How to Confuse Motor Control: Passive Muscle Shortening after Contraction in Lengthened Position Reduces the Muscular Holding Stability in the Sense of Adaptive Force

Abstract

Adaptation to external forces relies on a well-functioning proprioceptive system including muscle spindle afferents. Muscle length and tension control in reaction to external forces is most important regarding the Adaptive Force (AF). This study investigated the effect of different procedures, which are assumed to influence the function of muscle spindles, on the AF. Elbow flexors of 12 healthy participants (n = 19 limbs) were assessed by an objectified manual muscle test (MMT) with different procedures: regular MMT, MMT after precontraction (self-estimated 20% MVIC) in lengthened position with passive return to test position (CL), and MMT after CL with a second precontraction in test position (CL-CT). During regular MMTs, muscles maintained their length up to 99.7% ± 1.0% of the maximal AF (AFmax). After CL, muscles started to lengthen at 53.0% ± 22.5% of AFmax. For CL-CT, muscles were again able to maintain the static position up to 98.3% ± 5.5% of AFmax. AFisomax differed highly significantly between CL vs. CL-CT and regular MMT. CL was assumed to generate a slack of muscle spindles, which led to a substantial reduction of the holding capacity. This was immediately erased by a precontraction in the test position. The results substantiate that muscle spindle sensitivity seems to play an important role for neuromuscular functioning and musculoskeletal stability.

Another Way to Confuse Motor Control: Manual Technique Supposed to Shorten Muscle Spindles Reduces the Muscular Holding Stability in the Sense of Adaptive Force in Male Soccer Players

“…The maximal holding capacity was almost halved after the CL-procedure compared with regular testing (without any conditioning): the muscles started to lengthen already at ~53% of the maximal force, which was then reached during eccentric motion. This substantial and highly significant effect was cancelled by the short second contraction in the test position [14]. Furthermore, the peak value of the trial (AF max ) was still reached under isometric conditions (AFiso max ≈ AF max ) as for regular MMTs.…”

“…That specific adaptation of the neuromuscular system to an external force has not been sufficiently considered in motor science so far. Due to its adaptive character, we have introduced the term Adaptive Force (AF) for it [5][6][7][8][9][10][11][12][13][14]. Research over the last few years has shown that this Brain Sci.…”

“…The authors presumed that the second contraction tightened the slacked muscle spindles again [21]. Since research on shortened or slacked muscle spindles is mainly aimed at reflex reactions [15,17,[21][22][23][24], we recently performed a study investigating the behavior of AF after such procedures [14]. The AF was assessed by an objectified manual muscle test (MMT).…”

“…AF parameters were assessed by an objectified MMT in reaction to the manual spindle technique compared with regular MMT (without preceding spindle manipulation). The MMT was objectified by a handheld device that records force and limb position simultaneously [8][9][10][11][12][13][14]. Based on the results of the previous study [14], it was hypothesized that the holding capacity (AFiso max ) would be significantly reduced after spindle technique, whereas the AF max would not be affected.…”

“…The MMT was objectified by a handheld device that records force and limb position simultaneously [8][9][10][11][12][13][14]. Based on the results of the previous study [14], it was hypothesized that the holding capacity (AFiso max ) would be significantly reduced after spindle technique, whereas the AF max would not be affected. Additionally, the onset of oscillations in the force signal during the MMT was expected to be on a significantly higher force level following spindle technique compared with regular MMT.…”