Increased force following muscle stretching and simultaneous fibre shortening: Residual force enhancement or force depression - That is the question?
Residual force enhancement (rFE) describes the increase in isometric force following muscle stretching compared to the corresponding isometric force. Even though rFE is consistently observed in isolated muscle preparations, it is not always observed in human skeletal muscle. This inconsistency might be associated with disociations between length changes in muscle tendon units (MTUs) and fibres. This prompted the question if there is rFE for conditions where the MTU is stretched while fibres shorten. Rabbit tibialis anterior (TA) MTUs (n = 4) were stretched and the isometric forces following stretching were compared to corresponding forces from isometric reference contractions. Unique combinations of stretch speed and activation were used to create conditions of continuous fibre shortening during MTU stretch. Mean force was increased (18 ± 2%) following MTU stretching compared to the isometric reference forces. Without fibre length measurements, this result would be referred to as rFE. However, fibre shortening in the reference contractions was always greater than for the eccentric stretch contractions, suggesting that the observed increase in force might be caused by less residual force depression (rFD) in the stretch tests compared to the reference contractions. However, the work performed by fibre shortening was similar between the reference and the MTU stretch contractions, suggesting that rFD was similar for both experimental conditions. Therefore, we conclude that we observed rFE in the absence of contractile element stretching. However, a lack of knowledge of the molecular mechanisms that distinguish rFE from rFD prevents an unequivocal pronouncement of what caused the enhanced forces after active muscle stretching.