Amount and structure of force variability during short, ramp and sustained contractions in males and females

Abstract

1The aim of this study was to investigate the effect of gender differences on force 2 variability assessed by means of linear and nonlinear estimators during short duration,

Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

“…An unanimous finding among previous studies assessing three-dimensional steadiness of static forces is a monotonic increase of the SD of the three force components when increasing target forces (Hong et al 2007;Svendsen and Madeleine 2010;Salomoni and GravenNielsen 2012), which is further supported by the present data. This increase of SD with target force is related to the orderly recruitment of motor units: In order to increase muscle force, larger motor units are recruited (Henneman et al 1965), which produce larger and unfused twitches, resulting in increased force fluctuations (Jones et al 2002).…”

“…Since the CoP time-series is inversely proportional to the task-related force, the total excursion calculated over the CoP time-series can also be considered a normalized parameter. Furthermore, supporting the idea that fluctuations in task-related and tangential forces are governed by similar, or synergistic, mechanisms (Hong et al 2007;Svendsen and Madeleine 2010;Salomoni and Graven-Nielsen 2012), the present results show increased total excursions of the CoP and the CV of force during pain compared to baseline for all the assessed muscle groups (Fig 7B, D), as predicted by the second hypothesis.…”

“…This increase of SD with target force is related to the orderly recruitment of motor units: In order to increase muscle force, larger motor units are recruited (Henneman et al 1965), which produce larger and unfused twitches, resulting in increased force fluctuations (Jones et al 2002). As the resultant force represents the spatial summation of activity from different muscles, each contributing to fluctuations on its own direction of action (Kutch et al 2008), an overall increase in muscle activity modulates task-related and tangential forces similarly (Hong et al 2007;Svendsen and Madeleine 2010) , as observed in Fig. 5.…”

“…While previous investigations were limited to unidirectional force assessments during contractions of the upper limbs at moderate force levels (20% -35% of maximal voluntary force, MVC), force fluctuations are known to depend on muscle architecture and on the level of exerted force , and studies using exercise-induced delayed onset muscle soreness as pain model for the elbow flexors reported greater impairments during force levels under 10% MVC (Semmler et al 2007). The assessment of thee-dimensional force fluctuations has been so far limited to baseline and fatiguing contractions, showing similar modulation of task-related and tangential force components (Hong et al 2007;Svendsen and Madeleine 2010;Salomoni and GravenNielsen 2012), but it is unclear if the three components of force fluctuations are similarly affected by pain. This investigation may provide new insight on how pain-related changes in muscle activity affect the stability of three-dimensional forces during daily life activities.…”

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

“…Previous studies have focused on unidirectional force variability [Bandholm et al, 2008;Missenard et al, 2009] and recent investigations included three-dimensional force recordings obtaining additional information regarding motor control performance analysing the variability of tangential forces Graven-Nielsen, 2012a, 2012b;Svendsen and Madeleine, 2010]. The muscle contribution to the force output can be estimate by surface electromyography, which is a non-invasive technique assessing global activity of motor unit populations [Disselhorst-Klug et al, 2009;Farina et al, 2004].…”