every 200 m) with video to calculate total mechanical power output. Together with gross efficiency and aerobic kinetics, obtained in separate trials, data were used to calculate aerobic and anaerobic power output profiles. An energy flow model was applied to SP, simulating a range of pacing strategies, and a theoretically optimal pacing profile was imposed in a second race (IM). Results Final time for IM was ∼2 s slower than SP. Total power distribution per lap differed, with a higher power over the first 300 m for IM (637.0 (49.4) vs 612.5 (50.0) W). Anaerobic parameters did not differ. The faster first lap resulted in a higher aerodynamic drag coefficient and perhaps a less effective push-off. Conclusion Experienced athletes have a well-developed performance template, and changing pacing strategy towards a theoretically optimal fast start protocol had negative consequences on speed-skating technique and did not result in better performance.

Human skeletal muscle fibre types and force: velocity properties

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this relationship is greater in the range of 3–8 rad · s−1 during knee extension compared to slower or faster angular knee extensor speeds. A mathematical model of the force: velocity properties of muscle with various combinations of fast- and slow-twitch fibres may provide insight into why specific velocities may give better predictions of fibre type distribution. In this paper, a mathematical model of the force: velocity relationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribution and that the greatest fibre type discrimination in human knee extensor muscles should occur with measurement of power output at an angular velocity just greater than 7 rad · s−1. Measurements of torque: angular velocity relationships for knee extension on an isokinetic dynamometer and fibre type distribution in biopsies of vastus lateralis muscles were made on 31 subjects. Peak power and optimal velocity were determined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and optimal velocity using the Hill equation gave the best correlation with fibre type distribution (r > 0.5 for peak power or optimal velocity and percentage of fast-twitch fibres). The results of this study confirm that prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters.

Norepinephrine increases canine skeletal muscle OO2 during recovery

The purpose of this study was to investigate the effect of norepinephrine on the rate of O2 uptake (VO2) of the denervated canine gastrocnemius-plantaris muscle group in situ. In seven experiments, VO2 and developed tension were measured without and with norepinephrine infusion during rest, during contractions at 1.0 Hz, and during recovery. In six additional experiments with two consecutive rest-contraction-recovery periods, no norepinephrine was given during the second sequence. During rest, VO2 was increased by norepinephrine. Changes in VO2 during contractions were small, but the arteriovenous O2 content difference was significantly greater during norepinephrine infusion. The most significant finding was that net recovery VO2 was increased 40% by norepinephrine infusion. The ratio of net recovery VO2 to the VO2 during the preceding contraction period was significantly increased from 0.78 +/- 0.07 (mean +/- SEM) to 1.23 +/- 0.07 (mean +/- SEM) by norepinephrine infusion. This increase in net recovery VO2 could be due to either a direct metabolic effect of norepinephrine during recovery or to hypoxia during the preceding contractions. In either case, the data indicated that norepinephrine can produce large increases in muscle recovery VO2; this supports the notion that catecholamines may make a significant contribution to post-exercise recovery VO2.