Nonlinear relationship between heat production and force during voluntary contractions in humans

The rate of temperature rise (dT/dt) in the vastus lateralis muscle of seven subjects was measured at four to five locations in each muscle during voluntary isometric contractions ranging from 10 to 90% of maximal voluntary contraction (MVC) force. dT/dt increased from 3.1 +/- 1.1 mK/s at 10% MVC to 14.5 +/- 1.3 mK/s at 90% MVC. In the typical subject, the increase in dT/dt with force was markedly higher between 30 and 70% MVC than in the upper and lower force ranges. The estimated ratio between heat rate in active muscle and force was six times higher at 10% MVC than at 90% MVC, indicating a markedly increasing economy of contraction with increasing force. The lower contraction economy at low forces may be explained by an increased rate of energy turnover associated with force generation and relaxation when motor units are contracting at low firing rates. Hence, we argue that recruitment and rate coding may have a profound effect on the economy of contraction at different force levels.