Blood pressure, heart rate and EMG in low level static contractions

The purposes of the present investigation were: to evaluate a possible use of changes in the mean spectral frequency (MSF) of the EMG power spectra as a measure of reflex cardiovascular responses originating from the muscles during static exercise; and to study the relation between muscle fibre composition, EMG, and the cardiovascular response. Heart rate (HR), arterial blood pressure (BP), myoelectric signal (EMG), and intramuscular temperature (Tm) were measured during prolonged static contractions in five healthy male subjects (25-44 yrs). Two studies were performed. In study I constant EMG contraction muscle force in the first 5 s of the knee-extensor contraction was set to 20% MVC, (maximal voluntary contraction), and in the rest of the 5 min contraction the myoelectric signal was kept constant by visual feedback from an oscilloscope. In study II, (constant force contraction) two 1 h 7% MVC isometric contractions of the elbow-flexors and extensors were performed on two separate days. During the 5 min constant EMG contraction, the force fell from 20 to 11% MVC, mean BP (MAP) increased from 97 +/- 5 to 120 +/- 4 mmHg (P less than 0.01), and the EMG MSF decreased from 87 +/- 16 to 66 +/- 9 Hz (P less than 0.01). The decrease in MSF was strongly correlated to the increase in MAP (r = 0.96, P less than 0.01). The intramuscular temperature showed a small increase from 34.3 degrees C to 35.3 degrees C (P less than 0.01). During the 1 h constant force contraction involving m. triceps, MAP increased from 104 +/- 10 to 120 +/- 12 mmHg, with a simultaneous decrease in the EMG MSF from 96 +/- 11 to 70 +/- 19 Hz and an increase in the EMG amplitude (247% of the initial value). In the contractions involving m. biceps, however, both MAP and EMG MSF remained almost unchanged, but EMG amplitude increased (197% of the initial value). Very modest changes in HR were observed: 63 +/- 6 to 66 +/- 6 beats min-1 and 61 +/- 5 to 59 +/- 7 beats min-1 in the contractions involving m. triceps and m. biceps, respectively. The intramuscular temperature increased simultaneously, 1.3 degrees C and 0.7 degrees C in m. triceps and m. biceps, respectively. The results from the constant EMG contractions indicate the existence of a common 'trigger' for both the increase in BP and the decrease in EMG MSF; and the extracellular [K] is put forward as a candidate.(ABSTRACT TRUNCATED AT 400 WORDS)