Different joint positions for biarticular muscles may affect force and muscular activity during single-joint exercises. The aim of this study was to compare the maximal isometric contractions and muscle activation in 2 different knee and ankle positions during prone leg curl exercise in trained subjects. Fifteen resistance-trained men (27 +/- 4 years, 178.80 +/- 5.72 cm, 86.87 +/- 12.51 kg) were recruited. The peak force (PF) and muscle activation of biceps femoris, gastrocnemius lateralis (GL), and soleus lateralis (SL) were measured during knee flexion at 0 and 90 degrees and maximal dorsiflexion (D) or plantarflexion (P). Three maximal voluntary isometric contractions of 5 seconds were performed for each combination of knee and ankle positions. Two-way repeated-measures analysis of variances were used for all dependent variables. For PF, there was a significant difference between ankle positions (D x P) at 90 degrees (p = 0.009) and knee positions (0 x 90 degrees) for D (p < 0.001) and P (p < 0.001). Peak force was greater with the knee at 0 degrees and the ankle maximally dorsiflexed. For GL, there was a significant difference between ankle (D x P) at 0 degrees (p = 0.002) and knee positions (0 x 90 degrees) for D (p = 0.005). Gastrocnemius lateralis activation was greater with the knee at 90 degrees of flexion and the ankle maximally dorsiflexed. For SL, there was a significant difference between ankle positions (D x P): at 90 degrees (p = 0.001) and at 0 degrees (p = 0.002). Soleus lateralis is more active in plantarflexion irrespective of the knee joint position. Isometric contractions with full knee extension produce more strength regardless of the ankle position; neither the knee position nor the ankle position may influence the activity of the hamstrings.

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