Which of the following external forces is used by the practitioner to best elongate soft tissue?

The choice of tensile as the best external force for elongating soft tissue is correct because tensile force refers to the application of a pulling or stretching force. This type of force is specifically effective in creating tension within the muscle fibers and connective tissues, leading to elongation. When a practitioner effectively applies tensile force during massage techniques such as traction or stretching, it allows for increased flexibility and range of motion in the soft tissues, which can help alleviate muscle tightness and improve overall mobility.

In contrast, the other forces play different roles in tissue response. Friction is typically used to create heat and improve circulation but does not primarily serve to elongate tissues. Compression force, on the other hand, involves pressing or applying weight onto tissues, which often leads to a decrease in tissue length rather than elongation. Shear force acts parallel to the tissue surface and is often used to mobilize fascia but is not aimed at elongation either. Hence, tensile force is uniquely suited for promoting the lengthening of soft tissue within the scope of massage therapy.