Force : Velocity Properties of Mouse Muscles

Abstract

THE force : velocity properties of extensor digitorum longus (EDL) and soleus (SOL) muscles of the mouse have been determined at birth and four weeks after birth. The methods and recording equipment used in this work have been described in detail elsewhere¹. Contractions of muscles in four-week-old animals were recorded in vivo; the anaesthetic was 100 mg pentobarbital sodium/kg body-weight injected intraperitoneally followed by about 40 per cent of the initial dose every hour. Neonatal muscles were examined in vitro; in these conditions the average decline in the maximum isometric tetanic tension (P₀) in the course of a series of measurements lasting several hours was only 7 per cent for EDL and 4 per cent for SOL. The speed of shortening was determined from distance : time curves of after-loaded isotonic tetanic contractions. These contractions were recorded with light levers during repetitive indirect stimulation at the optimal frequency (fo). The equivalent masses of the levers were 22 mg and 114 mg for muscles from neonatal and four-week-old animals respectively. All measurements were made with the muscles at 35° C to 35.5° C. Curves describing the relationship between the speed of shortening and the load were fitted to the data for each muscle by the method of least squares using Hill's equation² ((P + a)V = b(P₀ − P), where P₀ = maximum isometric tetanic tension, P = load, V = speed of shortening, and a and b are constants). The anatomical arrangement of tendon and muscle fibres in mouse EDL and SOL is the same as that described for rat muscles¹.