Independence of reaction time and response force control during isometric leg extension

Progressive resistance, whole body long-axis rotational training improves kicking motion motor performance

OBJECTIVES

To evaluate lower extremity muscle activation, peak resultant ground reaction force (GRF) production and quickness during performance of a kicking motion following progressive resistance, whole body long-axis rotational training.

DESIGN

Randomized, controlled study.

SETTING

Kinesiological research laboratory.

PARTICIPANTS

Thirty-six healthy subjects were assigned to a training (Group 1) or to a control (Group 2) group.

MAIN OUTCOME MEASURES

Time-synchronized EMG (1000 Hz), peak resultant GRF (1000 Hz) and two-dimensional kinematic (60 Hz) data were collected as subjects responded to an audio cue by kicking a cone. Group mean change differences (MCD) were compared using independent sample t-tests. Fisher's exact tests were used to determine group differences in the proportion of subjects that displayed earlier activation responses post-training.

RESULTS

Group 1 MCD revealed earlier gluteus maximus, gluteus medius, rectus femoris, medial hamstrings, and biceps femoris activation timing than Group 2 (P ≤ 0.006) and more Group 1 subjects displayed earlier activation of these muscles post-training (P ≤ 0.041). Group 1 MCD also revealed earlier peak resultant GRF timing and improved "kick quickness" than Group 2 (P ≤ 0.014) and more Group 1 subjects displayed earlier response timing for these variables post-training (P = 0.035).

CONCLUSION

Progressive resistance, whole body long-axis rotational training may improve performance during sports movements that require quick, integrated trunk-lower extremity function.

Grip force control during gait initiation with a hand-held object

When walking with a hand-held object, grip force is coupled in an anticipatory manner to changes in inertial force resulting from the accelerations and decelerations of gait. However, it is not known how grip and inertial forces are organized at the onset of gait, and if the two forces are coupled in the early phases of gait initiation. Moreover, initiating walking with an object involves the coordination of anticipatory postural (e.g., ground reaction force changes) and grasping adjustments. The aim of this study was to investigate the relationship of ground reaction, grip, and inertial force onsets, and the subsequent development of the coupling of grip and inertial forces during gait initiation with a hand-held object. Ten subjects performed gait initiation with a hand-held object following predictable and unpredictable start signals. We found that ground reaction and grip force onsets were closely linked in time regardless of the predictability of the start signal. In the early period of gait initiation, the grip force started to increase prior to inertial force changes. While the strength of the coupling of grip and inertial forces was moderate in this early phase, it increased to values observed during steady-state gait after the swing foot left the ground. The early grip force increase and the coupling of grip and inertial forces represent an anticipatory control process. This process establishes an appropriate grip-inertial force ratio to ensure object stability during acceleration after foot-off and maintains this increased ratio thereafter. The results suggest that grasping and whole body movements are governed by a common internal representation.