ABSTRACT
Mechanical strain is central to both Wolff’s law and Frost’s “Mechanostat” model of bone. However, few empirical studies have considered the adaptation of soft connective tissue structures, such as tendon, to load. Although there is evidence that markers of tendon inflammation and collagen synthesis may peak as late as 24 hours post-exercise (Langberg et al., 2002; Miller et al., 2005), there has been minimal systematic research conducted on the acute effect of strain stimulus on the in vivo properties or morphology of human tendon. In one of the few studies performed thus far, Kubo et al. (2002) noted a 27% decrease in the Achilles tendon stiffness immediately following five minutes of isometric plantarflexion exercise and hypothesized that mechanical loading invoked a biphasic response in tendon, in which postexercise remodeling resulted in a transient period of mechanical weakness prior to full recovery. While the study did not include measures of tendon morphology to support their claim, a recent investigation employing magnetic resonance imaging (MRI) has shown a transient increase in Achilles tendon volume (12%–17%) and intratendinous signal (23%–31%) 30 minutes after exercise (Shalabi et al., 2004a; Shalabi et al., 2004b). The authors proposed that the greater dimensions and increased water content served a protective mechanism, which effectively lowered the stress within the tendon during exercise. However, this is not a biologically plausible mechanism, as an increase in the water content of tendon would not influence the tensile stress borne by the collagen fibres, but would influence tendon diameter and apparent stiffness (modulus). Moreover, given that both experiments employed cross-sectional study designs, the time-course of the adaptive change in tendon morphology to exercise remains unknown. The aim of the current study, therefore, was to describe the time-course of the acute change in the sagittal thickness of the Achilles tendon in response to a bout of resistive exercise.
