1. The effect of the different phases of the menstrual cycle on skeletal muscle strength, contractile properties and fatiguability was investigated in ten young, healthy females. Results were compared with a similar group on the combined (non-phasic) oral contraceptive pill (OC). Cycle phases were divided into the early and mid-follicular, mid-cycle (ovulatory) and mid- and late luteal. Cycle phases were estimated from the first day of the menstrual bleed. 2. Subjects were studied weekly through two complete cycles. Measurements included quadriceps and handgrip maximum voluntary isometric force and the relaxation times, force-frequency relationship and fatigue index of the quadriceps during percutaneous stimulation at a range of frequencies from 1 to 100 Hz. 3. In the women not taking the OC there was a significant increase of about 11% in quadriceps and handgrip strength at mid-cycle compared with both the follicular and luteal phases. Accompanying the increases in strength there was a significant slowing of relaxation and increase in fatiguability at mid-cycle. No changes in any parameter were found in the women taking the OC. 4. The changes in muscle function at mid-cycle may be due to the increase in oestrogen that occurs prior to ovulation.
The fatiguability of the quadriceps muscle was investigated in 10 male subjects (25-40 yrs), with inter-individual differences in fibre composition of their vastus lateralis muscles (range 25-65% fast twitch, FT, muscle fibres). Fatiguability was assessed as the decline in maximal force (in % of initial values) with 50 repeated isokinetic knee-extensions at fast angular velocity (3.14 rad/s). Each contraction lasted 0.5 s and the rest periods were about 0.7 s. Every subject was tested on two occasions and the standard deviation for a single determination of fatiguability was 1.4%. The decline in force after 50 contractions was on the average about 45%. The individual values varied, however, and a linear correlation was present between fatiguability and % FT fibres (r = 0.86, p less than 0.01). It was concluded that development of fatigue in human skeletal muscle performing repeated fast dynamic contractions with maximal effort was most marked in muscles with a high proportion FT fibres. This finding was in conformity with earlier results from animal skeletal muscle preparations.
Fatiguability
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The contractile properties of single motor units of rat plantaris were measured in situ 7 days following muscle partial denervation, achieved by section of radicular nerve L4. Partially denervated muscles weighed less, generated weaker twitch and tetanic forces, and contained denervated fibers, as evidenced by indirect/direct stimulation force ratios less than 1. Fast motor units (over 90% of unit pool) showed elevated twitch and tetanic responses (222% and 171% of controls, respectively) and elevated twitch-to-tetanic force ratios. Although partial denervation did not alter the mean fatiguability of fast motor units, fewer proportions of units remained in the extreme categories of fatigue resistance, with a clustering of units in the intermediate ranges. Slow units, while showing elevated twitch and tetanic responses, did not change in fatiguability. Glycogen depletion of the fibers of two fast motor units in partially denervated muscles revealed the presence of fibers varying in size, and in staining intensities for succinate dehydrogenase and ATPase, within the same motor unit, as a result of motoneurone sprouting.
Injury prevalence data, muscle strength, and fatiguability differ between males and females. In addition, arm spatial orientation affects muscle activation and strength of the shoulder muscles. Nevertheless, little research has been conducted in relation to the shoulder rotator muscles comparing men and women. Therefore, the main aim of of this study was to perform a comparative investigation between two arm spatial orientations (45 and 90 of abduction in the frontal plane) during a fatigue assessment of the internal rotator (IR) and external rotator (ER) shoulder muscles. Secondly, the interaction between sex and dominance with muscular performance was assessed.
Differences in shoulder performance fatiguability between sexes are affected by arm position, arm dominance and muscle groups. In agreement with the literature, performance values in males were approximately 50% higher than in females. However, the amount of IF was no different between both sexes. Based on findings in literature, it could be suggested that this is due to differences between males and females in motor control and/or coordination strategies during repetitive tasks. In addition, we also observed the IR muscles to be significantly stronger than the ER muscles. It has long been established in literature that these observations are due to the muscle-size differences between both muscle groups, where the IR muscles can produce a larger amount of force due to the larger cross-sectional area. Results of our study found similar ER:IR ratios compared to previous reports.
The main findings of this study established major differences between males and females in shoulder performance fatiguability with arm position, dominance and muscle group playing an important role. Previous findings have established that the arm spatial orientation affects muscle activation and strength of the shoulder muscles [24, 34]. In agreement, our results observed significant differences in C.Perf and BR between the angle of abduction and muscle group in both males and females. However, induced fatigue did not establish any differences between any of the comparisons. Nevertheless, it is well recognised that overhead athletes are at a higher risk of shoulder related injuries with the ligaments around the shoulder being weakened due to the overload, repetitive stress and fatigue [35, 36]. This is due to a reduction in the subacromial strength, as a result of the higher angles of abduction [7], especially when the rotator cuff muscles are in a fatigued state [37]. We found no differences in induced fatigue, but it must be noted that our participants did not undergo a repetitive stress/overload over an extended time-period.
One should have a high suspicion for MG when a patient's history and main signs and symptoms suggest variable muscle weakness and fatigability that worsens in the evening or with prolonged use, and improves with rest. A quick test in the office can be to ask the patient to look up and hold that position, and then observe if there is fatiguability of the levator muscle such that the upper eyelids start to drift downwards while the patient is looking up. The definitive diagnosis is made through various clinical, pharmacological and serologic tests.
Citation: Guerrini Usubini A, Bottacchi M, Bondesan A, Caroli D, Castelnuovo G and Sartorio A (2022) A three-week in-hospital multidisciplinary body weight reduction program exerts beneficial effects on physical and mental health and fatiguability of elderly patients with obesity. Front. Aging Neurosci. 14:1054941. doi: 10.3389/fnagi.2022.1054941
The dorsiflexor muscles in people with SCI are significantly more fatiguable than those in AB controls, but decreases in muscle excitability do not seem to be an important contributor to the increased fatiguability. The mechanisms behind the increased fatigue must lie distal to the muscle membrane.
Although the mechanism for the increased muscle fatiguability in the SCI population is unknown, a current theory of neuromuscular fatigue in the able-bodied (AB) population involves a decrease in muscle excitability due to increased extracellular K+. The sodium potassium pump (Na+/K+ ATPase) is largely responsible for maintaining concentration gradients of Na+ and K+ across the sarcolemmal membrane, allowing for muscle excitability to be maintained during repetitive activation. When an action potential arrives at the membrane, depolarization is required to elicit a contraction. Given the small extracellular space surrounding muscle fibers, repeated depolarization can quickly lead to significant changes in the extracellular concentration of K+, which would be predicted to affect membrane excitability.4
To date, the potential role of muscle excitability in contributing to fatiguability in paralyzed muscle has only been examined in the soleus muscle. Shields2 concluded that changes in muscle membrane excitability were not solely responsible for muscle fatigue as decreases in M-wave amplitude did not coincide with decreases in torque. A later study, also in the soleus muscle, found a 20% decrease in M-wave amplitude, a 40% increase in M-wave duration, no change in M-wave area, and an 80% decrease in peak torque (PT) after intermittent tetanic stimulation in participants with chronic, but not acute (
M-wave amplitude was greater in the AB group at baseline, which supports earlier findings13 and is likely a reflection of the significant muscle atrophy after SCI. The fact that M-wave amplitude and area stayed relatively constant over the course of fatigue in the SCI group suggests that the previously reported decrease in Na+/K+ pump concentration after SCI does not contribute to muscle fatiguability.5 Muscle membrane excitability seemed to be maintained quite effectively in paralyzed muscle during the 2-min fatigue protocol. Although there was an increase in M-wave area in the SCI group, this was likely associated with the increase in M-wave duration rather than increased Na+/K+ pump activity, as there was no corresponding increase in M-wave amplitude. The increasing M-wave duration in the SCI group during recovery is suggestive of a slowing of action potential conduction across the sarcolemma membrane, a finding that has been demonstrated previously in the soleus muscle.7
As predicted, the SCI group showed increased fatigue compared with the AB group as evidenced with an increased fatigue index for both PT and TT. This increased fatiguability in paralyzed muscle has been reported previously in the soleus and quadriceps muscle groups.1, 2, 3 However, based on the M-wave results of this study, and from earlier work,2, 7 a failure in Na+/K+ pump activity does not appear to play a significant role in the increased fatiguability during a relatively short period of maximum stimulation in people with SCI. This was an unexpected finding given the reported decrease in Na+/K+ pump concentration after SCI.5 One possible explanation may be that the previously reported decrease in Na+/K+ pump concentration in the vastus lateralis muscle is not transferable to the tibialis anterior muscle. An alternative possibility is that despite a decrease in pump concentration, an increase in the efficiency of the remaining Na+/K+ pumps may assist in maintaining membrane excitability. As muscle excitability does not seem to be playing a major role in the fatiguability of paralyzed muscle, it is important to consider the involvement of other mechanisms for fatigue. Muscle oxidative capacity, which provides an indication for the potential for aerobic metabolism, has been previously shown to decrease after SCI and has been correlated with an increase in fatiguability in the tibialis anterior8, 9 and vastus lateralis.10 Another common theory of muscle fatigue involves the impaired control of calcium (Ca2+) release into the sarcoplasmic reticulum.15 Alterations in the function and concentration of Ca2+ATPase have been suggested after SCI to explain the slowing of relaxation time12 and changes in sarcoplasmic reticulum Ca2+ATPase isoform expression.11 be457b7860
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