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According to a randomized clinical trial {1}, isometric contractions are more analgesic than isotonic contractions for tendon pain. The explanation they give is as follows:

The antinociceptive effect of motor activation may be explained by several potential centrally mediated mechanisms. Motor centers have direct projections to the dorsal horn {35} #8667 and also to midbrain centers that are themselves powerful generators of descending inhibition. Completing maximal or near maximal effort without tissue damage reinforces positive effects {36} #8596. Furthermore, fatiguing tasks that require effort and demand attention compete with pain to influence the output generator{37} #8648 {38}; #8650. However, these mechanisms may not explain the differences between isometric and isotonic muscle contractions as both protocols required fatiguing motor activation with cognitive demand. Therefore, they [sic] may also be differences in the local tissue response to the 2 types of contractions (such as nociceptive receptor stimulation or change to the biochemical environment) and/or unique differences in the cortical activation pattern between isometric and isotonic muscle contractions. Previous studies have reported a hypoalgesic effect of exercise depending on the type; aerobic exercise reduced perception of experimentally induced pain in healthy people (effect size for threshold d = 0.41, intensity d = 0.59), dynamic/isotonic exercise (threshold d = 0.83, intensity d = 0.75) and isometric exercise (threshold d = 1.02, intensity d = 0.72),{39} #8668.

The explanation above is inconclusive: they make a guess that there may be differences in the local tissue response to the 2 types of contractions (such as nociceptive receptor stimulation or change to the biochemical environment) and/or unique differences in the cortical activation pattern between isometric and isotonic muscle contraction. But this isn't clearly supported by any evidence.

Why are isometric contractions more analgesic than isotonic contractions for tendon pain?

Note that the randomized control trial reported in {1} had only 20 participants, so it might happen that other randomized control trials contradict the findings. If you are aware of randomized control trials confirming or infirming the results presented in {1} that isometric contractions are more analgesic than isotonic contractions, please leave a comment, write an answer or edit the question.

This tweet poll https://twitter.com/ProfJillCook/status/1127911783937613824 (mirror) seem to indicate that isometrics help tendon pain, but it is quite informal and doesn't compare against isotonic exercises:

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References:

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