Author(s): TSAI, M.S.1,2, DOMROES, T.1,2, PENTIDIS, N.1,2, KOSCHINSKI, S.1,2, SCHROLL, A., BOHM, S.1,2, ARAMPATZIS, A.1,2, MERSMANN, F.1,2, Institution: HUMBOLDT-UNIVERSITÄT ZU BERLIN, Country: GERMANY, Abstract-ID: 289

INTRODUCTION:
Human tendons adapt to their mechanical environment. The acute to loading response shows a positive net collagen synthesis after 24 to 48 hours (1), which could be a particularly effective window for further stimulation. An increase of loading volume may increase the anabolic response, yet there are indications for a ceiling-effect (1). Therefore, in the present longitudinal study we examined the hypotheses that (a) repeated loading after 24 to 48 h of recovery (i.e., high frequency loading) elicits faster and greater adaptation effects than when the net anabolic state may already decline (i.e., ≥ 48 h recovery; low frequency), and (b) that human tendons show a non-linear dose-response relationship, with greater effects of a high compared to low but not intermediate loading volume.
METHODS:
We assigned 91 adult men to either a control or an intervention group. In the latter, the two legs were randomly assigned to one of five Achilles tendon (AT) high-intensity loading protocols (i.e., 90% maximum voluntary contraction, about 4.5 to 6.5% tendon strain) that were systematically modified in terms of loading frequency (i.e., sessions per week) and overall loading volume (i.e., total time under loading). Before the intervention, after eight and 16 weeks, AT mechanical properties were determined using inverse dynamics and ultrasonography. The cross-sectional area (CSA) and length of the free AT were measured using magnetic resonance imaging pre- and post-intervention. The data was analyzed using a linear mixed model.
RESULTS:
There were significant increases of muscle strength (average over all protocols: 11%), normalized AT stiffness (31%) and CSA of the free AT (4%) in the intervention group (p < 0.05), yet with no marked differences between protocols (p > 0.05). While no clear effects were found considering the temporal coordination of loading and overall loading volume, combining the data from each of the two high and low volume protocols (i.e., with high and low frequency, respectively), greater changes in stiffness were found following low compared to high volume loading (p = 0.04). In all protocols, the major changes in normalized AT stiffness occurred within the first eight weeks of the intervention.
CONCLUSION:
The present study provides first experimental evidence that the adaptation of the human AT mechanical and morphological properties in men is rather independent of the temporal coordination of loading and recovery and can be effectively stimulated with a comparatively low loading volume (i.e., 180 seconds per week at 4.5 to 6.5% tendon strain). An increase in loading volume does not seem to further promote tendon adaptation, which supports the idea of a “ceiling-effect” in the acute responses to loading. Thus, low-volume loading programs seem to be the more time-efficient and attractive approach for the practical field.

REFERENCES:
1) Magnusson et al. 2010, Nat Rev Rheumatol 6, 262–268