Background Following stretch of an active muscle, muscle force is enhanced, which is known as residual force enhancement (rFE). As earlier studies found apparent corticospinal excitability modulations in the presence of rFE, this study aimed to test whether corticospinal excitability modulations contribute to rFE. Methods Fourteen participants performed submaximal plantar flexion stretch-hold and fixed-end contractions at 30% of their maximal voluntary soleus muscle activity in a dynamometer. During the steady state of the contractions, participants either received subthreshold or suprathreshold transcranial magnetic stimulation (TMS) of their motor cortex, while triceps surae muscle responses to stimulation were obtained via electromyography (EMG), and net ankle joint torque was recorded. B-mode ultrasound imaging was used to confirm muscle fascicle stretch during stretch-hold contractions in a subset of participants. Results Following stretch of the plantar flexors, an average rFE of 7% and 11% was observed for contractions with subthreshold and suprathreshold TMS, respectively. 41–46 ms following subthreshold TMS, triceps surae muscle activity was suppressed by 19–25%, but suppression was not significantly different between stretch-hold and fixed-end contractions. Similarly, the reduction in plantar flexion torque following subthreshold TMS was not significantly different between contraction conditions. Motor evoked potentials, silent periods and superimposed twitches following suprathreshold TMS were also not significantly different between contraction conditions. Discussion As TMS of the motor cortex did not result in any differences between stretch-hold and fixed-end contractions, we conclude that rFE is not linked to changes in corticospinal excitability.

23Following active muscle stretch, a muscle's force capacity is increased, which is known as 24 residual force enhancement (rFE). As earlier studies found modulations of cortical excitability 25 in the presence of rFE, this study aimed to test whether corticospinal drive contributes to rFE. 26 Fourteen participants performed submaximal plantar flexion stretch-hold and fixed-end 27 contractions at 30% of their maximal voluntary soleus muscle activity in a dynamometer. 28 During the steady state of the contractions, participants either received subthreshold or 29 suprathreshold transcranial magnetic stimulation (TMS) of their motor cortex while triceps 30 surae muscle responses to stimulation were obtained by electromyography (EMG) and net 31 plantar flexion torque was recorded. B-mode ultrasound imaging was used to confirm muscle 32 stretch during stretch-hold contractions in a subset of participants. Following stretch of the 33 plantar flexors, an average rFE of 7% and 11% was observed for contractions with 34 subthreshold and suprathreshold TMS, respectively. 42-46 milliseconds following 35 subthreshold TMS, triceps surae muscle activity was suppressed by 19-24%, but no difference 36 in suppression was found between contraction conditions. Similarly, the reduction in plantar 37 flexion torque following subthreshold TMS was not different between contraction conditions. 38Motor evoked potentials, silent periods and superimposed twitches following suprathreshold 39 stimulations were also not different between contraction conditions. As stimulations of the 40 motor cortex by TMS did not result in any differences between stretch-hold and fixed-end 41 contractions, we conclude that corticospinal drive does not contribute to the increased torque 42 production in the presence of rFE following active muscle stretch. 43 44 New & Noteworthy 45 This study tested whether corticospinal drive contributes to the increased torque capacity in 46 the presence of rFE. Through subthreshold and suprathreshold TMS of the motor cortex, 47 51 52 Keywords: eccentric contraction, active muscle stretch, neural control, cortical excitability, 53 torque enhancement 54 55It is well known that stretch of an active muscle results in increased force production during 56 the isometric steady state following stretch compared with the steady-state force produced at 57 the same muscle length and activation level during a fixed-end contraction. This is referred to 58 as residual force enhancement (rFE), which was initially investigated in situ (Abbott and 59 Aubert, 1952) and in isolated muscle fibres (Edman et al., 1982). Edman et al. (1982) 60 suggested that rFE after active muscle stretch results from non-uniformities in sarcomere 61 lengths. Since then, a variety of studies have investigated the development of rFE and 62 suggested additional potential underlying mechanisms. These suggestions include stretch-63 induced increases in the number of attached cross-bridges, an increase in the average cross-64 bridge force and/or the attachment...