Should pre–measurement physical activity be standardized in muscle thickness and stiffness evaluations? – A randomized controlled four arm cross-over study

High standardization is of crucial relevance for reliability in imaging diagnostics. When quantifying muscle properties (muscle thickness and stiffness) by ultrasound or myotonometry, internal validity can be compromised by examiner-related factors and participant biologic variability. A frequently neglected source of bias is pre-measurement activity, which may acutely alter muscle perfusion and muscle blood inflow. The acute influence of different physical activity routines on tissue parameters was investigated in 30 healthy participants (16 m, 14f). Ten minutes before, immediately before, immediately after and 10 min retention of cycling, jogging, calf raises or control, muscle thickness and stiffness measurements via shear wave elastography (SWE) and myotonometry were measured. Reliability was excellent for muscle thickness (ICC = 0.94–1.00; CV = 1.7–9.1%), good-excellent for SWE stiffness (ICC = 0.68–0.97; CV = up to 26% for inter-day) and myotonometry (muscle ICC = 0.77–0.98; CV = 4.0–17% tendon 0.86–0.93 (CV = 11–17%). Muscle thickness significantly increased after calf raises (d = 1.60, 10.3%) and jogging (d = 0.60, 3.0%), without effects after cycling or control. Shear-wave elastography showed muscle stiffness decreased after calf raises (d=-0.73, -16.7%). Myotonometry indicated a stiffness increase (d = 1.04, 20.1%). The 10-minute retention showed consistent effects for muscle thickness (d = 0.80, 5.3%) and stiffness (SWE: d = 0.78, 21.1%, myotonometry: d=-0.82, -13.0%). Pre-measurement activity could systematically affect muscle thickness and stiffness with dependence on activity type and intensity. This highlights the importance of monitoring pre-measurement activity to minimize potential reliability issues as this, depending on several potential moderators, could enhance the random error if within sample pre-measurement activity is not standardized. Before ultrasound evaluation, for some activity (i.e. calf raises), > 10 min of rest was required to diminish this bias.