Griaučių raumenų jėgos depresijos po raumens sutrumpėjimo priklausomybė nuo nuovargio, postaktyvacinės potenciacijos ir atlikto darbo

It has been established that the steady-state isometric force following the muscle shortening of an activated muscle is smaller than the corresponding steady-state force obtained for a purely isometric contraction (Abbott, Aubert, 1952; Till et al., 2014). This phenomenon is commonly referred to as residual force depression (rFD) (Till et al., 2014). It is well established that rFD after muscle shortening increases with incremental shortening magnitudes (Herzog et al., 1998; Van Noten, Van Leemputte, 2009; 2011), increasing shortening force (De Ruiter et al., 1998), decreases with increasing shortening speeds (Van Noten, Van Leemputte, 2011), and incremental mechanical work done by the muscle during shortening (Kosterina et al. 2009; Lee, Herzog 2009). Also, rFD following shortening can be abolished instantaneously by deactivating the muscle just long enough for force to drop to zero (Herzog, Leonard, 1997). Although the phenomenon of rFD was introduced more than 60 years ago (Abbott, Aubert, 1952), there is still not a single unified definition of the term to this day. Several theories have been proposed as explanations for the underlying mechanism of rFD: (1) sarcomere non-uniformity (Edman et al., 1993); (2) reduced Ca2+ release from the sarcoplasmic reticulum immediately after muscle contraction (Edman, 1996) (3) increase in proton and inorganic phosphate concentration (Granzier, Pollack, 1989) (4) stress-induced cross-bridge inhibition (Maréchal, Plaghki, 1979). This data provides an opportunity for new studies to determine how rFD could be affected post activation potentiation (PAP) and low-frequency fatigue (LFF). It is a common fact that a short, intense exercise can trigger PAP and muscle fatigue. The most likely explanation for PAP is the phosphorylation of myosin regulatory light chains during the conditioning contractions that increase sensitivity of actin-myosin to Ca2+ released from sarcoplasmic reticulum (Palmer, Moore, 1987; McIntosh, 2003). Previous research has shown that PAP could be caused not only after a maximum voluntary isometric contraction or tetanic electrical stimulation (Sale, 2002), but also after concentric or eccentric contractions (Baudry, Duchateau, 2004). Therefore we hypothesize that rFD could depend on PAP. LFF is often observed after voluntary contractions, at both low-frequency (10 and 20 Hz), and high-frequency (50 Hz and higher) electrical stimulation (Dundon et al., 2008). It was first described by Edwards and colleagues (1977) and is characterised by a proportionate loss of force in response to low-versus high-frequency muscle stimulation. It is determined that the decrease of LLF is due to contractile activation and peripheral electromechanical communication disorder caused by the loss of Ca2+ released from the sarcoplasmic reticulum. The mechanisms for determining the LFF may be Na+ channel inactivation (Allen, 2005), reactive oxygen species, which reduces the depolarisation (Moopanar, Allen, 2005). It is well known that rFD is dependent on the extent of muscle force (De Ruiter et al., 1998), so we hypothesise that rFD may depend on LFF. The daily movements of the muscles are activated at low frequencies (5 - 30 Hz) (De Ruiter et al., 2003), so the answer to the question whether the LFF may affect FD is very significant. The purpose of this research was to analise the rFD relationship between muscle fatigue and PAP. Objectives: 1. To determine the effects of shortening-induced rFD in electrically activated (20 and 50 Hz) and maximal voluntary contractions human quadriceps muscle. 2. To assess the effects of muscle fatigue on rFD by stimulating the quadriceps muscle of the low (20 Hz) and high (50 Hz) stimulation frequencies, and maximal voluntary contractions. 3. To investigate the influence between rFD, mechanical work speed and of muscle shortening in fatigued muscle. 4. To examine if PAP could influence shortening-induced rFD and mechanical work. CONCLUSIONS 1. rFD at high (50 Hz) stimulation frequency and MVC were smaller than at low frequency rFD. 2. rFD depends on low-frequency electrical stimulation, but not from high and MVC after eccentric exercise of human quadriceps muscle. 3. rFD after muscle shortening depends on mechanical work during muscle shortening, but not from the speed of shortening in fresh and fatigued muscle. 4. rFD depends on PAP and mechanical work during muscle shortening