Increased rate of force Increased rate of force development and neural drive development and neural drive

of human skeletal muscle of human skeletal muscle

following resistance trainingfollowing resistance training Per Aagaard. Erik B. Simonsen, Jesper Per Aagaard. Erik B. Simonsen, Jesper

L. Andersen, Peter Magnusson, and L. Andersen, Peter Magnusson, and Poul Dyhre-Poulsen Poul Dyhre-Poulsen

J Appl PhysiolJ Appl Physiol 93: 1318-1326, 2002. 93: 1318-1326, 2002.

Explosive Strength & Explosive Strength & RFDRFD

Explosive strength can be defined as Explosive strength can be defined as the rate of rise in contractile force at the rate of rise in contractile force at the onset of contraction.the onset of contraction.

RFD is slope of the joint moment – time RFD is slope of the joint moment – time curve.curve.

RFD has functional significance in fast RFD has functional significance in fast and forceful contractions.and forceful contractions.

Explosive strength is positively related Explosive strength is positively related to the relative content of type II myosin to the relative content of type II myosin heavy chain (MHC) isoforms.heavy chain (MHC) isoforms.

EMG & Explosive EMG & Explosive StrengthStrength

RFD may be influenced by neural RFD may be influenced by neural activation, muscle size, and fiber activation, muscle size, and fiber composition.composition.

Enhanced neuromuscular drive has been Enhanced neuromuscular drive has been demonstrated after heavy-resistance demonstrated after heavy-resistance training.training.

Ballistic training has been shown to Ballistic training has been shown to increase doubletsincrease doublets

As a result, neural drive and RFD can be As a result, neural drive and RFD can be expected to increase in response to expected to increase in response to resistance training.resistance training.

Methods: Subjects & Methods: Subjects & TrainingTraining

15 untrained males15 untrained males 14 weeks of training (38 sessions)14 weeks of training (38 sessions) Hack squats, leg press, knee Hack squats, leg press, knee

extension, hamstring curls.extension, hamstring curls. Training loadsTraining loads

10-12 RM first 10 days10-12 RM first 10 days 3-10 RM middle sessions3-10 RM middle sessions 4-6 RM final 4 weeks of training (Heavy 4-6 RM final 4 weeks of training (Heavy

Loading)Loading)

Pre-Post Strength Pre-Post Strength MeasuresMeasures

Maximal isometric knee extensions at 70 d knee Maximal isometric knee extensions at 70 d knee angle were measured with isokinetic angle were measured with isokinetic dynamometer.dynamometer.

Torque data sampled at 1000 Hz, low-pass Torque data sampled at 1000 Hz, low-pass filtered at 15 Hz.filtered at 15 Hz.

Torque Onset defined as torque > 7.5 N•mTorque Onset defined as torque > 7.5 N•m Joint moments were gravity corrected.Joint moments were gravity corrected. RFD computed at: 0-30, 0-50, 0-100, 0-200 ms.RFD computed at: 0-30, 0-50, 0-100, 0-200 ms. Impulse computed at: 0-30, 0-50, 0-100, 0-200 Impulse computed at: 0-30, 0-50, 0-100, 0-200

ms.ms. Impulse and RFD normalized to peak isometric Impulse and RFD normalized to peak isometric

torque.torque. (NOTE) Peak torque will change due to training.(NOTE) Peak torque will change due to training.

Pre-Post EMG Pre-Post EMG EMG recorded from VL, VM & RF at 1000 Hz.EMG recorded from VL, VM & RF at 1000 Hz. IEMG computed at: 0-30, 0-50, 0-100, 0-200 IEMG computed at: 0-30, 0-50, 0-100, 0-200

ms.ms. AveEMG (MAV) computed at: 0-30, 0-50, 0-AveEMG (MAV) computed at: 0-30, 0-50, 0-

100, 0-200 ms.100, 0-200 ms. Rate of Rise (RER) of EMG computed at: 0-Rate of Rise (RER) of EMG computed at: 0-

30, 0-50, 0-75 ms.30, 0-50, 0-75 ms. Onset EMG was set at 70 ms before onset of Onset EMG was set at 70 ms before onset of

contraction (Torque > 7.5)contraction (Torque > 7.5) Statistics: Wilcoxon signed-rank test (non-Statistics: Wilcoxon signed-rank test (non-

parametric dependent t-test).parametric dependent t-test).

Fig. 1.   A: moment and raw (EMG) VL, VM, RF. Time = 0 corresponds to the onset of muscle contraction.

RFD was defined as the slope of the moment-time curve ( moment/ time) derived at time intervals of 0-30, 0-50, 0-100, and 0-200 ms.

B: moment and filtered EMG

Fig. 2.   Average moment-time curves before and after 14 wk of heavy-resistance training. Onset of contraction is denoted by solid circle. Dotted vertical lines indicate time intervals of 30, 50, 100, and 200 ms relative to the onset of contraction. Increases in peak isometric moment were observed posttraining in parallel with a steeper slope of the moment-time curve in the early time phase of muscle contraction. The increase in slope was reflected by a significant increase in contractile RFD, which was observed both in the initial (30 and 50 ms) and later (100 and 200 ms) phases of force rise.

Fig. 3.   Contractile RFD (means ± SE) before (open bars) and after (hatched bars) 14 wk of heavy-resistance strength training. RFD ( moment/ time) was calculated in time intervals of 0-30, 50, 100, and 200 ms ( time) from the onset of contraction. In addition, peak RFD was determined within the early contraction phase (0-200 ms). Pre- to posttraining differences: * P < 0.05 and ** P < 0.01

Fig. 4.   Contractile impulse (means ± SE) before (open bars) and after (hatched bars) 14 wk of heavy-resistance strength training. Contractile impulse, defined as the area covered by the moment-time curve ( Moment dt), was calculated in the time intervals of 0-30, 50, 100, and 200 ms from the onset of contraction. Pre- to posttraining differences: * P < 0.05 and ** P < 0.01.

Fig. 5.   Normalized moment-time curve, before (solid line) and after (dashed line) heavy-resistance training. Onset of contraction is denoted by the solid circle. Steeper slopes of the normalized moment-time curves were observed in the initial phase of force rise after training, as reflected by an increase in normalized RFD at 0- maximal voluntary contraction (MVC). In contrast, normalized RFD remained unchanged when determined at higher force levels (0-1/2 MVC, 0- MVC)

Fig. 6.   Normalized RFD (means ± SE) before (open bars) and after (hatched bars) 14 wk of heavy-resistance strength training. Normalized RFD was determined at normalized force intervals of 0- MVC, 0-1/2 MVC, and 0- MVC. Pre- to posttraining differences: * P < 0.05.

Fig. 7.   A: EMG signal amplitudes (means ± SE) before (open bars) and after (hatched bars) training.

B: rate of EMG rise (RER) (means ± SE) before and after training.

RER ( EMG/ time) was determined in time intervals ( time) of 0-30, 50, and 75 ms relative to the onset of EMG integration.

Pre- to posttraining differences: * P < 0.01 and ** P < 0.001.

Fig. 8.   Time needed to reach a given level of contractile impulse before and after training. The magnitude of contractile impulse is directly proportional to the velocity of the lower limb segment. Note that all curve points are located below the line of identity (dashed line), as it took significantly shorter time to reach a given value of contractile impulse after the period of training. It took 109 ms to reach an impulse of 10 N · m · s before training, whereas only 98 ms after training.

DiscussionDiscussion

The training induced gains in RFD The training induced gains in RFD and impulse were attributed to and impulse were attributed to enhanced neural drive in the early enhanced neural drive in the early phase of the contraction (0-200 ms).phase of the contraction (0-200 ms).

This increased neural drive at the This increased neural drive at the onset of contraction may be due to onset of contraction may be due to recruitment and/or discharge rate.recruitment and/or discharge rate.