This article will take a peek at some of the contraversial, and poorly understood areas concerning the neuromuscular system relating to resistance exercise.
Training to Failure
Whether or not training to failure is more productive seems to be individualistic, based on personal preference and other possible physical or mental factors. Nonetheless, there are some points that have been studied by research.
The "CNS" is wrongly blamed for failure during a 'set to failure' (1). In isolated bouts, studies do not support this idea. During a set of intermitant contractions, supraspinal output has been studied by supplementing the contractions with added electrical current (2,3). Even to the point of fatigue, the lacking point of force has been found to be locally in the muscular system (4, 18). Usually in the excitation-contraction coupling systems.
Long term, or too frequent use of 'training to failure' has been theoretically linked to more general CNS fatigue (23). Most likely changes in neurotransmitters similar to that found when a subject experiences any overload of emotional stress. My contention is that 'how' a person uses 'to failure' training, and 'how' they actually 'train to failure' is where the differnces lie. If one is relaxed and works the muscles until 'they' fail, less systematic stress would be induced. However, if one makes a set to failure into a "do or die, use your entire being to get the reps" scenario, this would be much more stressful to the CNS and endocrine system (15).
Volumes could be, and have been written on this subject. It has been thoroughly studied for decades. Basic highlights are these.
Human muscle fibers are employed in an 'all or none' fashion. Either there is enough signal to recruit a motor unit, where it 'fires', or there is not. Once a fiber is recruited, it fires with varying frequencies to alter it's force. A gradual increase in force would find a fiber recruited at it's lowest rate coding frequency. As force requirements increased, so would rate coding. This would increase the fiber's tension. However, if a fiber is recruited at a certain tension, and fatigue rises, rate coding will be increased only to 'maintain' that required tension.
Recruitment is orderly, based motor unit size (16, 17). It increases from smallest to largest. Momentary required force (5), based on the percentage of maximum momentary effort (6) dictates the required number of motor units for the task. The majority of larger motor units consist of FT fibers, although the fiber type is not the reason itself.
Full recruitment occurs before full force is reached (21, 22). Depending on the muscular structures involved, full recruitment occurs anywhere from 40-90% of full effort (7).
Increased force after full recruitment has occured, is generated by 'rate coding' (13), and synchronization of motor units. These are methods of neural manipulation where the central nervous systems 'fires' the fibers at a faster rate or has more motor units 'on' at any point in time. Typical rate coding frequencies can vary from 10-20 hz up to 50 hz or higher. Each fiber can still only generates it's momentary maximum tension, but this tension is displayed at a higher rate. This causes more fibers to be 'on' at any point in time, thus increasing the total net force.
This term used to describe how much of the maximum potential muscular force a subject can generate of their own volitional effort. Well motivated subjects are usually able to fully activate their musculature (8 ,10, 11). Those that cannot, are very close to full activation and can usually reach full activation with practice (9, 20). This means, most of us can not only recruit all motor units but reach full voluntary force. This is typically tested by comparing the maximum voluntary force generated with the maximum force attained by electrical stimulation.
Neural "strength gains"
This relates to the 'strength without size' concept, where performance and/or strength increases without apparant gains in cross sectional area. (CSA) Since most subjects can activate their recruited musculature fully, it would appear there is no room for strength increases based purely on neural mechansims, but there are, indeed, other means.
Subjects who cannot fully activate their musculature, will soon become able to (8, 9, 10, 11).
Reduction of antagonistic contractions will allow more of the generated force by the agonist(s) to be effective (19).
Coordination of stabilizer muscles will allow more effective and efficient contractions.
Increases in cardio-vascular fitness will allow a better 'clearing of metabolic wastes'.
Increased tolerance to pain will allow a more 'intense' performance
Note: 4-6 usually translate to an increase in TUL or reps, rather than actual strength per se'
Intra-muscular Temperature (Warm-ups)
Temperature is a factor of concern for both the nervous system, and the muscles. The nervous system operates more efficiently at a cooler temperature (14) than do the muscles themselves.
Hopefully this information can help explain some of the simpler ideas with the nervous system as it relates to muscular contractions and weight training activities. More extensive information can be attained from many college texts on the subject. Most of the preceeding information can be found, with relevant references, in Roger M. Enoka's "Neuromechanics of Human Movement" and Komi's text "Strength and Power in Sport". Many other references may be found in the neural link section on the HIT and Abbreviated Forum.
Duchateau & Hainaut, 1993; Gandevia, 1998 CNS not failure
Allen, McKenzie, & Gandevia, 1998 EMS proving CNS not failure
Bigland-Ritchie et al., 1982 EMS proving CNS not failure
Bigland-Ritch, Furbush, & Woods, 1986 failure in muscle
Darling & Hayes, 1983; Windhorst et al., 1986 momentary force recruitment
Cafarelli, 1988; Jones & Hunter, 1983 momentary force, effort recruitment
Deluca, LeFever, McCue & Xenakis, 1982; Kukulka & Clamann, 1981; Van Cutsem et al,. 1997
Yeu et al., 2000 full activation
Garfinkel and Cafarelli, 1992 full activation
Bellanger and McComas, 1981 full activation
Gandevia and McKenzie, 1988 full activation
F. Bellemare, J.J. Woods, R. Johansson and B. Bigland-Ritchie , 1983
B. Bigland-Ritchie, C. K. Thomas, C.L. Rice, J.V. Howarth and J.J. Woods ,1992
R.M. Enoka and D.G. Stuart 1992
H.J. Freund, H.J. Budingen and V. Dietz, 1975 orderly recruitment
Clamann H.P., Henneman E., 1976 orderly recruitment
J. Duchateau and K. Hainaut, 1985 failure in muscle
B. Carolan and E. Cafarelli, 1982
R.D. Herbert and S.C. Gandevia, 1996 full activation
Akataki K, Mita K, Watakabe M., 2004 full recruitment
Dario Farina1, Luigi Fattorini, Francesco Felici, and Giancarlo Filligoi, 2002 full recruitment
R.M. Enoka and D.G. Stuart, 1992 long term CNS overtraining