Muscle activation and its distribution within human triceps surae muscles

Abstract

The purposes of this study were 1) to quantify the volume of activated parts within a whole muscle and 2) to examine activated area distributions along the length of muscle. Seven male subjects performed five sets of 10 repetitions of a single-leg calf-raise exercise with the knee fully extended. Transverse relaxation time (T2)-weighted spin echo images were acquired before and immediately after the exercise. A range of pixels with a T2 greater than the mean +1 SD of the region of interest (ROI) from the preexercise image and pixels with a T2 lower than the mean + SD of the ROI from the postexercise image were defined as "active" muscle. The active muscle images were three dimensionally reconstructed, from which the volume of the activated muscle was determined for individual triceps surae (TS) muscles. Our data indicate that approximately 46% of the medial gastrocnemius (MG) muscle was activated during the exercise, with activation of the lateral gastrocnemius (LG) and soleus (Sol) muscles being approximately 35%. In the MG, distal portions had a greater percentage area of activated muscle than the proximal portions (P < 0.05), which was consistent with the results regarding electromyogram activity. In contrast, regional activation differences were not observed in the LG and Sol. These findings suggest that the amounts of activated muscle and its distribution would be different among TS muscles.

Muscle Architecture and its Relationship to Muscle Circulation

“…Another possibility for the different behavior in LG might be a lesser contribution to the force generation at the ankle joint. There has been no direct evidence for this, but the study by Kinugasa, et al, (2005) showed that the activation of the muscles was less (35%) in LG compared to MG (46%). If this finding implies a smaller increase in force in LG than MG with an increase in contraction level, the absence of comparable decrease in blood volume in LG would be attributable to the lower resistance in the artery supplying to LG compared with MG.…”

“…However, this explanation was not applicable to different responses in blood volume between LG and SOL because the activation of the SOL was similar to the LG (Kinugasa, et al, 2005). Possibly, different responses in blood volume to a change in the muscle length (Yokozawa, et al, 2002) may be involved…”

Muscle Functional MRI as an Imaging Tool to Evaluate Muscle Activity

“…The superior spatial resolution of MRI provides a unique opportunity to study multiple muscles and demonstrate whether the target muscle has been activated, how effectively it has been activated, and whether substitution has occurred. Studies have evaluated muscle activity during exercise of the lower (knee extension, ankle extension and flexion, running, and cycling), 29,35,38,39,47,50 and upper extremities, as well as the spine. 10,15,25,42,43,53,56,61 Other studies have evaluated the impact on muscle activity of altering the parameters (type of contraction, velocity, and intensity) of the exercise being performed.…”

Partitioning locomotor energy use among and within muscles Muscle blood flow as a measure of muscle oxygen consumption

“…Thus, this technique can indicate the relative volume of active muscle fibers, particularly under conditions of high-energy demand (Armstrong and Laughlin, 1985;Armstrong et al, 1986) or prolonged fatiguing contractions (English and Weeks, 1987), but cannot with any certainty be quantitatively related to aerobic energy use. Similarly, using magnetic resonance imaging to measure the transverse relaxation time estimates the volume of muscle that was active during a preceding exercise bout (Meyer and Prior, 2000;Kinugasa et al, 2005), but cannot specify the amount of energy used within this volume.…”