Effects of plyometric and isometric training on muscle and tendon stiffness in vivo

Abstract

The purpose of this study was to compare the effects of plyometric and isometric training on tendon properties during ramp and ballistic contractions and muscle stiffness under passive and active conditions. Eleven subjects completed 12 weeks (3 days/week) of a unilateral training program for the plantar flexors. They performed plyometric training on one side (PLY) and isometric training on the other side (ISO). Active muscle stiffness in the medial gastrocnemius muscle was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions. Passive muscle stiffness was also calculated from estimated passive muscle force and fascicle length during slow passive stretching. Stiffness and hysteresis of tendon structures were measured using ultrasonography during ramp and ballistic contractions. Passive muscle stiffness and tendon hysteresis did not change for PLY or ISO. Active muscle stiffness significantly increased for PLY, but not for ISO. Tendon stiffness during ramp and ballistic contractions increased significantly for ISO, but not for PLY. In addition, tendon elongation values at force production levels beyond 100 N during ballistic contractions increased for PLY. These results suggest that plyometric training (but not isometric training) enhances the extensibility of tendon structures during ballistic contractions and active muscle stiffness during fast stretching, and these changes may be related to improved performances during stretch‐shortening cycle exercises.

Isometric training and long‐term adaptations: Effects of muscle length, intensity, and intent: A systematic review

“…Additionally, the included studies only compared isometric training at ~55 and 90% of MVIC which leaves a large range of potential intensities. However, previous interventions have reported large increases (17.5%‐61.6%, ES = 0.57‐4.9, P < 0.05) in tendon stiffness following training between 70% and 100% of MVIC . Therefore, it might be that a minimum intensity of ~70% MVIC is required to induce meaningful changes in tendon thickness and stiffness.…”

“…Conversely, healthy increases in tendon thickness and stiffness in response to exercise have been found to be region specific and may have rehabilitative, pre‐habilitative, and performance benefits . For instance, heavy (resistance) training can lead to an increase in maximal muscular force and rate of force development by increasing tendon stiffness, thus reducing the electromechanical delay . Additionally, increased tendon stiffness through chronic loading can be due to increased tendon CSA without alterations in viscoelastic properties, potentially improving safety when performing ballistic movements .…”

Changes in joint, muscle, and tendon stiffness following repeated hopping exercise

“…The range of motion of the ankle from touchdown to the lowest position (ROM) was calculated. The ankle joint torque during the drop jump was estimated from the following equation (Kawakami et al, ; Kubo et al, ; Kubo et al, ; Suzuki et al, ):…”

“…Therefore, the active muscle stiffness obtained using this methodology represented the intrinsic characteristic of muscle under an active condition without any potential neural effects. According to cross‐sectional and longitudinal studies (Kubo et al, ; Kubo et al, ), we have investigated training‐induced changes in active muscle stiffness. Furthermore, we recently proposed a method to evaluate active muscle stiffness including the effect of the stretch reflex from changes in the torque and fascicle length during slower stretch velocity (peak angular velocity was 100 deg·sec −1 ) (Kubo et al, ).…”

“…In these studies, the elongation rates of tendons during the measurements of tendon properties were markedly lower than those during SSC exercises. Our recent studies showed that tendon properties (elongation and hysteresis) during ballistic contractions were different from those during ramp contractions (Kubo et al, 2017;Kubo, 2018;Kouno et al, 2019). Accordingly, we need to revalidate the effects of repeated SSC exercises on tendon properties during ballistic as well as ramp contractions.…”