Advertisement

Changes in serum fast and slow skeletal troponin I concentration following maximal eccentric contractions

      Abstract

      Objectives

      We tested the hypothesis that fast skeletal muscle troponin I (fsTnI) concentration in serum would increase more than those of slow skeletal muscle troponin I (ssTnI) after eccentric exercise of the elbow flexors using a sensitive blood marker to track fibre specific muscle damage.

      Design

      Observational comparison of response in a single experimental group.

      Methods

      Eight young men (26.4 ± 6.2 years) performed 210 (35 sets of 6) eccentric contractions of the elbow flexors on an isokinetic dynamometer with one arm. Changes in serum fsTnI and ssTnI concentrations, serum creatine kinase (CK) activity, and maximal voluntary isometric contraction torque (MVIC) before and 1, 2, 3, 4 and 14 days following exercise were analysed by a Student–Newman–Keuls multiple comparison test. The relationship between serum CK activity and fsTnI or ssTnI concentrations was determined using a Pearson's product moment correlation.

      Results

      Significant (P < 0.05) decreases in MVIC and increases in serum CK activity and fsTnI were evident after exercise, but ssTnI did not change. The time course of changes in fsTnI was similar to that of CK, peaking at 4 days post-exercise, and the two were highly correlated (r = 0.8).

      Conclusions

      Increases in serum fsTnI concentrations reflect muscle damage, and it seems likely that only fast twitch fibres were damaged by eccentric contractions.

      Keywords

      1. Introduction

      Repeated eccentric contractions result in muscle damage that is characterised by morphological alterations of muscle fibres such as infiltration of inflammatory cells to muscle fibres and endomysium, missing of dystrophin or desmin staining, and/or ultrastructural alteration of myofilaments and/or intermediate filaments.
      • Fridén J.
      • Lieber R.L.
      Eccentric exercise-induced injuries to contractile and cytoskeletal muscle fibre components.
      • Gibala M.J.
      • MacDougall J.D.
      • Tarnopolsky M.A.
      • et al.
      Changes in human skeletal muscle ultrastructure and force production after acute resistance exercise.
      • Lauritzen F.
      • Paulsen G.
      • Raastad T.
      • et al.
      Gross ultrastructural changes and necrotic fiber segments in elbow flexor muscles after maximal voluntary eccentric action in humans.
      • Raastad T.
      • Owe S.G.
      • Paulsen G.
      • et al.
      Changes in calpain activity muscle structure, and function after eccentric exercise.
      Indirect markers of muscle damage include delayed onset muscle soreness, prolonged large decreases in muscle strength and range of motion, and abnormalities in magnetic resonance (e.g., increases in T2 relaxation time) or ultrasound (e.g., increases in echo intensity) images.
      • Nosaka K.
      • Clarkson P.M.
      Changes in indicators of inflammation after eccentric exercise of the elbow flexors.
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      • Foley J.M.
      • Jayaraman R.C.
      • Prior B.M.
      • et al.
      MR measurements of muscle damage and adaptation after eccentric exercise.
      Additionally, serum or plasma markers of muscle damage such as creatine kinase (CK) and myoglobin are also used to determine the extent of muscle damage induced by eccentric contractions.
      • Nosaka K.
      • Clarkson P.M.
      • Apple F.S.
      Time course of serum protein changes after strenuous exercise of the forearm flexors.
      • Sorichter S.
      • Puschendorf B.
      • Mair J.
      Skeletal muscle injury induced by eccentric muscle action: muscle proteins as markers of muscle fiber injury.
      However, CK and myoglobin are not necessarily specific for skeletal muscle damage.
      • Ebbeling C.B.
      • Clarkson P.M.
      Exercise-induced muscle damage and adaptation.
      It has been reported that type II (fast twitch) fibres are more susceptible to muscle damage induced by maximal eccentric contractions than type I (slow twitch) fibres.
      • Lieber R.L.
      • Fridén J.
      Selective damage of fast glycolytic muscle fibres with eccentric contraction of the rabbit tibialis anterior.
      • Macpherson P.C.
      • Schork M.A.
      • Faulkner J.A.
      Contraction-induced injury to single fiber segments from fast and slow muscles of rats by single stretches.
      • Shepstone T.N.
      • Tang J.E.
      • Dallaire S.
      • et al.
      Short-term high- vs. low-velocity isokinetic lengthening training results in greater hypertrophy of the elbow flexors in young men.
      However, neither serum or plasma CK activity nor myoglobin concentration can identify the muscle fibre type damaged by eccentric contractions, and other indirect markers of muscle damage such as muscle strength and imaging techniques can only provide a gross indication of the extent of muscle damage.
      Skeletal troponin I has been proposed as a muscle fibre specific and sensitive marker of skeletal muscle damage.
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal troponin I as a marker of exercise-induced muscle damage.
      There are three isoforms of troponin I, cardiac, fast and slow.
      • Wilkinson J.M.
      • Grand R.J.A.
      Comparison of amino acid sequence of troponin I from different striated muscles.
      The cardiac isoform is found exclusively in the myocardium and is currently the gold standard for detecting myocardial injury.
      • Mair J.
      • Thome-Kromer B.
      • Wagner I.
      • et al.
      Concentration time courses of troponin and myosin subunits after acute myocardial infarction.
      The fast (fsTnI) and slow skeletal troponin I isoforms (ssTnI) are found exclusively in adult fast and slow twitch skeletal muscle fibres, respectively.
      • Wilkinson J.M.
      • Grand R.J.A.
      Comparison of amino acid sequence of troponin I from different striated muscles.
      Most human limb skeletal muscles consist of both slow (type I) and fast (type II) muscle fibres. This is not an exception for the elbow flexors such that biceps brachii surface and deep regions are reported to be 42% type I and 58% type II, and 51% type I and 49% type II, respectively.
      • Johnson M.A.
      • Polgar J.
      • Weightman D.
      • et al.
      Data on the distribution of fibre types in thirty-six human muscles: An autopsy study.
      Thus, it is possible to gain a more exact indication of the type of muscle fibres damaged by measuring fsTnI and ssTnI.
      • Wilkinson J.M.
      • Grand R.J.A.
      Comparison of amino acid sequence of troponin I from different striated muscles.
      Recently, a new diagnostic assay for measuring serum concentrations of fsTnI and ssTnI isoforms has been established.
      • Simpson J.A.
      • Labugger R.
      • Collier C.
      • et al.
      Fast and slow skeletal troponin I in serum from patients with various skeletal muscle disorders: a pilot study.
      • Simpson J.A.
      • Labugger R.
      • Hesketh G.G.
      • et al.
      Differential detection of skeletal troponin I isoforms in serum of a patient with rhabdomyolysis: markers of muscle injury?.
      In this study, we investigated changes in fsTnI and ssTnI concentrations following exercise consisting of maximal eccentric contractions of the elbow flexors. We hypothesised that the concentrations of fsTnI would increase more than those of ssTnI following eccentric exercise.

      2. Methods

      Eight men with no upper body resistance training in the prior six months volunteered for this study. The institutional human research ethics committee approved the study, and the study conformed to the Declaration of Helsinki for medical research involving human participants. All volunteers provided written informed consent prior to participation. The mean (±SD) age, weight, and height were 27.5 ± 6.3 years, 67.9 ± 11.0 kg and 1.74 ± 0.08 m, respectively. Each volunteer was asked to refrain from altering his normal diet and not to consume anti-inflammatory medication prior to and during the experimental period. The time of day for the exercise session and testing for each participant was maintained as close as possible with a maximum of 2 h variation due to scheduling.
      Each participant was positioned on an isokinetic dynamometer (Cybex 6000, Ronkonkoma, NY, U.S.A.) with the exercised arm supported at 45° of shoulder flexion on an arm curl bench. Each subject performed a bout of eccentric exercise of the elbow flexors with the exercise protocol consisting of 35 sets of 6 maximal voluntary eccentric contractions from a flexed (60°) to a fully extended position (0°) with a 90 s rest period between sets. The rest between contractions was 12 s with a passive arm movement whilst a lever arm of the dynamometer returned the arm to the flexed position at 10° s−1. Participants were encouraged throughout the eccentric contractions to apply maximal resistance against the lever arm. In addition, each participant was provided with visual feedback and verbal encouragement to maximise torque output for each contraction.
      Maximal voluntary isometric contraction (MVIC) torque of the elbow flexors of the exercised arm was measured at an elbow joint angle of 90° in the same position on the same apparatus as that used for the exercise. Participants were verbally encouraged to perform two maximal contractions, holding each contraction for 4 s with a 30-s rest between contractions. The peak torque of each contraction was obtained, and the higher value was used for further analysis. The measurements were taken before, immediately after, and 1, 2, 3, 4 and 14 days following exercise.
      An 8.5 mL blood sample was drawn from the antecubital vein using a standard venipuncture technique into a serum separating tube (SST II Advance, Becton Dickinson Vacutainer Systems, UK) before, and 1, 2, 3, 4, and 14 days following exercise. The sample was allowed to clot at room temperature for 10 min before being centrifuged for 10 min at 3000 rpm and 4 °C to obtain serum. The serum was separated by pipette into four 1.5 mL aliquots and stored at −80 °C for later analysis.
      Serum creatine kinase (CK) activity was determined by a Hitachi Modular PT automated clinical chemistry analyser (Roche, Germany) with a commercially available Roche Diagnostics Reagent (Mannheim, Germany). The normal reference range using this method is <200 IU L−1. Serum fast (fsTnI) and slow skeletal troponin I isoform (ssTnI) concentrations were determined by arrangement with Quest Diagnostics (California, USA) using enzyme-linked immunosorbent assays. The sensitivity of the method has been previously compared to western blot analysis,
      • Simpson J.A.
      • Labugger R.
      • Collier C.
      • et al.
      Fast and slow skeletal troponin I in serum from patients with various skeletal muscle disorders: a pilot study.
      however disclosure of information pertaining to the exact antibodies and their sensitivities is restricted by commercial confidentiality. The sensitivity of the antibodies was carefully tested and the reliability of each antibody (ssTnI, fsTnI) had been established.
      Changes in the dependent variables from the pre-exercise value were analysed by a Student–Newman–Keuls multiple comparison test. A Pearson's product moment correlation was used to determine the relationship between serum fsTnI or ssTnI concentration and CK activity using data collapsed across all time points between pre and 4 day post-exercise. The significance was set at P < 0.01 and all data are presented as mean ± standard deviation (SD).

      3. Results

      A large variability among participants was observed for their responses to eccentric exercise such that the magnitude of decrease in MVIC torque from pre to 1 day post-exercise ranged from 38–65%, and serum CK activity increased from baseline (109–893 IU L−1) to 517–12,823 IU L−1 4 days post-exercise (Table 1). For the average of the 8 participants, MVIC torque decreased significantly 1 day after exercise (−50%) and remained significantly lower than the baseline at 4 days post-exercise (−34%), but recovered by 14 days post-exercise. Serum CK activity increased significantly after eccentric exercise, was highest 4 days post-exercise, and returned to the baseline by 14 days post-exercise (Fig. 1a ). As shown in Table 1, a large variability in serum fsTnI and ssTnI concentrations among participants was observed for the baseline and the responses to exercise. Fig. 1b shows the average (±SD) changes in serum fsTnI and ssTnI concentrations before and after eccentric exercise. Serum fsTnI increased significantly from baseline (5.5 ± 5.2 ng/mL) at 1 (14.6 ± 12.8 ng/mL) to 4 (89.5 ± 71.4 ng/mL) days following eccentric exercise, and returned to the baseline by 14 days post-exercise (2.6 ± 4.3 ng/mL); however, ssTnI did not change significantly from baseline (13.1 ± 11.8 ng/mL) following exercise (1 day post: 12.6 ± 8.8 ng/mL, 4 day post: 10.4 ± 9.6 ng/mL). A significant correlation (r = 0.80) between serum CK activity and fsTnI concentration was present (Fig. 2).
      Table 1Serum creatine kinase activity (CK), fast skeletal muscle troponin I isoform concentration (fsTnI) and slow skeletal muscle tropnin I isoform concentration (ssTnI) before (pre), and 1 and 4 days after eccentric exercise of the elbow flexors for each participant (1–8).
      VariableParticipantTime
      Pre1 day post4 days post
      CK (IU/L)15477544149
      21092141725
      3416454810
      42428818381
      5311676517
      616780412,823
      72055103065
      889316662364
      fsTnI (ng/mL)114.620.8151.5
      26.737.3157.3
      30.80.812.9
      411.222.8158.1
      50.82.93.3
      62.87.3152.7
      70.84.152.7
      86.520.727.3
      ssTnI (ng/mL)128.921.718.2
      227.223.817.3
      35.88.64.2
      424.723.628.2
      52.34.41.7
      69.89.37.1
      72.94.63.7
      Figure thumbnail gr1
      Fig. 1Mean (±SD) changes in serum creatine kinase (CK) activity (a), fast (fsTnI) and slow (ssTnI) skeletal muscle troponin I concentrations (b) before (pre) and 1, 2, 3, 4, and 14 days following eccentric exercise. *Indicates a significant (p < 0.05) difference from the pre-value.
      Figure thumbnail gr2
      Fig. 2Relationship between serum creatine kinase (CK) activity and fast skeletal muscle troponin I (fsTnI) concentration from the pooled data (pre, 1, 2, 3, and 4 days post).

      4. Discussion

      To the best of our knowledge, this is the first study to report changes in skeletal muscle fibre type-specific troponin I concentrations in serum following eccentric exercise of the elbow flexors. This study presents three novel findings: (1) fsTnI, not ssTnI, increased significantly after eccentric exercise, (2) the time course of changes in fsTnI was similar to that of serum CK activity, and (3) serum CK activity and fsTnI concentration were highly correlated.
      The large and prolonged decreases in MVIC torque and increases in serum CK activity indicate that the eccentric exercise induced muscle damage, which was consistent with previous studies.
      • Nosaka K.
      • Clarkson P.M.
      Changes in indicators of inflammation after eccentric exercise of the elbow flexors.
      • Rama D.
      • Margaritis I.
      • Orsetti A.
      • et al.
      Troponin I immunoenzymometric assays for detection of muscle damage applied to monitoring a triathlon.
      It has been documented that fast-twitch skeletal muscle fibres are more vulnerable to muscle damage induced by eccentric contractions;
      • Lieber R.L.
      • Fridén J.
      Selective damage of fast glycolytic muscle fibres with eccentric contraction of the rabbit tibialis anterior.
      • Macpherson P.C.
      • Schork M.A.
      • Faulkner J.A.
      Contraction-induced injury to single fiber segments from fast and slow muscles of rats by single stretches.
      • Shepstone T.N.
      • Tang J.E.
      • Dallaire S.
      • et al.
      Short-term high- vs. low-velocity isokinetic lengthening training results in greater hypertrophy of the elbow flexors in young men.
      however this was based on histological analyses, which requires an invasive procedure (muscle biopsy). It was assumed that most muscle fibres were recruited during the maximal eccentric contractions.
      • Beltman J.G.M.
      • Sargeant A.J.
      • van Mechelen W.
      • et al.
      Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions.
      Thus, if slow-twitch and fast-twitch muscle fibres were equally susceptible to eccentric contraction-induced muscle damage, we should have seen similar increases in both ssTnI and fsTnI following the eccentric exercise. However, no significant changes in ssTnI were evident following the exercise (Fig. 1b). The susceptibility of fast-twitch fibres to exercise induced injury is also seen in models of statin-induced myopathy.
      • Vassallo J.D.
      • Janovitz E.B.
      • Wescott D.M.
      • et al.
      Biomarkers of drug-induced skeletal muscle injury in the rat: troponin I and myoglobin.
      The significant increases in fsTnI in the absence of an increase in ssTnI suggest that only fast-twitch fibres in the elbow flexor muscles were damaged by the eccentric exercise. It is known that fast-twitch or type II muscle fibres have a shorter optimum length for force generation than slow-twitch or type I muscle fibres.
      • Brockett C.L.
      • Morgan D.L.
      • Gregory J.E.
      • et al.
      Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat.
      Furthermore, the Z-line structure of type II fibres is thinner than that of type I fibres, and the number of M-bridges (3 vs. 5) in type II fibres is less than that of type I fibres.
      • Fridén J.
      • Seger J.
      • Ekblom B.
      Sublethal muscle fibre injuries after high-tension anaerobic exercise.
      • Fridén J.
      • Sjostrom M.
      • Ekblom B.
      Myofibrillar damage following intense eccentric exercise in man.
      It seems that these characteristics of the fast-twitch fibres are associated with their greater susceptibility to eccentric contraction-induced muscle damage.
      Serum CK activity is not specific to skeletal muscle fibres; however, its skeletal muscle isoform (CK-MM) is dominant when increases in CK activity occur in the blood following eccentric exercise of the elbow flexors.
      • Apple F.S.
      • Hellsten Y.
      • Clarkson P.M.
      Early detection of skeletal muscle injury by assay of creatine kinase MM isoforms in serum after acute exercise.
      The time course of changes of CK and fsTnI were similar (Fig. 1), and a high correlation between CK and fsTnI was observed (Fig. 2). It has been documented that CK is released through the plasma membrane due to an increase in membrane permeability;
      • Nosaka K.
      • Clarkson P.M.
      • Apple F.S.
      Time course of serum protein changes after strenuous exercise of the forearm flexors.
      however, because of the large molecular mass of CK (80 kDa), large increases in serum CK (e.g. >1000 IU L−1) are probably the result of muscle fibre necrosis.
      • Newham D.J.
      • McPhail G.
      • Mills K.R.
      • et al.
      Ultrastructural changes after concentric and eccentric contractions of human muscle.
      Considering the molecular weight of TnI (18.5 kDa),
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      • Wilkinson J.M.
      • Grand R.J.A.
      Comparison of amino acid sequence of troponin I from different striated muscles.
      it seems likely that the cause of increases in fsTnI in the blood is similar to that of CK, necrosis of muscle fibres initiated by mechanical damage to plasma membrane, myofilamants, and/or intermediate filaments. CK and sTnI are considered to be large proteins, and Lindena et al.
      • Lindena J.
      • Küpper W.
      • Friedel R.
      • et al.
      Lymphatic transport of cellular enzymes from muscle into the intravascular compartment.
      stated that large proteins released into the interstitial space from cells cannot enter the microvascular endothelium directly but enter the lymphatic vessels before reaching the blood circulation. It is reasonable to assume that CK and fsTnI enter the blood stream via the lymphatic system. This may explain the delayed large increases in CK and fsTnI after eccentric exercise. Although the early increase in fsTnI could be attributed to release from the total soluble pool,
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      the prolonged and delayed increase is more likely related to a secondary disruptive mechanism which causes the degradation of the troponin complex or myofilaments.
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      Large inter-subject variability in changes in serum CK activity and fsTnI concentration were seen in the present study. A large intersubject variability in CK responses following eccentric exercise has been reported in previous studies.
      • Clarkson P.M.
      • Ebbeling C.
      Investigation of serum creatine kinase variability after muscle-damaging exercise.
      • Nosaka K.
      • Clarkson P.M.
      Variability in serum creatine kinase response after eccentric exercise of the elbow flexors.
      Importantly, a subject who had a large CK increase after eccentric exercise also showed a large increase in fsTnI (Fig. 2). Rama et al.
      • Rama D.
      • Margaritis I.
      • Orsetti A.
      • et al.
      Troponin I immunoenzymometric assays for detection of muscle damage applied to monitoring a triathlon.
      reported a strong association between plasma sTnI concentration and CK activity in blood samples of 12 competitors following a triathlon; however, it is possible that the increases in sTnI and CK were due to a combination of myocardial and skeletal muscle damage. Sorichter et al.
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      also showed that total muscle-specific troponin I in plasma correlated positively with CK activity, with peak values occurring 24 h post eccentric exercise of the knee extensors. It should be noted that their assay was not able to distinguish between fast, slow and cardiac TnI. In the present study, serum CK activity and fsTnI concentration were higher 4 days compared with 1 day post-exercise. Because the magnitude of increase in CK in the present study was much greater than that reported by Sorichter et al.,
      • Sorichter S.
      • Mair J.
      • Koller A.
      • et al.
      Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
      the difference in the time course of changes in CK and sTnI between the studies was likely due to differences in the exercise protocol, muscles utilised during exercise and the magnitude of muscle damage. Nevertheless, our results provide further support to the relationship between CK and sTnI, in particular fsTnI.

      5. Conclusions

      In summary, this study reports that eccentric exercise of the elbow flexors causes damage-induced release of fsTnI, the levels correlating with changes in serum CK activity. Adoption of sTnI as a biomarker of muscle damage will allow for better understanding of muscle damage and adaptation induced by eccentric contractions.

      Practical implications

      • The measurements of fsTnI and ssTnI concentrations in the blood can provide information about muscle fibre-specific damage.
      • Exercise practitioners should be aware that maximal eccentric contractions induce muscle damage mainly to fast-twitch muscle fibres.
      • When sTnI analysis is not possible, CK activity in the blood appears to provide similar information to that of sTnI, but muscle fibre specificity cannot be determined by CK.

      Acknowledgements

      The authors declare that there are no conflicts of interests or financial contributions.

      References

        • Fridén J.
        • Lieber R.L.
        Eccentric exercise-induced injuries to contractile and cytoskeletal muscle fibre components.
        Acta Physiol Scand. 2001; 171: 321-326
        • Gibala M.J.
        • MacDougall J.D.
        • Tarnopolsky M.A.
        • et al.
        Changes in human skeletal muscle ultrastructure and force production after acute resistance exercise.
        J Appl Physiol. 1995; 78: 702-708
        • Lauritzen F.
        • Paulsen G.
        • Raastad T.
        • et al.
        Gross ultrastructural changes and necrotic fiber segments in elbow flexor muscles after maximal voluntary eccentric action in humans.
        J Appl Physiol. 2009; 107: 1923-1934
        • Raastad T.
        • Owe S.G.
        • Paulsen G.
        • et al.
        Changes in calpain activity muscle structure, and function after eccentric exercise.
        Med Sci Sports Exerc. 2010; 42: 86-95
        • Nosaka K.
        • Clarkson P.M.
        Changes in indicators of inflammation after eccentric exercise of the elbow flexors.
        Med Sci Sports Exerc. 1996; 28: 953-961
        • Sorichter S.
        • Mair J.
        • Koller A.
        • et al.
        Skeletal muscle troponin I release and magnetic resonance imaging signal intensity changes after eccentric exercise-induced skeletal muscle injury.
        Clin Chim Acta. 1997; 262: 139
        • Foley J.M.
        • Jayaraman R.C.
        • Prior B.M.
        • et al.
        MR measurements of muscle damage and adaptation after eccentric exercise.
        J Appl Physiol. 1999; 87: 2311-2318
        • Nosaka K.
        • Clarkson P.M.
        • Apple F.S.
        Time course of serum protein changes after strenuous exercise of the forearm flexors.
        J Lab Clin Med. 1992; 119: 183-188
        • Sorichter S.
        • Puschendorf B.
        • Mair J.
        Skeletal muscle injury induced by eccentric muscle action: muscle proteins as markers of muscle fiber injury.
        Exerc Immunol Rev. 1999; 5: 5-21
        • Ebbeling C.B.
        • Clarkson P.M.
        Exercise-induced muscle damage and adaptation.
        Sports Med. 1989; 7: 207-234
        • Lieber R.L.
        • Fridén J.
        Selective damage of fast glycolytic muscle fibres with eccentric contraction of the rabbit tibialis anterior.
        Acta Physiol Scand. 1988; 133: 587-588
        • Macpherson P.C.
        • Schork M.A.
        • Faulkner J.A.
        Contraction-induced injury to single fiber segments from fast and slow muscles of rats by single stretches.
        Am J Physiol: Cell Physiol. 1996; 271: C1438-C1446
        • Shepstone T.N.
        • Tang J.E.
        • Dallaire S.
        • et al.
        Short-term high- vs. low-velocity isokinetic lengthening training results in greater hypertrophy of the elbow flexors in young men.
        J Appl Physiol. 2005; 98: 1768-1776
        • Sorichter S.
        • Mair J.
        • Koller A.
        • et al.
        Skeletal troponin I as a marker of exercise-induced muscle damage.
        J Appl Physiol. 1997; 83: 1076-1082
        • Wilkinson J.M.
        • Grand R.J.A.
        Comparison of amino acid sequence of troponin I from different striated muscles.
        Nature. 1978; 271: 31-35
        • Mair J.
        • Thome-Kromer B.
        • Wagner I.
        • et al.
        Concentration time courses of troponin and myosin subunits after acute myocardial infarction.
        Coron Artery Dis. 1994; 5: 865-872
        • Johnson M.A.
        • Polgar J.
        • Weightman D.
        • et al.
        Data on the distribution of fibre types in thirty-six human muscles: An autopsy study.
        J Neurol Sci. 1973; 18: 111-129
        • Simpson J.A.
        • Labugger R.
        • Collier C.
        • et al.
        Fast and slow skeletal troponin I in serum from patients with various skeletal muscle disorders: a pilot study.
        Clin Chem. 2005; 51: 966-972
        • Simpson J.A.
        • Labugger R.
        • Hesketh G.G.
        • et al.
        Differential detection of skeletal troponin I isoforms in serum of a patient with rhabdomyolysis: markers of muscle injury?.
        Clin Chem. 2002; 48: 1112-1114
        • Rama D.
        • Margaritis I.
        • Orsetti A.
        • et al.
        Troponin I immunoenzymometric assays for detection of muscle damage applied to monitoring a triathlon.
        Clin Chem. 1996; 42: 2033-2035
        • Beltman J.G.M.
        • Sargeant A.J.
        • van Mechelen W.
        • et al.
        Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions.
        J Appl Physiol. 2004; 97: 619-626
        • Brockett C.L.
        • Morgan D.L.
        • Gregory J.E.
        • et al.
        Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat.
        J Appl Physiol. 2002; 92: 1104-1110
        • Fridén J.
        • Seger J.
        • Ekblom B.
        Sublethal muscle fibre injuries after high-tension anaerobic exercise.
        Eur J Appl Physiol. 1988; 57: 360
        • Fridén J.
        • Sjostrom M.
        • Ekblom B.
        Myofibrillar damage following intense eccentric exercise in man.
        Int J Sports Med. 1983; 4: 170-176
        • Vassallo J.D.
        • Janovitz E.B.
        • Wescott D.M.
        • et al.
        Biomarkers of drug-induced skeletal muscle injury in the rat: troponin I and myoglobin.
        Toxicol Sci. 2009; 111: 402-412
        • Apple F.S.
        • Hellsten Y.
        • Clarkson P.M.
        Early detection of skeletal muscle injury by assay of creatine kinase MM isoforms in serum after acute exercise.
        Clin Chem. 1988; 34: 1102-1104
        • Newham D.J.
        • McPhail G.
        • Mills K.R.
        • et al.
        Ultrastructural changes after concentric and eccentric contractions of human muscle.
        J Neurol Sci. 1983; 61: 109-122
        • Lindena J.
        • Küpper W.
        • Friedel R.
        • et al.
        Lymphatic transport of cellular enzymes from muscle into the intravascular compartment.
        Enzyme. 1979; 24: 120-131
        • Clarkson P.M.
        • Ebbeling C.
        Investigation of serum creatine kinase variability after muscle-damaging exercise.
        Clin Sci. 1988; 75: 257-261
        • Nosaka K.
        • Clarkson P.M.
        Variability in serum creatine kinase response after eccentric exercise of the elbow flexors.
        Int J Sports Med. 1996; 17: 120-127