Residual force enhancement is affected more by quadriceps muscle length than stretch amplitude
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Curated by eLife
Evaluation Summary:
The authors have systematically examined relationships between muscle length and force potentiation in young adults using very carefully conducted and controlled measurements by dynamometry and estimated using patellar tendon shear wave speed. The paper should be of interest to those who study human performance.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #2 agreed to share their name with the authors.)
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Abstract
Little is known about how muscle length affects residual force enhancement (rFE) in humans. We therefore investigated rFE at short, long, and very long muscle lengths within the human quadriceps and patellar tendon (PT) using conventional dynamometry with motion capture (rFE TQ ) and a new, non-invasive shear-wave tensiometry technique (rFE WS ). Eleven healthy male participants performed submaximal (50% max.) EMG-matched fixed-end reference and stretch-hold contractions across these muscle lengths while muscle fascicle length changes of the vastus lateralis (VL) were captured using B-mode ultrasound. We found significant rFE TQ at long (7±5%) and very long (12±8%), but not short (2±5%) muscle lengths, whereas rFE WS was only significant at the very long (38±27%), but not short (8±12%) or long (6±10%) muscle lengths. We also found significant relationships between VL fascicle length and rFE TQ ( r =0.63, p=0.001) and rFE WS ( r =0.52, p=0.017), but relationships were not significant between VL fascicle stretch amplitude and rFE TQ ( r =0.33, p=0.126) or rFE WS ( r =0.29, p=0.201). Squared PT shear-wave-speed-angle relationships did not agree with estimated PT force-angle relationships, which indicates that estimating PT loads from shear-wave tensiometry might be inaccurate. We conclude that increasing muscle length rather than stretch amplitude contributes more to rFE during submaximal voluntary contractions of the human quadriceps.
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Evaluation Summary:
The authors have systematically examined relationships between muscle length and force potentiation in young adults using very carefully conducted and controlled measurements by dynamometry and estimated using patellar tendon shear wave speed. The paper should be of interest to those who study human performance.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #2 agreed to share their name with the authors.)
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Reviewer #1 (Public Review):
This manuscript presents results of a study in young adult male subjects of the effect of muscle length on force potentiation of human quadriceps muscle under very carefully controlled conditions that attempt to ensure that force measurements are not influenced by position of the femur and tibia relative to the axis of rotation of the dynamometer used to directly record leg extension forces. Direct measurements of knee torques are compared to an indirect measure of force based on patellar tendon shear waves. Inferences about muscle length based on knee angles at the time of contraction are independently confirmed by evaluation of muscle fascicle length using ultrasound techniques to further control conditions across subjects. Force potentiation is measured at three joint angles providing three different …
Reviewer #1 (Public Review):
This manuscript presents results of a study in young adult male subjects of the effect of muscle length on force potentiation of human quadriceps muscle under very carefully controlled conditions that attempt to ensure that force measurements are not influenced by position of the femur and tibia relative to the axis of rotation of the dynamometer used to directly record leg extension forces. Direct measurements of knee torques are compared to an indirect measure of force based on patellar tendon shear waves. Inferences about muscle length based on knee angles at the time of contraction are independently confirmed by evaluation of muscle fascicle length using ultrasound techniques to further control conditions across subjects. Force potentiation is measured at three joint angles providing three different muscle lengths for study. Voluntary contractions were performed using EMG activity to provide a target for subject effort. Young healthy male subjects were studied. Strengths of the study include a very careful and systematic approach to measuring muscle force generation at three different muscle lengths. Challenges to applying patellar tendon shear wave speed as a measure of tendon load were identified. The data add to the weight of evidence that muscle length determines force potentiation of the quadriceps muscle in human subjects.
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Reviewer #2 (Public Review):
The authors made a careful and critical series of measurements using human dynamometry. In particular they were careful to derive the best estimate of quadriceps muscle force that they could, correcting the torque measured by the dynamometer with detailed dynamic estimates of the moment arm during knee extension movements. Figure 6 shows how much knee angle changes without any change in position of the external dynamometer arm. They used the root mean squared emg signal to signal to the subject what level of muscle activation was required. Quadriceps muscle force was also estimated by a technique established by others to measure the speed of a mechanical impulse within the patellar tendon. The squared velocity of this impulse is claimed to correlate with the transmitted force.
The authors need to make this …
Reviewer #2 (Public Review):
The authors made a careful and critical series of measurements using human dynamometry. In particular they were careful to derive the best estimate of quadriceps muscle force that they could, correcting the torque measured by the dynamometer with detailed dynamic estimates of the moment arm during knee extension movements. Figure 6 shows how much knee angle changes without any change in position of the external dynamometer arm. They used the root mean squared emg signal to signal to the subject what level of muscle activation was required. Quadriceps muscle force was also estimated by a technique established by others to measure the speed of a mechanical impulse within the patellar tendon. The squared velocity of this impulse is claimed to correlate with the transmitted force.
The authors need to make this clearer in their presentation and to make clearer when they refer to the speed of the wave, and when they refer to the squared speed that is supposed to correlate with force.
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