Mechanical demand and multijoint control during landing depend on orientation of the body segments relative to the reaction force

The purpose of this study was to determine how diverse momentum conditions and anatomical orientation at contact influences mechanical loading and mul tijoint control of the reaction force during landings. Male collegiate gymn asts (n = 6) performed competition style landings (n = 3) of drop jumps, fr ont saltos, and back saltos from a platform (0.72 m) onto landing mats (0.1 2 m). Kinematics (200 fps), reaction forces (800 Hz) and muscle activation patterns (surface EMG, 1600 Hz) of seven lower extremity muscles were colle cted simultaneously. Between-task differences in segment orientation relati ve to the reaction force contributed to significant between-task difference s in knee and hip net joint moments (NJM) during the impact phase. During t he stabilization phase, ankle, knee, and hip NJMs acted to control joint fl exion. Between-task differences in muscle activation patterns indicated tha t gymnasts scaled biarticular muscle activation to accommodate for between- task differences in NJM after contact. Activation of muscles on both sides of the joint suggests that impedance like control was used to stabilize the joints and satisfy the mechanical demand imposed on the lower extremity. B etween-subject differences in the set of muscles used to control total body center of mass (TBCM) trajectory and achieve lower extremity NJMs suggests that control of multijoint movements involving impact needs to incorporate mechanical objectives at both the total body and local level. The function al consequences of such a control structure may prove to be an asset to gym nasts, particularly when required to perform a variety of landing tasks und er a variety of environmental constraints. (C) 2001 Elsevier Science Ltd. A ll rights reserved.