european college of sport science

OP-PH17 Physiology 17
ing-Sorensen muscular endurance test with 15 degrees forward bend was used: NIRS and EMG were recorded bilaterally from the center
of the erector spinae muscle at the level L3.
For analysis of the NIRS data, least square regression lines were computed and inflection points between different slopes were identified.
For analysis of the EMG the median frequency (MDF) of the power spectrum was computed by the Fast Fourier Transform on sequential
epochs. Additionally, we used a novel nonlinear approach by tracing the changes of the instantaneous frequency with maximum power
(MPF) in the Morlet wavelet based time-frequency distribution (mTFD).
Results: An identical pattern was found in all NIRS and EMG measurements. Muscle oxygenation showed an initial rapid decrease (RD)
followed by a long lasting plateau (LP). During the RD phase both MDF and MPF remained constant on a high frequency level. During the
LP phase both MDF and MPF showed a linear decrease, mostly beginning at half time of the entire contraction. The time ratio of RD/LP
was significantly correlated to the frequency decrease.
Discussion: Our results contradict studies reporting coincidence and correlation of the decay of oxygenation during the RD phase and
decrease of MDF in the EMG (1). In our study, during this first phase (RD) the decay of oxygenation was not correlated with myoelectrical
activity. Only during the second LP phase of the fatiguing isometric contractions the EMG power spectrum shifted to lower frequencies.
Although the local tissue oxygen content now remained constant, the developing fatigue was characterized by this myoelectrical alteration.
Thus, we strongly recommend the use of nonlinear time series analysis, e.g. wavelet power spectrum, for tracking the instantaneous
frequency changes in the EMG.
References
1) Felici F, Quaresima V, Fattorini L, Sbriccoli P, Filligoi GC, Ferrari M. J Electromyogr Kinesiol 2009; 19(2), e1-e11
EFFECTS OF INCREASED EXTRACELLULAR POTASSIUM AND LACTIC ACID ON DYNAMIC MUSCLE CONTRACTIONS IN
ISOLATED RAT SOLEUS MUSCLES
OVERGAARD, K.
UNIVERSITY OF AARHUS
Introduction: It is well known that in muscles, increased extracellular potassium concentration ([K+]o) reduces excitability and isometric
force. [K+]o is, therefore, considered an important factor in muscle fatigue. The effects of [K+]o on dynamic muscle function parameters
such as maximal power or maximal unloaded velocity (Vo) are, however, not extensively studied. Knuth et al. (2006) observed that a
reduction in muscle fiber pH to 6.2 gave a larger decrease in maximal power than expected from the loss of isometric force. This was
explained by pH affecting maximal unloaded velocity (Vo) and the curvature of the FV relation. In contrast, the effects of high [K+]o on
muscle isometric force and excitability have been shown to be alleviated by lowering pH with lactic acid (LA) (Nielsen et al., 2001). The
present study was undertaken to investigate the effects of [K+]o and combinations of [K+]o and LA on dynamic muscle function.
Methods: Soleus muscles from 4 wk old Wistar rats were isolated and incubated in standard K.R. buffer at 30°C initially containing 4 mM
K+ and equilibrated with 95% O2/5% CO2 (pH 7.4). Muscles were mounted on a force/length controlled dynamometer (305B, Aurora
inc., Canada) at Lo. Muscles were stimulated every 10 min with 1.5 sec pulse trains (60 Hz) to obtain tetanic activation. Muscles were held
isometric until force was fully developed, where after the resistance was reduced to preset force levels in brief steps (0.2s sec) and length
changes were recorded over time. Data points for force and velocity were used to construct FV curves by fitting to the Hill equation.
Maximal power was calculated from the Hill plots. During the experiment [K+]o was increased from 4 to 9 mM and subsequently 20 mM
LA was added (reducing buffer pH to 6.9).
Results: Increasing [K+]o from 4 to 9 mM reduced maximal power by 64±7% (mean±sd), which was significantly more than the 50±10%
reduction seen in isometric force (P0.05). When 20 mM LA was added to the 9 mM K+ buffer, the isometric force and
maximal power recovered completely to the level initially obtained in 4 mM K+. LA did not significantly affect Vo.
Discussion: Maximal power was substantially decreased by increasing [K+]o and this decrease was relatively larger than the reduction in
isometric force. Since Vo was not reduced by 9 mM K+ the relatively large reduction in power may be due to an increased curvature in
the FV relation. The effect of 20 mM LA added to muscles in 9 mM K+ was a complete recovery of dynamic contractile function. At the
concentrations used here, LA protects dynamic muscle function against the deleterious effects of high [K+]o, and no fatiguing effects of LA
were found in muscles contracting dynamically.
References
Nielsen OB, de Paoli FV, Overgaard, K. (2001). J Physiol, 536, 161-166.
Knuth ST, Dave H, Peters JR, Fitts RH. (2006) J Physiol, 575, 887-899.
TRAINING INDUCED CHANGES IN TENDON MECHANICAL AND MATERIAL PROPERTIES ARE RELATED TO MUSCLE
HYPERTROPHY, NOT TO STRENGTH GAINS.
SEYNNES, O.R., ERSKINE, R.M., MAGANARIS, C.N., NARICI, M.V.
MANCHESTER METROPOLITAN UNIVERSITY
The design of tendinous structures determines their interaction with the muscle (2) and the coordination of the changes occurring in both
structures is critical to the preservation of muscle-tendon unit function. To get a better understanding of the effect of chronic overloading
on human tendon, we investigated the relationships between the structural and mechanical adaptations of the patellar tendon and of
the quadriceps femoris muscle in response to strength training.
Fifteen healthy male subjects (20 ± 2 yrs) underwent 9 weeks of knee extension resistance training. Tendon load-deformation properties
were measured with ultrasonography during a ramp maximal isometric contraction, and tendon cross-sectional area (CSA) was determined
along the entire length of the tendon by using magnetic resonance imaging (MRI). Maximal isometric force was measured at the
optimal knee joint angle of force production. The physiological CSA (PCSA) of the quadriceps femoris muscle group was obtained by
combining measurements of muscle volume (MRI) and architecture (ultrasound).
Following training, muscle force and PCSA increased by 31% (P < 0.0001) and 7% (P < 0.01), respectively. Increases in tendon CSA were
observed in regions corresponding to 20%, 30%, 60%, 90% and 100% of the tendon length (5-6%, P < 0.05), and tendon stiffness and
modulus increased by 24% (P < 0.001) and 20% (P < 0.01), respectively. None of the changes in tendon mechanical properties or CSA
were related to the increase in maximal force. However, we observed a positive correlation between the increase in quadriceps PCSA
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