The effects of dietary creatine supplements on the contractile properties of rat soleus and extensor digitorum longus muscles

Daily creatine supplements (0.258 g kg(-1)) were administered to adult male Wistar rats (n = 7) in the drinking water. Age matched rats (n = 6) acted as controls. After 5-6 days, contractile properties were examined in solens and extensor digitorum longus (EDL) muscle strips in vitro at 30 degreesC. In soleus muscles, creatine supplements decreased the half-relaxation time of the isometric twitch from 53.6 +/- 4.3 ms in control muscles to 48.4 +/ - 5.5 ms but had no effect on twitch or tetanic tension or on twitch contra ction time. In EDL muscles twitch tension, tetanic tension, twitch contract ion and half-relaxation times were ah unaffected by creatine supplements. C reatine supplements increased the fatigue resistance of the soleus muscles but had no effect on that of the EDL muscles. After a 5 min low-frequency f atigue test, tension (expressed as a percentage of initial tension) was 56 +/- 3% in control soleus muscles, whereas that in the creatine-supplemented muscles was 78 +/- 6% (P < 0.01). In the EDL muscles, the corresponding va lues were 40 <plus/minus> 2% and 41 +/- 9%, respectively. The force potenti ation which occurred in the EDL muscles during the initial 20-30 s of the f atigue test was 170 +/- 10 % of initial tension in the control muscles 24 s after the initial stimulus train but was reduced (P < 0.01) to 130 <plus/m inus> 20% in the creatine-supplemented muscles. In conclusion, soleus muscl e endurance was increased by creatine supplements. EDL endurance was unaffe cted but force potentiation during repetitive stimulation was decreased.