The first test aimed to find the optimal angles for the hip and the knee to be set as initial conditions for the subsequent experiments. This analysis was conducted in one day through a series of measurements at different angles on a group of 8 subjects (Figure 2 a). This first step allowed for the identification of the optimal starting machine settings for quantifying at best muscle strength of knee extensors and flexors in a standing position.
While subjects underwent maximal force recordings for knee extensors and flexors using the machine, at the same time, surface electromyographic measurements (FREEEMG, BTS Bioengineering) at the rectus femoris, vastus medialis, long head of the biceps femoris, and semitendinosus were performed. After appropriate skin cleaning, two electrodes were attached by an expert operator 0.02 m apart (center-to-center) on the skin, above each muscle, halfway between the center of the belly and the distal myotendinous junction.
These measurements were made by varying the hip and knee initial inclination. In the literature, many devices set the hip at a fixed flexion angle of 90°. In these conditions, it is known that the maximal extensor force can be obtained with the knee flexed between 60° and 70°, while the maximal flexor force can be obtained with the knee flexed at about 40° [36 (link),37 (link)]. Thus, in our analysis, we first fixed the knee flexed to 70° and varied the hip’s flexion from −30 to +40°, recording the maximal extensor force. Then, we repeated the procedure with knee flexion fixed at 40°, recording the maximal flexor force. The results obtained allowed for the setting of the optimal hip angles, which resulted in +20° for the extensors and +30° for the flexors (see the results section) after establishing the optimal hip flexion angles. Finally, we reevaluated the strength and muscle activation profiles as the knee angle changed, namely 30, 40, and 50° for both flexors and extensors, in order to find the knee angles that produced the maximum strength of the two muscle groups for these specific angles of the hip. Each test was performed two times, in which we acquired MVIC value recorded by the exoskeleton and maximum contraction value (MCV) extracted by EMG signal at a sampling rate of 1 kHz.
While subjects underwent maximal force recordings for knee extensors and flexors using the machine, at the same time, surface electromyographic measurements (FREEEMG, BTS Bioengineering) at the rectus femoris, vastus medialis, long head of the biceps femoris, and semitendinosus were performed. After appropriate skin cleaning, two electrodes were attached by an expert operator 0.02 m apart (center-to-center) on the skin, above each muscle, halfway between the center of the belly and the distal myotendinous junction.
These measurements were made by varying the hip and knee initial inclination. In the literature, many devices set the hip at a fixed flexion angle of 90°. In these conditions, it is known that the maximal extensor force can be obtained with the knee flexed between 60° and 70°, while the maximal flexor force can be obtained with the knee flexed at about 40° [36 (link),37 (link)]. Thus, in our analysis, we first fixed the knee flexed to 70° and varied the hip’s flexion from −30 to +40°, recording the maximal extensor force. Then, we repeated the procedure with knee flexion fixed at 40°, recording the maximal flexor force. The results obtained allowed for the setting of the optimal hip angles, which resulted in +20° for the extensors and +30° for the flexors (see the results section) after establishing the optimal hip flexion angles. Finally, we reevaluated the strength and muscle activation profiles as the knee angle changed, namely 30, 40, and 50° for both flexors and extensors, in order to find the knee angles that produced the maximum strength of the two muscle groups for these specific angles of the hip. Each test was performed two times, in which we acquired MVIC value recorded by the exoskeleton and maximum contraction value (MCV) extracted by EMG signal at a sampling rate of 1 kHz.
Free full text: Click here
