In vivo tendon forces correlate with activity level and remain bounded: evidence in a rabbit flexor tendon model.

While some tendons and ligaments in the lower extremity develop peak forces proportional to the intensity of activity (Komi 1990; Komi et al., 1992; Korvick et al., 1996), others maintain a steady force regardless of activity level (Herzog et al., 1993; Prilutsky et al., 1994). Investigators (Biewener et al., 1988; Korvick et al., 1996) have also shown that peak knee and ankle tendon forces approach one-quarter to one-third of ultimate or failure force values. In the rabbit flexor digitorum profundus (FDP) tendon model we tested several hypotheses, chiefly that peak in vivo forces not only increase with increasing activity but do not exceed one-third of their ultimate or failure values. The FDP tendon was instrumented in three animals, and each rabbit subjected to an experimental design involving three activity levels. Peak tensile forces and rates of rise and fall in tendon force increased significantly with increasing activity (p < 0.01). Further, the tendon maintained a non-zero force level throughout all trials. For the most vigorous activity, inclined hopping, tensile forces and stresses were, on average, within 30% of the tendon's ultimate force and stress values, respectively. Such in vivo measurements in different tendon systems should help investigators better understand the recruitment and contribution of important muscle-tendon units to joint stability and gait.