Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions

Abstract

Voluntary activation levels during lengthening, isometric, and shortening contractions (angular velocity 60 degrees/s) were investigated by using electrical stimulation of the femoral nerve (triplet, 300 Hz) superimposed on maximal efforts. Recruitment of fiber populations was investigated by using the phosphocreatine-to-creatine ratio (PCr/Cr) of single characterized muscle fibers obtained from needle biopsies at rest and immediately after a series of 10 lengthening, isometric, and shortening contractions (1 s on/1 s off). Maximal voluntary torque was significantly higher during lengthening (270 +/- 55 N.m) compared with shortening contractions (199 +/- 47 N.m, P < 0.05) but was not different from isometric contractions (252 +/- 47 N.m). Isometric torque was higher than torque during shortening (P < 0.05). Voluntary activation level during maximal attempted lengthening contractions (79 +/- 8%) was significantly lower compared with isometric (93 +/- 5%) and shortening contractions (92 +/- 3%, P < 0.05). Mean PCr/Cr values of all fibers from all subjects at rest were 2.5 +/- 0.6, 2.0 +/- 0.7, and 2.0 +/- 0.7, respectively, for type I, IIa, and IIax fibers. After 10 contractions, the mean PCr/Cr values for grouped fiber populations (regardless of fiber type) were all significantly different from rest (1.3 +/- 0.2, 0.7 +/- 0.3, and 0.8 +/- 0.6 for lengthening, isometric, and shortening contractions, respectively; P < 0.05). The cumulative distributions of individual fiber populations after either contraction mode were significantly different from rest (P < 0.05). Curves after lengthening contractions were less shifted compared with curves from isometric and shortening contractions (P < 0.05), with a smaller shift for the type IIax compared with type I fibers in the lengthening contractions. The results indicate a reduced voluntary drive during lengthening contractions. PCr/Cr values of single fibers indicated a hierarchical order of recruitment of all fiber populations during maximal attempted lengthening contractions.

Familiarisation to maximal recumbent eccentric cycling

“…We demonstrated a trend for increased RF and MG activation during the initial stages of familiarisation to maximal, recumbent, eccentric cycling. Similar findings have also been observed in isolated eccentric contractions and attributed to a reduction in neural inhibition [18,19]. Neural inhibition is thought to limit muscle activation to protect the muscle-tendon unit from high forces that would otherwise cause damage.…”

Neural control of lengthening contractions

“…Whereas the peak force that can be evoked from isolated fibres and whole muscles in animals is usually 50-80% greater during lengthening contractions (Edman, 1988;Katz, 1939;Morgan et al, 2000), the peak force achieved in untrained humans during maximal voluntary contraction (MVC) is usually either comparable or only modestly greater (<40%) for lengthening contractions versus isometric or slow shortening contractions. For example, no significant difference was observed in peak force between lengthening and isometric contractions in young adults for the knee extensors (Amiridis et al, 1996;Babault et al, 2001;Beltman et al, 2004;Seger and Thorstensson, 2000;Westing et al, 1991), ankle plantar flexors (Pinniger et al, 2000) and elbow flexors (Colson et al, 1999), whereas a slightly greater force was reported during lengthening actions for the knee extensors (Aagaard et al, 2000;Kellis and Baltzopoulos, 1998;Reeves et al, 2009), elbow flexors (Linnamo et al, 2006) and plantar flexors (Duclay et al, 2011). In contrast, the force produced during lengthening contractions by the ankle dorsiflexors is substantially greater (∼30-50%) than that during isometric contractions Pasquet et al, 2000;Reeves and Narici, 2003).…”

“…To avoid these confounding factors when comparing muscle activation during the two anisometric contractions, many studies have used isokinetic dynamometers to control the force and joint angular velocity (Aagaard et al, 2000;Amiridis et al, 1996;Babault et al, 2001;Baudry et al, 2007;Beltman et al, 2004;Duclay and Martin, 2005;Duclay et al, 2011;Grabiner and Owings, 2002;Pasquet et al, 2006;Pinniger et al, 2003;Westing et al, 1990). Nonetheless, the strategy employed by the central nervous system (CNS) when resisting the force imposed by a torque motor can differ slightly from that used when lowering an inertial load to match an imposed trajectory (Duchateau and Enoka, 2008).…”

“…First, when individuals perform maximal isokinetic actions, EMG amplitude recorded with surface electrodes is often less during lengthening contractions than during shortening contractions performed at the same speed (Aagaard et al, 2000;Amiridis et al, 1996;Kellis and Baltzopoulos, 1998;Komi et al, 2000;Tesch et al, 1990;Westing et al, 1991); the difference is even greater when the lengthening contraction is not preceded by a maximal isometric contraction (Komi et al, 2000). Second, the level of voluntary activation assessed by superimposing a single stimulus or a brief train of electrical pulses over the muscle or its motor nerve during the force plateau of a MVC is often, but not always (Babault et al, 2001), depressed during lengthening contractions (Amiridis et al, 1996;Beltman et al, 2004;Westing et al, 1990). For example, Beltman and colleagues (2004) observed a deficit in voluntary activation of 21% during a maximal lengthening contraction at constant speed with the quadriceps femoris compared with a deficit of only 7% and 8% during an isometric and shortening MVC, respectively (Fig.…”

Myofibrillar and collagen protein synthesis in human skeletal muscle in young men after maximal shortening and lengthening contractions

“…The HCl was evaporated under nitrogen, and the amino acids were purified by ion exchange chromatography on Dowex H ϩ resin. Incorporation of [1,[2][3][4][5][6][7][8][9][10][11][12][13] C]leucine into protein was determined by gas chromatography combustionisotope ratio mass spectrometry as previously described (16).…”