Isometric Contraction of Arm Flexor Muscles as a Method of Evaluating Cardiac Vagal Tone in Man

Abstract

1. We have previously shown that brief voluntary isometric contractions of upper arm flexor muscles performed for one respiratory cycle elicit a significant decrease in the R-R interval. The present study was designed to determine if similar changes are produced by non-voluntary electrically evoked contractions and, if so, to establish the consistency and repeatability of the associated changes in the R-R interval. 2. The heart rate (R-R interval) response to voluntary or non-voluntary brief isometric contraction equivalent to 40% of the maximum voluntary contraction was studied in 10 healthy young male subjects during controlled ventilation at supine rest. 3. The absolute values of R-R intervals occurring in any one of 10 arbitrary phases of a respiratory cycle were measured and plotted by a computer. 4. Both voluntary and non-voluntary contractions elicited similar changes in heart rate and R-R interval, which were greater during expiration than during inspiration. 5. This confirms our previous finding that the magnitude of the R-R interval changes, with brief isometric contraction, is positively related to the degree of cardiac vagal tone. 6. Analysis of the variability between repeated tests initiated in either inspiration or expiration revealed that there was significantly less variability with the electrically induced contraction. 7. It was concluded that electrically induced contractions of 40% maximal voluntary contraction are a viable alternative to voluntary contractions and provide a more controllable means of measuring cardiac vagal withdrawal.

GABAergic mechanisms involved in the vagally mediated heart rate response to muscle contraction as revealed by studies with benzodiazepines

“…It is known from previous studies that muscle contraction in man causes an immediate withdrawal of vagal influence on the chronotropic control of the heart, since it is blocked by atropine [12,15,19]. This effect has been previously exploited in our laboratory, in order to look at the actions of 60 % MVC muscle contractions upon cardiac vagal tone, in the absence of respiratory variations in cardiac vagal outflow [4].…”

“…Respiratory movements were recorded from a strain gauge placed around the chest at the level of the base of the sternum. In addition isometric force produced by contractions of the elbow flexors was measured using a strain gauge with the arm held firmly in place in a frame so that the forearm was at right angles to the upper arm as described previously [2,4].…”

Variabilidade da frequência cardíaca em mulheres com hipermobilidade articular

“…Estudos demonstram que tanto a contração, como o alongamento do músculo esquelético, ativa fibras nervosas aferentes, no indivíduo normal, ocasionando um aumento da frequência cardíaca (FC) por diminuição do tônus vagal cardíaco, levando assim a uma diminuição da VFC 19 .…”

Influence of different characteristics of sport on heart rate recovery in elite athletes

“…Pokazano je da grupa izdržljivosti ima značajno niže vrednosti srčane frekvencije u miru u odnosu na preostale tri grupe. S obzirom na niže vrednosti srčane frekvencije u miru, a bez značajne razlike u maksimalnoj srčanoj frekvenciji, sportisti koji vežbaju izdržljivost imaju veći raspon za povećanje u toku fizičke aktivnosti, označen kao rezerva srčane frekvencije (24). Osim toga, oporavak srčane frekvencije u prvom minutu nakon maksimalnog fizičkog opterećenja značajno je brži u grupi izdržljivosti u odnosu na preostale tri grupe.…”