Browsing by Author "Power, Geoffrey A."

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    Effects of fiber type on force depression after active shortening in skeletal muscle
    (Elsevier, 2015-07-16) Joumaa, Vénus; Power, Geoffrey A.; Hisey, Brandon; Caicedo, A.; Stutz, J.; Herzog, Walter
    The aim of this study was to investigate force depression in Type I and Type II muscle fibers. Experiments were performed using skinned fibers from rabbit soleus and psoas muscles. Force depression was quantified after active fiber shortening from an average sarcomere length (SL) of 3.2µ m to an average SL of 2.6 µm at an absolute speed of 0.115f iber length/s and at a relative speed corresponding to 17% of the unloaded shortening velocity (V0) in each type of fibers. Force decay and mechanical work during shortening were also compared between fiber types. After mechanical testing, each fiber was subjected to myosin heavy chain (MHC) analysis in order to confirm its type (Type I expressing MHC I, and Type II expressing MHC IId). Type II fibers showed greater steady-state force depression after active shortening at a speed of 0.115 fiber length/s than Type I fibers (14.5±1.5% versus 7.8±1.7%). Moreover, at this absolute shortening speed, Type I fibers showed a significantly greater rate of force decay during shortening and produced less mechanical work than Type II fibers. When active shortening was performed at the same relative speed (17% V0), the difference in force depression between fiber types was abolished. These results suggest that no intrinsic differences were at the origin of the disparate force depressions observed in Type I and Type II fibers when actively shortened at the same absolute speed, but rather their distinct force-velocity relationships.
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    Mitigating the bilateral deficit: reducing neural deficits through residual force enhancement and activation reduction
    (European Journal of Applied Physiology, 2018-09) Mazara, Nicole; Herzog, Walter; MacDonald, Graham Z.; Power, Geoffrey A.
    The bilateral deficit (BLD) is characterized by a reduction in maximal voluntary torque during a bilateral contraction relative to the sum of left and right unilateral contractions. The BLD has been attributed to interhemispheric inhibition as a result of unilateral torque differences between limbs. If the BLD is the result of interhemispheric inhibition, lowering activation for a torque matching task, as shown in residual force enhancement (RFE), may help overcome the decrease in neural drive during bilateral contractions. Therefore, the purpose of the present study was to determine whether RFE could reduce the BLD.
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    The sag response in human muscle contraction
    (Springer, 2018-03-08) Smith, Ian C.; Ali, Jahaan; Power, Geoffrey A.; Herzog, Walter
    PURPOSE: We examined how muscle length and time between stimuli (inter-pulse interval, IPI) influence declines in force (sag) seen during unfused tetani in the human adductor pollicis muscle. METHODS: A series of 16-pulse contractions were evoked with IPIs between 1 × and 5 × the twitch time to peak tension (TPT) at large (long muscle length) and small (short muscle length) thumb adduction angles. Unfused tetani were mathematically deconstructed into a series of overlapping twitch contractions to examine why sag exhibits length- and IPI-dependencies. RESULTS: Across all IPIs tested, sag was 62% greater at short than long muscle length, and sag increased as IPI was increased at both muscle lengths. Force attributable to the second stimulus increased as IPI was decreased. Twitch force declined from maximal values across all IPI tested, with the greatest reductions seen at short muscle length and long IPI. At IPI below 2 × TPT, the twitch with highest force occurred earlier than the peak force of the corresponding unfused tetani. Contraction-induced declines in twitch duration (TPT + half relaxation time) were only observed at IPI longer than 1.75 × TPT, and were unaffected by muscle length. CONCLUSIONS: Sag is an intrinsic feature of healthy human adductor pollicis muscle. The length-dependence of sag is related to greater diminution of twitch force at short relative to long muscle length. The dependence of sag on IPI is related to IPI-dependent changes in twitch duration and twitch force, and the timing of peak twitch force relative to the peak force of the associated unfused tetanus.
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    The stretch-shortening cycle (SSC) revisited: residual force enhancement contributes to increased performance during fast SSCs of human m. adductor pollicis
    (Physiological Reports, 2015) Herzog, Walter; Seiberl, Wolfgang; Power, Geoffrey A.; Hahn, Daniel
    The stretch-shortening cycle (SSC) occurs in most everyday movements, and is thought to provoke a performance enhancement of the musculoskeletal system. However, mechanisms of this performance enhancement remain a matter of debate. One proposed mechanism is associated with a stretch-induced increase in steady-state force, referred to as residual force enhancement (RFE). As yet, direct evidence relating RFE to increased force/work during SSCs is missing. Therefore, forces of electrically stimulated m. adductor pollicis (n = 14 subjects) were measured during and after pure stretch, pure shortening, and stretch-shortening contractions with varying shortening amplitudes. Active stretch (30°, x = 161 6°s 1) caused significant RFE (16%, P < 0.01), whereas active shortening (10°, 20°, and 30°; x = 103 3°s 1, 152 5°s 1, and 170 5°s 1) resulted in significant force depression (9–15%, P < 0.01). In contrast, after SSCs (that is when active stretch preceded active shortening) no force depression was found. Indeed for our specific case in which the shortening amplitude was only 1/3 of the lengthening amplitude, there was a remnant RFE (10%, P < 0.01) following the active shortening. This result indicates that the RFE generated during lengthening affected force depression when active lengthening was followed by active shortening. As conventional explanations, such as the storage and release of elastic energy, cannot explain the enhanced steady-state force after SSCs, it appears that the stretch-induced RFE is not immediately abolished during shortening and contributes to the increased force and work during SSCs.