Increases in limb circumferences from training are usually

Increases in limb circumferences from training are usuallyassociated with muscular hypertrophy. Significant increases instrength accompanied by increased limb circumferences have beenreported due to isometric training (Kanehisa and Miyashita 1983a;Kitai and Sale 1989; Meyers 1967; Rarick and Larson 1958). However,changes in limb circumferences do not necessarily accompanyincreased strength (Ward and Fisk 1964). Neural adaptations alsoplaya role in increased strength due to isometric training.

Computerized tomography and magnetic resonance imaging (MRI) havebeen used to more directly determine muscle cross-sectional areachanges due to isometric training. Six weeks of isometric traininghas resulted in elbow-flexor cross-sectional area increases of 5.4%and isometric strength increases of 14.5%. Nevertheless, nosignificant correlation between the increased strength and cross-sectional area was shown (Davies et al. 1988). Eight weeks of knee-extensor isometric training resulted in a 28% increase in isometricstrength and a 14.6% increase in the cross-sectional area with asignificant correlation shown between the increased strength andincreased cross-sectional area (Garfinkel and Cafarelli 1992).Twelve weeks of training resulted in a significant increase of 8% inknee-extensor cross-sectional area and a 41 % increase in isometricstrength (Kubo et al. 2001). Whether hypertrophy occurs and theextent to which it occurs may vary from muscle to muscle, as shownby the following studies. Type I and II muscle fiber diameters inthe vast us lateralis did not change after 9 weeks of isometrictraining at 100% of MVMA (Lewis et al. 1984). Type I and II fiberareas increased in the soleus approximately 30% after 16 weeks ofisometric training with either 30% or 100% of MVMA (Alway,MacDougall, and Sale 1989; Alway, Sale, and MacDougall 1990),whereas only the type II fibers of the lateral gastrocnemiusincreased in area 30 to 40% after the identical training program(Alway, MacDougall, and Sale 1989; Alway, Sale, and MacDougall1990). Some data indicate that longer-duration muscle actions resultin greater gains in crosssectional area than shorter-duration muscleactions (Schott, McCully, and Rutherford 1995). Muscle cross-sectional area (computerized tomography) was determined before andafter training with four 30-second actions and four sets of 10repetitions each 3 seconds in duration. Even though total durationof the isometric muscle actions (120 seconds per training session)were identical between the two groups, the longer duration isometricactions resulted in a significant increase in quadriceps cross-sectional area (10-11%), whereas the shorter-duration muscularactions resulted in nonsignificant increases (47%) in quadricepscross-sectional area. Similar to increases in isometric strength,the blood flow occlusion and resulting increase in intramuscularmetabolite concentrations may be related to the greater increase inmuscle cross-sectional area with the longer-duration isometricactions. Another study indicates 100% MVMAs results in significantlygreater hypertrophy than 60% MVMAs (Kanehisa et al. 2002). In thisstudy, training lasted 10 weeks and consisted of either 12 muscleactions at 100% MVMA with each action lasting 6 seconds or 4 actionsat 60% MVMA with each action lasting 30 seconds. Muscle size changeswere measured using magnetic resonance imaging. Collectively, theseresults indicate hypertrophy is dependent on both isometric muscleaction duration and intensity (percent of MVMA used in training).Muscle protein synthesis in the soleus after an isometric action at40% of MVMA to fatigue (approximately 27 minutes) increasessignificantly by 49% (Fowles et al. 2000). This finding supports theefficacy of isometric actions inducing muscle hypertrophy.Collectively, this information indicates that muscular hypertrophyof both the type I and type II muscle fibers can occur fromisometric training with submaximal and maximal muscle actions.Increases in strength may also occur due to neural adaptations.Isometric training can bring about increases in hypertrophy andneural adaptations, both of which can increase strength.