Residual force enhancement within the human quadriceps is greatest during submaximal stretch-hold contractions at a very long muscle length

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 (rFETQ) and a new, non-invasive shear-wave tensiometry technique (rFEWS). 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 rFETQ at long (7±5%) and very long (12±8%) but not short (2±5%) muscle lengths, whereas rFEWS 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 rFETQ (r=0.63, p=.001) and rFEWS (r=0.52, p=.017), but relationships were not significant between VL fascicle stretch amplitude and rFETQ (r=0.33, p=.126) or rFEWS (r=0.29, p=.201). PT shear-wave speed-angle relationships did not agree with estimated quadriceps muscle 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.

Residual force enhancement decreases when scaling from the single muscle fibre to joint level in humans

“…The in-vivo rFE values observed in the present study (1-12% for the fascicle-matching method and 4-19% for the joint-matching method) are within previously reported values of up to 34% rFE in the knee extensors for maximal voluntary contractions 13,22,24,30,35 . Furthermore, the magnitudes of rFE we observed in single fibres (6-46%) are within previously reported ranges in single fibres from the human vastus lateralis 8 .…”

“…To identify the site for ultrasound measurements, a line was drawn from the greater trochanter to the lateral condyle of the left femur using indelible ink, followed by a transverse line marking 50% of the distance between those landmarks 22,27,28 . With the participant seated in the dynamometer and the leg fully extended, a 65-mm wide Telemed linear array probe (LV8-4L65S-3; Frequency: 8 MHz; Image depth: 60 mm) was placed just proximal to the 50% line on the lateral aspect of the vastus lateralis (Figure 1), then the probe position that produced the clearest view of muscle fascicles with the superficial and deep aponeuroses parallel in the image was determined.…”

“…Ultrasonographic recording of fascicle length during rFE experiments 13,[22][23][24] , in conjunction with muscle biopsies, provides the opportunity to gain a better understanding of how rFE at the single muscle fibre level scales to joint-level voluntary contractions in humans.…”

“…Therefore, to translate in-vivo joint-level rFE experiments to the single fibre level, one could match fascicle length, rather than joint angle, between the isometric steady-states of the stretch-hold and fixed-end contractions. Ultrasonographic recording of fascicle length during rFE experiments 13,22–24 , in conjunction with muscle biopsies, provides the opportunity to gain a better understanding of how rFE at the single muscle fibre level scales to joint-level voluntary contractions in humans.…”