Residual force enhancement in humans: Is there a true non‐responder?

Abstract

When an active muscle is stretched and kept isometrically active, the resulting force is enhanced compared to a purely isometric reference contraction at the same muscle length and activity; a generally accepted muscle property called residual force enhancement (rFE). Interestingly, studies on voluntary muscle action regularly identify a significant number of participants not showing rFE. Therefore, the aim was to unmask possible confounders for this non‐responsive behavior. Ten participants performed maximum voluntary isometric plantarflexion contractions with and without preceding stretch. Contractions were accompanied by the assessment of voluntary activation using the twitch‐interpolation technique. The same test protocol was repeated four additional times with a least on day rest in‐between. Additionally, at the first and fifth sessions, a submaximal tetanic muscle‐stimulation condition was added. At both muscle‐stimulation sessions mean rFE higher 10% (p < 0.028) was found. In contrast, during voluntary muscle action, individual participants showed inconsistent rFE across sessions and only one session (#3) had significant rFE (5%; p = 0.023) in group means. As all participants clearly had rFE in electrical stimulation conditions, structural deficits cannot explain the missing rFE in voluntary muscle action. However, we also did not find variability in voluntary activation levels or muscle activity as the confounding characteristics of “non‐responders.”

Residual force enhancement is affected more by quadriceps muscle length than stretch amplitude

“…We thus speculate there is no true non-responder. This speculation is supported by recent work by Paternoster et al, 2021 , who showed that voluntary rFE appeared inconsistently in the same participants across five testing sessions, whereas during submaximal tetanic electrical muscle stimulation, all participants consistently showed rFE. Indeed, having participants match muscle activity levels through voluntary effort, rather than through artificial stimulation, likely adds variability to rFE estimates (especially over a short duration analysis window like the one used here), and might contribute to the ‘non-responder’ phenomenon.…”

Eccentric exercise ≠ eccentric contraction

“…Our findings align with those of from Hisey et al and Bakenecker et al (40,66), who showed that rFE depends on the final muscle length rather than the fascicle stretch amplitude. Additionally, our findings showed that rFE strongly depends on the peak fascicle force during stretch (Figure 7C) as similarly reported by Bullimore et al (33) for the in-situ cat soleus and by Paternoster et al (36,37) for peak torque during in vivo human studies. However, using peak torque instead of peak fascicle force makes it challenging to draw clear conclusions related to rFE because similar peak torques occurred at different joint angles, MTU lengths, and fascicle lengths in our study (Figure 6, Table 2).…”

“…Differences in fascicle stretch amplitude during eccentric exercises might affect peak fascicle force during stretch, the residual force enhancement (rFE) following stretch (33)(34)(35), and the active and passive contributions to eccentric force production. This is because rFE was found to be related to the peak force and/or torque during stretch (33,36,37). However, it is unclear whether peak force is affected more by stretch amplitude or fascicle length (38).…”

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

“…We therefore speculate that there is no true non-responder. This speculation is supported by recent work from Paternoster et al (2021), who showed that voluntary rFE appeared inconsistently in the same participants across five testing sessions, whereas during submaximal tetanic electrical muscle stimulation, all subjects consistently showed rFE. Indeed, having participants match muscle activity levels through voluntary effort, rather than through artificial stimulation, likely adds variability to rFE estimates (especially over a short duration analysis window like the one used here), and might contribute to the 'non-responder' phenomenon.…”