An increase in muscle conduction velocity (CV), which has been shown to increase with muscle temperature (Jarcho et al. 1954), could be one of the factors determining the improvement in muscle performance following a rise in temperature. CV has been shown to correlate to the median frequency (MDF) of the surface EMG (sEMG) power spectrum (Lindstrom et al. 1970). The aim of this study was to observe the effect of increasing quadriceps muscle temperature on maximum voluntary contraction (MVC), peak power output and sEMG variables.
With ethics committee approval, eight volunteers (two female) (mean ± S.D.; age 22 ± 4 years; mass 72.5 ± 9.9 kg; height 1.73 ± 0.06 m) completed two trials (counter-balanced) on the same day, one control (CO), and the other preceded by an active cycling warm-up (WU) at 70 % ventilatory threshold (ventilatory equivalent method). Quadriceps muscle temperature, measured continuously from vastus lateralis with a flexible thermistor (Ellab, UK), was 33.8 ± 0.4 °C in CO compared with 36.8 ± 0.5 °C in WU (P < 0.05). Aural temperature, measured by an infrared tympanic thermistor (Braun, type 6013, Germany), was not different between conditions. Experimental trials consisted of three knee-extension MVCs (Kin-Com, USA) at a 90 deg angle with simultaneous recording of sEMG from the vastus lateralis, followed by three squat jumps performed on a force platform (Kistler, Switzerland). sEMGs were analysed in the frequency domain as MDF and in the time domain as root mean square (RMS). All data were compared using a Student’s paired t test, with an α level of 0.05.
MDF was 59.2 ± 14.1 Hz in CO compared with 67.2 ± 11.8 Hz in WU (P < 0.05), while RMS was lower in CO compared with WU (0.65 ± 0.28 vs. 0.56 ± 0.19 V, respectively; P < 0.05). MVC was not different (465.7 ± 107.6 vs. 490.1 ± 117.2 N), whilst peak power output during the squat jump was significantly higher in the WU trial (3324 ± 866 vs. 3569 ± 919 W; P < 0.05).
These data show MDF to be altered with increased temperature, which would relate to a greater CV. This may translate into faster activation of individual muscle fibres, thus partly explaining the increase in power output. The increased MDF and the decreased RMS in WU may reflect a smaller amplitude and shorter duration of the action potential, possibly because of a decreased time for Na+ diffusion into the cell (Rutkove, 2001).
All procedures accord with current local guidelines.