Author(s): GILMORE, N., HAYDEN, C., KOSITSKY, A., FELDSTEIN, J., BAAR, K. , Institution: UNIVERSITY OF CALIFORNIA DAVIS, Country: UNITED STATES, Abstract-ID: 1810

INTRODUCTION:
Tendons are mechanosensitive tissues that adapt in response to tensile strain, but the optimal stimulus for tendon adaptation is unknown. Isometric loading shows at least as much promise as eccentric loading but is understudied. We previously showed low-intensity isometric loading can improve tendon function and strength in a retrospective analysis of finger strength training data on rock climbers (PMID: 39560837), supported by our work on engineered ligaments, which indicates tenocytes are responsive to short (<10 min) load bouts irrespective of load magnitude (PMID: 28332110). Our hypothesis was that moderate- or long-duration isometric load would lead to improved tendon strength.
METHODS:
We performed a 3-week training study on adult rats using hindlimb electrical stimulation (stim) to induce controlled repeated isometric muscle contraction to determine how tendon adapts to different load magnitudes and durations with training. Each rat performed three 11-min training sessions per week (9 total). Protocols were matched for total stim time but varied in stim intensity: maximal or “high” load group vs. low intensity (50% of max) “low” load group and stim duration: (12 reps x 10s sets vs. 4 reps x 30s sets; 120s total). The three study groups (rep stim duration x intensity) were 10s x High, 10s x Low, and 30s x Low. Following training, we performed tendon tensile testing, then analyzed matrix stability using differential scanning calorimetry (DSC) and collagen content. The loading was optimized to the Achilles, a load bearing tendon, but we also analyzed the tibialis anterior (TA) tendon, which, due to stimulator placement in the posterior compartment of the distal hindlimb, received less muscle activation.
RESULTS:
Positive tendon adaptation was most evident in 30s x Low, with a trend toward higher maximal tensile load (MTL; p=0.0671) and an increased failure stress (p=0.0022) vs. control. By contrast, maximal loading was detrimental to the tendon: maximal tensile load (p=0.0106) and reduced failure stress (p=0.0005) in 10s x High vs. the contralateral control leg. Cross-sectional area (CSA) only increased in 10s x High (Achilles p=0.0753 (trend), TA p=0.0001) vs. control. Collagen (%) only changed in TA, indicating training can modestly impact CSA, but it is not the primary driver for the mechanical adaptations observed. The most apparent adaptations with DSC were on the proximal (muscle) end: proximal Achilles enthalpy (p=0.0165) and TA onset of melting temp (p=0.0105) increased in 30s x Low vs. control, and TA melt curve peak temp decreased in proximal TA (p=0.0309) indicating damage.
CONCLUSION:
Both load intensity and duration influenced the adaptive response of tendon to load, with the 30s x Low group exhibiting a superior adaptive response to training. The effect of training adaptation was greater in the TA, which as a positional tendon is subjected to much lower tensile loads in vivo than the Achilles.