The impact of loading, unloading, ageing and injury on the human tendon

Abstract

A tendon transfers force from the contracting muscle to the skeletal system to produce movement and is therefore a crucial component of the entire muscle-tendon complex and its function. However, tendon research has for some time focused on mechanical properties without any major appreciation of potential cellular and molecular changes. At the same time, methodological developments have permitted determination of the mechanical properties of human tendons in vivo, which was previously not possible. Here we review the current understanding of how tendons respond to loading, unloading, ageing and injury from cellular, molecular and mechanical points of view. A mechanistic understanding of tendon tissue adaptation will be vital for development of adequate guidelines in physical training and rehabilitation, as well as for optimal injury treatment.

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

“…The primary function of the tendon is to transfer forces between bone and muscle, facilitating joint motion . Although originally assumed to be inert, tendinous structures can experience adaptations and are capable of significant architectural adaptations from habitual loading and injury . Injured tendons tend to be less stiff, despite increased thickness due to a shift in viscoelastic properties .…”

“…Additionally, tendinopathy negatively affects tendon structure, leading to increased vascularization and overall thickness . Although long‐term alteration in tendon morphology is minimal in healthy, mature human tissue, tendons can increase in stiffness to optimize the time and magnitude of force transmission between muscle and bone . 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 .…”

“…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 .…”

“…Therefore, isometric training should occur predominantly at relatively LMLs as there is a clear advantage for improving muscle volumes (Figure ) and strength throughout a range of motion . Additionally, large increases in tendon stiffness following LML have been reported, which would likely reduce electromechanical delay and therefore improve RFD . Furthermore, LML isometric training may have beneficial effects on the length‐tension relationship, although greater evidence is needed to solidify optimal angle as a key variable in performance and injury prevention .…”

“…Resistance training is widely utilized as a component of physical preparation for populations ranging from elite strength and power athletes to injured members of the general public . Commonly documented resistance training adaptations include increased muscle mass, tendon quality, strength, power, and range of motion, delaying muscular fatigue, and improving voluntary activation . Dynamic movements incorporating the stretch‐shortening cycle (SSC) comprise the overwhelming majority of resistance training programs .…”