Shortening‐induced force depression in human adductor pollicis muscle

Abstract

The effects of single isovelocity shortening contractions on force production of the electrically stimulated human adductor pollicis muscle were investigated in seven healthy male subjects. Redeveloped isometric force immediately following isovelocity shortening was always depressed compared with the isometric force recorded at the same muscle length but without preceding shortening. The maximal isometric force deficit (FD) was (mean ± s.e.m.) 37 ± 2 % after 38 deg of shortening at 6.1 deg s−1. The FD was positively correlated with angular displacement (r2 > 0.98) and decreased with increasing velocity of the shortening step. Stimulation at 20 Hz instead of 50 Hz reduced absolute force levels during the contractions to about 73 % and the FD was decreased to a similar extent. Eighty‐nine per cent of the velocity‐related variation in the FD could be explained by the absolute force levels during shortening. FD was largely abolished by allowing the muscle to relax briefly (approximately 200 ms), a time probably too short for significant metabolic recovery. At all but the highest velocities there was a linear decline in force during the latter part of the isovelocity shortening phase, suggesting that the mechanisms underlying FD were active during shortening. Our results show that shortening‐induced force deficit is a significant feature of human muscle working in situ and is proportional to the work done by the muscle‐tendon complex. This finding has important implications for experimental studies of force‐velocity relationships in the intact human.

Temperature Effect on the Rates of Isometric Force Development and Relaxation in the Fresh and Fatigued Human Adductor Pollicis Muscle

“…When the thumb was fully adducted its length axis was parallel with the length axis of the index finger and this position was defined as 0 deg thumb angle. All measurements were performed at a thumb angle of 51 deg, which is the optimum for force production, although the angle-force relationship is very flat over a range (38-76 deg) of thumb angles (De Ruiter et al 1998). …”

Variation in isometric force after active shortening and lengthening and their mechanisms: a review

“…Literature data point to the idea that the intensity of the decline of isometric force is directly influenced by the extent of muscle shortening, the shortening speed and the amount of force produced during the shortening phase (2,6,7,14,15,16,33). This can be seen in the summary of the studies shown in Table 1, which also shows that the employed force control, in most studies, is made by frequency of the stimulation produced in the sarcomere, or to muscle fiber.…”

“…Thus, it is proposed that this non-uniformity would lead to depression of isometric force following an active pre shortening because while some sarcomeres would resume its original shape and length (observed during isometric contraction reference), others remain shortened even after the removal of stimulus (2,6,7,14,15,16,32,34,35,36,40,51).…”

“…stimulation combined with maximum voluntary dynamic contractions (5). Furthermore, devices have relied on complex servo motors coupled to force transducers that bear little resemblance to human movement pattern (2,11,14,15,16,17,18,19,20,21,22,23,24,25,26).…”

“…Moreover, the FD cannot be completely eliminated after a maneuver of muscle relaxation by deactivation nor manipulated by shortening with fast speed, preceded by a shortening with slow and steady speed, which would result in a reduction of the isometric force to zero (6,7,15,42). stimulation combined with maximum voluntary dynamic contractions (5).…”