The effects of two forms of isometric training on the mechanical properties of the triceps surae in man.

The training effects of rhythmic and sustained isometric contractions on the contractile characteristics of the triceps surae have been investigated in four healthy subjects over a period of 8 weeks. One leg (ST) of each subject was trained by performing repeated daily sustained (1 min) isometric contractions at 30% of maximal voluntary contraction force (MVC), and the other using rhythmic isometric contractions at 100% of MVC. The protocol was so arranged that the total area under the training force/time curves was the same for each limb. Electrically evoked maximal twitch (Pt) and tetanic (Po) tensions of the triceps surae were measured weekly on both legs using a standard procedure. The results showed that the rhythmic regime increased the MVC at the rate of 5.5% per week and the sustained training increased MVC at the rate of 3.3% per week. Twitch and tetanic tensions were not altered by either regime. However, only training using sustained contractions produced a progressive increase in endurance as measured by performance in a fatigue test. The results suggest that increases in voluntary strength resulting from short term isometric training are not necessarily associated with a rise in the intrinsic involuntary capacity of muscle fibres to generate force.