Oxygen consumption of gastrocnemius medialis muscle during submaximal voluntary isometric contractions with and without preceding stretch

www.nature.com/scientificreports OPEN Received: 30 December 2016 Accepted: 9 May 2017 Published: xx xx xxxx Oxygen consumption of gastrocnemius medialis muscle during submaximal voluntary isometric contractions with and without preceding stretch F. K. Paternoster1, D. Hahn2,3, F. Stöcker1, A. Schwirtz1 & W. Seiberl 1 After an active muscle stretch, maintaining a certain amount of force in the following isometric phase is accompanied by less muscle activation compared to an isometric contraction without preceding active stretch at the corresponding muscle length. This reduced muscle activation might be related to reduced metabolic costs, such as the oxidative metabolism. Hence, the aim of this study was to clarify if mechanisms associated with stretch-induced activation reduction (AR) also influence oxygen consumption of voluntary activated human muscles after active stretch. Plantarflexion torque of 20 subjects was measured during 1) purely isometric and 2) active stretch contractions (26°, 60°/s), at a submaximal torque level of 30% MVC. Oxygen consumption (mV·O2) of gastrocnemius medialis (GM) was estimated by near-infrared spectroscopy while applying arterial occlusion. Since the overall group did not show AR at GM after active stretch (p > 0.19), a subgroup was defined (n = 10) showing AR of 13.0 ± 10.3% (p = 0.00). However, for both purely isometric and active contractions mV·O2 was the same (p = 0.32). Therefore, AR triggered by active stretch did not affect mV·O2 of active human muscle. In 1952, Abbott and Aubert made experiments on the toad sartorius muscle and found enhanced forces in the isometric phase after active stretch compared to purely isometric contractions. The final isometric muscle length as well as the muscle stimulus intensity were the same for the two conditions1. This finding of the so-called “residual force enhancement” (RFE) provided the basis for a continual research. Despite the growing number of studies, the underlying mechanism(s) of RFE is (are) still unknown2–5. From a phenomenological perspective the muscular feature is known to be independent of stretch velocity6 but sensitive to the stretch amplitude1, 7. RFE has been proven for the entire force-length relationship8, 9 and has been verified in in vivo studies for maximum voluntary10–12 and submaximal voluntary contractions13–16, for small10 and large human muscles17 as well as for multi-joint movements11, 14, 15. Beside the aforementioned lack of knowledge regarding the origin of RFE, especially studies on humans performing voluntary contractions reported a discrepancy between the total number of subjects involved in a study and those who showed enhanced forces after active stretch, referred to as responder vs. non-responder phenomenon15, 18, 19. In 2005, the definition of RFE was extended to submaximal contractions while keeping the applied force constant20. In 2005, Oskouei and Herzog found changes in the EMG signal in the isometric phase after active stretch, compared to pure isometric contractions at the same muscle length, resulting in a lower EMG signal for the isometric contraction preceded by an active stretch. Also in later work, this enhanced neuromuscular efficiency was assumed to be beneficial in terms of reduced metabolic cost during muscle contraction after active stretch13, 21. However, only one study directly tested this hypothesis of reduced metabolic cost after active stretch on a muscle fibre level22. These authors demonstrated a reduction in the ATPase activity per unit force for the isometric contraction after active stretch, compared to the purely isometric contraction for a skinned fibre from rabbit 1 Biomechanics in Sports, Technical University of Munich, Munich, Germany. 2Human Movement Science, RuhrUniversity Bochum, Bochum, Germany. 3School of Human Movement and Nutrition Sciences, University of Queensland, Brisbane, Australia. Correspondence and requests for materials should be addressed to F.K.P. (email: ) Scientific Reports | 7: 4674 | DOI:10.1038/s41598-017-04068-y 1 www.nature.com/scientificreports/ Figure 1. Schematic timeline of the performed experiment, measuring plantar-flexion torque. After the subject preparation, the participant did two maximum voluntary contractions (3–5 s) at the starting (13.3 ± 0.4° dorsiflexion) and reference position (13.0 ± 0.4° plantar flexion) in randomized order. These contractions were followed by either isometric (13.0 ± 0.4° plantar flexion) or active stretch contractions (Range of motion: 26.3 ± 0.4°. Angular velocity: 60°s−1) executed in randomized order with an activation level of 30% MVC. Rest between the different contractions was set to a minimum of 3 minutes30. psoas muscle. They suggested that the average force per cross bridge or the engagement of a passive structure is responsible for this enhanced fibre efficiency. However, it is unclear if these results can be transferred to a complex system like in vivo human muscle contraction. As one of the main methods of energy production in skeletal muscle is the oxidative metabolism, enhanced economisation triggered by active stretch contraction would positively influence the energy demands inside the muscle. A possibility to monitor processes of muscle metabolism non-invasively is provided by the use of near-infrared spectroscopy systems (NIRS). Such optical measurement systems are used in a variety of different settings, ranging from isometric to dynamic muscle contractions measuring variant muscles23–25. One of the first experiments calculating muscle oxygen consumption (mV̇ O2) using light at different wavelength were done by Millikan26 on the cat soleus. These days, using NIRS devices in combination with arterial occlusion enables the possibility to estimate mV̇ O2 in active in vivo muscles27–29. Little is known regarding in vivo muscle oxygen consumption within the field of residual force enhancement and it is unclear if the rare information about metabolic benefits of RFE can be transferred to human muscle action. Therefore, the purpose of this study was to estimate oxygen consumption of gastrocnemius medialis (GM) during isometric, submaximal plantar flexion, with and without a preceding active stretch. The submaximal contractions were chosen because most everyday movements are based on non-maximal efforts. Methods Subjects. Twenty healthy male subjects (29 ± 4 y, 80 ± 7 kg, 183 ± 6 cm) with no history of ankle joint injuries or neurological disorders participated in the study. The subjects had a mean adipose tissue thickness of 4.0 ± 1.9 mm at the gastrocnemius medialis. The study was approved by the local Ethics Committee of the Technical University of Munich and conducted according to the declaration of Helsinki. Subjects voluntarily participated and gave written informed consent. Experimental Protocol. Previous to the experiment, subjects performed a training session to become familiar with the testing setup and procedure, especially to (...truncated)