I received this question from one of my students who is currently undertaking the Master Flexibility Trainer course: "Why is measuring tissue behaviour so difficult?" I thought it was a great question and would also make for an interesting blog post, so I'll share my answer here.
Being able to measure a given tissue's mechanical properties accurately is crucial to understanding how it functions. ("Mechanical properties" means how the tissue behaves when subjected to force.) However, the tests themselves which are used to perform such measurements can change the behaviour of the tissue. So naturally, this presents researchers with quite the problem!
Good measurement in science strikes a fine balance between quantifying our observations as accurately as possible and acknowledging that error is everywhere, and so no methodology can give us perfect results. Therefore, researchers typically approach measurement in three ways: in vitro, in situ, and in vivo. Each method has its advantages and disadvantages.
In vitro translates from Latin into English as "within glass." While this may conjure images of test-tube experiments, in vitro refers to any investigation conducted in an artificial environment. The sample of tissue (e.g., bone, connective tissue, muscle) is isolated and soaked in a solution that is maintained at body temperature. Although the solution mimics the sample's natural environment, the test is performed outside of the body, in an artificial environment, and so is referred to as an in vitro experiment. The beauty of in vitro tests is that we can directly measure the sample. However, to measure it directly, the researcher must remove it from its native habitat. Thus, questions remain about the validity of applying data from in vitro tests to living human beings.
In situ translates from Latin into English as "in the original place." In situ tests are conducted outside the body, but the samples retain more of the cells' original natural environment. For example, whereas an in vitro skeletal muscle sample will isolate the muscle cells and remove all other structures, in situ samples will keep the surrounding structures like fascia, blood vessels, and nerves. A large majority of our current knowledge regarding musculoskeletal physiology was gathered this way. While in situ measurements better replicate the human body's internal conditions, it is essential to remember that they are still artificial tests.
In vivo translates from Latin into English as "in the living." Many people assume that performing tests within a living, breathing human body provides the best data. But getting that data is incredibly difficult. For example, even if researchers can successfully record data, they must consider the possibility that the transducers inserted into the body may have interfered with the data sample. There is also the substantial hurdle of ethical issues. For example, obtaining good data about the mechanical properties of musculoskeletal injury is very challenging because no decent ethics committee would authorise a study that purposely tried to injure test subjects. While some researchers have successfully measured in vivo mechanical forces and technology continues to improve, always keep in mind that most in vivo experiments involving musculoskeletal tissues have been done using animals. As with results from in vitro experiments, we need to exercise caution when applying findings from animal models to human beings.