The interaction of trunk-load and trunk-position adaptations on knee anterior shear and hamstrings muscle forces during landing

Abstract

Context: Because anterior cruciate ligament (ACL) injuries can occur during deceleration maneuvers, biomechanics research has been focused on the lower extremity kinetic chain. Trunk mass and changes in trunk position affect lower extremity joint torques and work during gait and landing, but how the trunk affects knee joint and muscle forces is not well understood.

Objective: To evaluate the effects of added trunk load and adaptations to trunk position on knee anterior shear and knee muscle forces in landing.

Design: Crossover study.

Setting: Controlled laboratory environment.

Patients or other participants: Twenty-one participants (10 men: age = 20.3 +/- 1.15 years, height = 1.82 +/- 0.04 m, mass = 78.2 +/- 7.3 kg; 11 women: age = 20.0 +/- 1.10 years, height = 1.72 +/- 0.06 m, mass = 62.3 +/- 6.4 kg).

Intervention(s): Participants performed 2 sets of 8 double-leg landings under 2 conditions: no load and trunk load (10% body mass). Participants were categorized into one of 2 groups based on the kinematic trunk adaptation to the load: trunk flexor or trunk extensor.

Main outcome measure(s): We estimated peak and average knee anterior shear, quadriceps, hamstrings, and gastrocnemius forces with a biomechanical model.

Results: We found condition-by-group interactions showing that adding a trunk load increased peak (17%) and average (35%) knee anterior shear forces in the trunk-extensor group but did not increase them in the trunk-flexor group (peak: F(1,19) = 10.56, P = .004; average: F(1,19) = 9.56, P = .006). We also found a main effect for condition for quadriceps and gastrocnemius forces. When trunk load was added, peak (6%; F(1,19) = 5.52, P = .030) and average (8%; F(1,19) = 8.83, P = .008) quadriceps forces increased and average (4%; F(1,19) = 4.94, P = .039) gastrocnemius forces increased, regardless of group. We found a condition-by-group interaction for peak (F(1,19) = 5.16, P = .035) and average (F(1,19) = 12.35, P = .002) hamstrings forces. When trunk load was added, average hamstrings forces decreased by 16% in the trunk-extensor group but increased by 13% in the trunk-flexor group.

Conclusions: Added trunk loads increased knee anterior shear and knee muscle forces, depending on trunk adaptation strategy. The trunk-extensor adaptation to the load resulted in a quadriceps-dominant strategy that increased knee anterior shear forces. Trunk-flexor adaptations may serve as a protective strategy against the added load. These findings should be interpreted with caution, as only the face validity of the biomechanical model was assessed.

Figure 1

Lateral view of marker set. Medial markers on the right leg and left sides of the trunk and pelvis are not shown. The clusters of markers on the foot (3), lower leg (4), and femur (4) were used to track motion of the lower extremity during dynamic landing trials. Markers on the right and left anterior-superior iliac spines and sacrum were used to track the motion of the pelvis. A group of 4 markers on the posterior trunk tracked motion of the trunk segment.

Figure 2

Representative trunk flexion and shear forces in each trunk adaptation group. Data are presented for A, 1 individual in the trunk flexor adaptation group and B, 1 individual in the trunk extensor group. Positive values represent trunk extension and anterior shear forces. Negative values represent trunk flexion and posterior shear forces. Knee shear forces are normalized to body weight.

Figure 3

Peak and average anterior shear forces across conditions and between groups. Vertical bars represent 95% confidence intervals. ^a Indicates increase across no-load to trunk-load conditions. ^b Indicates group differences in the trunk-load condition. ^c Indicates group differences in the no-load condition. ^a–c Collectively used to explain the condition-by-group interactions for both peak and average variables (both P < .01).

Figure 4

Peak and average hamstrings forces across conditions and between groups. Vertical bars represent 95% confidence intervals. ^a Indicates increase across no-load to trunk-load conditions. ^b Indicates group differences in the trunk-load condition. ^c Indicates group differences in the no-load condition. ^a–c Collectively used to explain the condition-by-group interactions for both peak (P < .05) and average variables (P < .01).

Figure 5

Knee muscle force responses across conditions and between groups. Arrows indicate increases or decreases based on the results of the 2 × 2 analyses of variance for average muscle-force–dependent variables.