European Journal of Applied Physiology and Occupational Physiology

, Volume 60, Issue 5, pp 346-352

First online:

Twitch contractile adaptations are not dependent on the intensity of isometric exercise in the human triceps surae

  • Stephen E. AlwayAffiliated withSchool of Health, Physical Education and Recreation, The Ohio State University
  • , Digby G. SaleAffiliated withDepartments of Medicine and Physical Education, McMaster University
  • , J. Duncan MacDougallAffiliated withDepartments of Medicine and Physical Education, McMaster University

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Ultrastructural and twitch contractile characteristics of the human triceps surae were determined in six healthy but very sedentary subjects before and after 16 weeks of isometric training at 30% maximal voluntary contraction (MVC). Following training, twitch contraction time was approximately 16% shorter, although no differences were observed in one-half relaxation time or peak twitch torque. Percent fibre type was not changed by training. The mean area of type I and type II fibres in the soleus increased by approximately 30% but only type II fibres showed an increase in area in the lateral gastrocnemius (30%). Despite such changes in fibre area the volume density of the sarcoplasmic reticulum-transverse tubular network averaged 3.2 ± 0.6% and 5.9 ± 0.9% in type I and type II fibres respectively, before and after training in the two heads of the gastrocnemius. The results indicate that contractile adaptations to isometric training at 30% MVC were limited to twitch contraction time and were not directly related to changes in percent fibre distribution or the volume of sarcoplasmic reticulum and transverse tubules in either type I or type II fibres. The data further demonstrate that substantial fibre hypertrophy is achieved by training with low-intensity contractions.

Key words

Skeletal muscle Isometric training Contactile properties Sarcoplasmic reticulum