A. de Haan

Temperature Effect on the Rates of Isometric Force Development and Relaxation in the Fresh and Fatigued Human Adductor Pollicis Muscle

The purpose of the present study was to investigate the effect of temperature on the rates of isometric force development and relaxation in electrically activated fresh and fatigued human adductor pollicis muscle. Following immersion of the lower arm for 20 min in water baths of four different temperatures, muscle temperatures were approximately 37, 31, 25 and 22°C. Maximal isometric force was reduced by 16.8 ± 1.5% at 22°C. The stimulation frequency‐force and ‐rate of force development relationships were shifted to the left at lower temperatures. Q10 values for the maximal rates of force development and relaxation, and the times for 100 to 50% and 50 to 25% force relaxation, were about 2.0 between 37 and 25°C and about 3.8 between 25 and 22°C. However, the time for 50 to 25% force relaxation had a relatively high Q10 value between 25 and 22°C (6.9) and this parameter also appeared to be more sensitive to fatigue compared to the other indices of relaxation. Nevertheless, the effect of fatigue on all parameters decreased with cooling over the entire (37‐22°C) temperature range.

Changes in velocity of shortening, power output and relaxation rate during fatigue of rat medial gastrocnemius muscle

The force-velocity characteristics of rat medial gastrocnemius muscle have been determined by measuring the force sustained during constant velocity releases of the muscle stimulated in situ at an ambient temperature of 26°C. The velocity of unloaded shortening was determined using the “slack” test and rate of relaxation from the half time of force loss at the end of stimulation. Measurements were first made on fresh muscles using short contractions and then during a series which consisted of a 15 s contraction (fatigued muscle), followed by 15 min recovery and a 1 s contraction (recovered muscle). After a 5 min recovery period the sequence was repeated. Comparison was made between the fatigued and recovered state in each preparation in order to allow for any change in the preparation during the course of the experiment. After 15 s contraction the fatigued muscles showed a marked reduction in all parameters measured. In fatigued muscles the isometric force fell to 48±15% (mean±SD) and there was a decrease in maximum velocity of shortening to 66%. These changes in the force-velocity relationship were accompanied by slowing of relaxation so that the half time of relaxation nearly doubled. The consequence of these changes was that the maximum power output was reduced by a much greater extent that was the isometric force (75% vs. 52%). It is suggested that the changes in force-velocity characteristics reflect a reduction in cross-bridge cycling in fatigued muscle.

Clinical and molecular overlap between myopathies and inherited connective tissue diseases

This review presents an overview of myopathies and inherited connective tissue disorders that are caused by defects in or deficiencies of molecules within the extracellular matrix (ECM). We will cover the myopathies caused by defects in transmembrane protein complexes (dystroglycan, sarcoglycan, and integrins), laminin, and collagens (collagens VI, XIII, and XV). Clinical characteristics of several of these myopathies imply skin and joint features. We subsequently describe the inherited connective tissue disorders that are characterized by mild to moderate muscle involvement in addition to the dermal, vascular, or articular symptoms. These disorders are caused by defects of matrix-embedded ECM molecules that are also present within muscle (collagens I, III, V, IX, lysylhydroxylase, tenascin, fibrillin, fibulin, elastin, and perlecan). By focussing on the structure and function of these ECM molecules, we aim to point out the clinical and molecular overlap between the groups of disorders. We argue that clinicians and researchers dealing with myopathies and inherited connective tissue disorders should be aware of this overlap. Only a multi-disciplinary approach will allow full recognition of the wide variety of symptoms present in the spectrum of ECM defects, which has important implications for scientific research, diagnosis, and for the treatment of these disorders.

Variability in fibre properties in paralysed human quadriceps muscles and effects of training

A spinal cord injury usually leads to an increase in contractile speed and fatigability of the paralysed quadriceps muscles, which is probably due to an increased expression of fast myosin heavy chain (MHC) isoforms and reduced oxidative capacity. Sometimes, however, fatigue resistance is maintained in these muscles and also contractile speed is slower than expected. To obtain a better understanding of the diversity of these quadriceps muscles and to determine the effects of training on characteristics of paralysed muscles, fibre characteristics and whole muscle function were assessed in six subjects with spinal cord lesions before and after a 12-week period of daily low-frequency electrical stimulation. Relatively high levels of MHC type I were found in three subjects and this corresponded with a high degree of fusion in 10-Hz force responses (r=0.88). Fatigability was related to the activity of succinate dehydrogenase (SDH) (r=0.79). Furthermore, some differentiation between fibre types in terms of metabolic properties were present, with type I fibres expressing the highest levels of SDH and lowest levels of α-glycerophosphate dehydrogenase. After training, SDH activity increased by 76±26% but fibre diameter and MHC expression remained unchanged. The results indicate that expression of contractile proteins and metabolic properties seem to underlie the relatively normal functional muscle characteristics observed in some paralysed muscles. Furthermore, training-induced changes in fatigue resistance seem to arise, in part, from an improved oxidative capacity.

Novel pattern of editing regions in mitochondrial transcripts of the cryptobiid Trypanoplasma borreli.

In mitochondria of Kinetoplastida belonging to the suborder Trypanosomatina, the nucleotide sequence of transcripts is post‐transcriptionally edited via insertion and deletion of uridylate residues. In order to shed more light on the evolutionary history of this process we have searched for editing in mitochondrial RNAs of Trypanoplasma borreli, an organism belonging to the suborder Bodonina. We have cloned and sequenced a 5.3 kb fragment derived from a 37 kb mitochondrial DNA molecule which does not appear to be a part of a network structure and have found genes encoding cytochrome c oxidase (cox) subunit 1, cox 2 and apocytochrome (cyt) b, and genes encoding the small and large subunit mitoribosomal RNAs. The order in which these genes occur is completely different from that of trypanosomatid maxicircle genes. The 5′ and 3′ termini of both the cytb and cox1 gene are cryptic, the protein coding sequences being created by extensive insertion/deletion of Us in the corresponding mRNA sections. Phylogenetic analyses of the protein and ribosomal RNA sequences demonstrated that the separation between T.borreli and Trypanosomatina was an early event, implying that U‐insertion/deletion processes are ancient. Different patterns of editing have persisted in different lineages, however, since editing of cox1 RNA and of relatively small 3′‐terminal RNA sections is not found in trypanosomatids. In contrast, cox2 RNA which is edited in trypanosomatids by the insertion of four Us, is unedited in T.borreli.

No acute effects of short-term creatine supplementation on muscle properties and sprint performance.

Abstract In a double-blind, placebo, controlled study, we investigated the acute effects of short-term oral creatine supplementation (20 g · day−1 for 6 days) on muscle activation, fatigue and recovery of the m. quadriceps femoris during electrical stimulation, and on maximal performance during sprint cycling. The quadriceps muscles of 23 well-trained rowers were stimulated at different frequencies (10, 20, 50, 100, 150 and 200 Hz). Furthermore, 40 repetitive, electrically stimulated (duration 220 ms, stimulation frequency 150 Hz) concentric contractions were imposed at a constant angular velocity of 180° · s−1 over a range of 50° (from 90 to 140° knee angle), each extension/flexion cycle lasting 1200 ms. To determine recovery, torque was measured at 20, 50, 80, 120, 180 and 300 s after the last contraction. In addition, two maximal 30-s sprints were performed on a cycle ergometer with 4 min rest in between. Following short-term creatine supplementation, body mass [mean (SEM)] increased (P < 0.05) from 85.7 (2.7) kg to 87.3 (2.9) kg. Creatine supplementation had no effect on maximal voluntary isometric torque and muscle activation, or on fatigue and recovery of dynamic exercise. There was also no significant effect on peak power, time to peak power and work to peak power, or total work during both sprints on the cycle ergometer. It was concluded that short-term oral creatine supplementation resulted in increased body mass, but did not enhance muscle performance or maximal output during sprint cycling.

Muscle economy of isometric contractions as a function of stimulation time and relative muscle length

For rat medial gastrocnemius muscle economy (i.e. the ratio of time integral of force and total energy-rich phosphate consumption) was calculated. Muscles in situ at 35°C were stimulated to perform either one continuous or several repetitive isometric contractions at one muscle length in the range from 70% to 130% of optimum muscle length for force generation. Whereas during one continuous contraction economy increased, no differences in economy were found between 6, 12 or 18 successive contractions. Economy during intermittent exercise was always lower than during continuous exercise. The difference in economy is a result of different rates of metabolism, whereas no difference was found for force generation. Economy was highest at optimum muscle length for force generation and decreased at muscle lengths smaller as well as greather than optimum muscle length. Force-dependent energy consumption was calculated by substracting the force-independent part (obtained by extrapolation) from total energy consumption. The calculated force produced per μmol force-dependent energy-rich phosphate consumption was similar in muscles stretched beyond optimum length. In contrast, a decreasing amount of force per μmol force-dependent energy-rich phosphate consumption was observed at lengths smaller than optimum length.

Effects of training on contractile properties of paralyzed quadriceps muscle

Effects of two different training regimens on the contractile properties of the quadriceps muscle were studied in six individuals with spinal cord injury. Each subject had both limbs trained with the two regimens, consisting of stimulation with low frequencies (LF) at 10 HZ or high frequencies (HF) at 50 HZ; one limb of each subject was stimulated with the LF protocol and the other with the HF regimen. Twelve weeks of daily training increased tetanic tension by ∼20%, which was not significantly different between training regimens. Interestingly, after HF but not LF training, the unusual high forces at the low frequency range of the force–frequency relationship decreased, possibly due to a reduced activation per impulse. After LF but not HF training, force oscillation amplitudes declined (by 33%) as relaxation tended to slow, which may have opposed possible effects of reduced activation as seen after HF training. Finally, fatigue resistance also increased rapidly after LF training (by 43%) but not after HF training. These results indicate that different types of training may selectively change different aspects of function in disused muscles.

Myosin heavy chain isoform expression and high energy phosphate content in human muscle fibres at rest and post‐exercise.

1. The relationship between myosin heavy chain (MyHC) isoforms and high energy phosphate content was studied in human muscle fibres at rest and following maximal dynamic exercise lasting 25 s. 2. Single fibre fragments were characterized as type I, type IIA, type IIX or type IIAX. These latter fibres were subdivided into five groups on the basis of the proportion of MyHC IIX isoform present. 3. Resting ATP concentration in type I fibres was 10% lower than in type II fibres (P < 0.05), but no differences were found amongst type IIA, IIX and IIAX fibre groups. Phosphocreatine (PCr) content was lower in type I than in type II fibres (P < 0.01) and, amongst type II fibres, increased progressively with the amounts of MyHC IIX expressed. 4. After 25 s of maximal dynamic exercise ATP concentration was reduced in all fibres. The decrease was approximately 25% in type I fibres and between 47 and 66% in the type II subgroups. 5. Post‐exercise PCr content was low in all fibre types. Fibre groups with the lowest post‐exercise PCr also had the lowest ATP and the highest inosine monophosphate contents. delta PCr (rest to post‐exercise) was smallest in type I fibres and showed a progressive increase in the type II fibre groups as the proportion of the faster IIX myosin heavy chain isoform increased.

Extramuscular myofascial force transmission for in situ rat medial gastrocnemius and plantaris muscles in progressive stages of dissection

SUMMARY The aim of this study was to establish the extent of extramuscular myofascial force transmission for dissected rat medial gastrocnemius (GM) and plantaris (PL) muscles. Initially, this was done with GM still connected to extramuscular connective tissue (general fascia, neuro-vascular tract and compartmental fascia). Neighbouring muscles were also connected to these tissues. In a later stage, it was dissected progressively until finally a fully dissected in situ GM was obtained, for which the neuro-vascular tract (i.e. the nerves, bloodvessels and the surrounding connective tissue) was the only extramuscular tissue left intact. Force of GM was measured not only at its distal tendon in progressive stages of dissection, but also at its dissected proximal tendon. In the stage where GM was still connected to extramuscular tissues, the experiments showed that up to 40.5±5.9% (mean ± s.e.m.) of the force exerted by the neighbouring PL muscle was transmitted onto the calcaneal bone, even when the PL tendon was not connected to this bone. After distal PL-tenotomy, a difference between proximally and distally measured forces of GM constituted evidence for myofascial force transmission. In the fully dissected in situ GM muscle, no relevant myofascial force transmission occurred in the reference position (the position of the GM origin corresponding to a knee angle of 120°). However, some myofascial force transmission occurred when the relative position of the origin of the fully dissected GM muscle was changed with respect to the neuro-vascular tract.